FACTORS AFFECTING TONALITY

Condenser microphones commonly used for professional audio come with diaphragms varying in diameter from under 5mm to over 25mm. The diaphragm’s diameter affects the performance in numerous ways. From a tonality point of view, however, the most significant affects are on the high frequency performance and the self-noise.

DIAPHRAGM DIAMETER

The high frequency limit of a condenser microphone with a circular diaphragm is primarily determined by the relationship between its diameter and the wavelength of the frequency being captured. At frequencies where the wavelength is considerably larger than the diameter, the diaphragm is too small to have any affect on the tonality of the captured sound. At frequencies where the wavelength is considerably smaller than the diameter, the diaphragm is big enough to be an obstacle and therefore have a significant affect on the tonality of the captured sound. And at frequencies where the wavelength is the same size as the diameter, a situation can occur where the diaphragm is receiving equal amounts of energy from both halves of the waveform (positive and negative), which cancel each other out and create a null (i.e. zero output). This frequency – where the wavelength is equal to the diameter of the diaphragm – theoretically determines the upper limit of a condenser microphone’s frequency response because it defines the first null point, but it’s not that straightforward…

Unlike the simple maths for determining a ribbon microphone’s high frequency limit [as shown in the second instalment], the high frequency limit of the condenser diaphragm is also affected by the angle of incidence and the way the diaphragm is mounted. For any given diaphragm, the high frequency limitations will be different in a side-address configuration than in an end-address configuration, and different again if mounted in a diffraction sphere or fitted with an equalising ring or diffusion grid.

It gets complicated when trying to define the upper frequency limit of a condenser microphone with a simple formula or rule. It is, however, worthwhile putting the concept of ‘wavelength’ into perspective with diaphragm diameter. The range of human hearing is considered to exist between 20Hz and 20kHz. Assuming a room temperature of 21°C, 20Hz has a wavelength of 17.2m – that’s how long one cycle is in the air. It’s significantly bigger than any practical microphone. At 20kHz, however, the wavelength is only 17.2mm long. It’s significantly smaller than the 25mm diaphragms commonly seen in dual-diaphragm microphones and is therefore affected by them.

Based on this knowledge, we’ll use 17mm (close enough to the wavelength of 20kHz) as a threshold point for differentiating between small and large diaphragms. There are no hard-and-fast rules for this, but condenser microphones with diaphragm diameters less than 17mm are generally described as SDCs (Small Diaphragm Condensers) while microphones with diaphragms of 20mm or more are generally described as LDCs (Large Diaphragm Condensers). Condenser microphones with diaphragm diameters less than 17mm usually exhibit extended high frequency response and better off-axis response when compared to those with diaphragms larger than 17mm.

Due to the very light weight of the diaphragm and the potential to make it very small, it is not difficult to make a condenser microphone with a frequency response extending up to 20kHz – the upper limit of human hearing. Maintaining a consistent response beyond 20kHz becomes more of a challenge, but is important for those who need to record sounds with content above the range of human hearing; whether to slow it down to create sound effects for movies and games, pitch-shift it down to analyze bat calls and insect sounds, or make ‘high resolution’ recordings for the audiophile market.

OFF-AXIS RESPONSE & TRANSIENTS

A sound arriving from directly in front of the microphone, perpendicular to the diaphragm, is considered to be on-axis, and is said to be arriving at the diaphragm from an angle of 0°. For the purpose of this discussion any sound arriving from an angle other than 0° can be considered to be off-axis because it is no longer coming directly on-axis. (This is discussed in a later instalment where we look at microphones’ polar responses and acceptance angles.)

As the diaphragm gets larger it becomes more directional at higher frequencies. As a result, sounds arriving significantly off-axis will be duller than sounds arriving on-axis. This is generally referred to as a poor off-axis response and results in a reduction of overall high frequency energy relative to low frequency energy captured by the diaphragm. Poor off-axis response at high frequencies is a characteristic of all large diaphragm mics and fuels the myth that large diaphragms have better low frequency performance than small diaphragms, when the reality is that they have worse high frequency performance (as discussed in the previous instalment, see ‘Subkick microphones’).

As the diaphragm gets smaller its off-axis response improves. This is one of the reasons why small diaphragm condensers are often preferred for main stereo pairs when making distant-miked recordings of orchestras, choirs, pipe organs and chamber music – all situations where sounds arriving from all directions (including early reflections, reverberation and audience reaction) need to be captured with a consistent tonality. If not, the sounds arriving off-axis will be duller than those arriving on-axis and result in a sound that could be described as ‘muddy’ or ‘roomy’.

A microphone’s off-axis response is not much of an issue when close-miking a single instrument, but it can be an issue if you are close-miking a number of instruments playing together in the same space. After doing what you can to minimise spill by the strategic use of instrument placement, baffles and polar response rejection (as explained in later instalments), you’ll want the remaining spill to be captured clearly so it doesn’t muddy up the sound when all the mics are blended together in the mix. The better off-axis response of smaller diaphragms can have a cumulatively positive effect in this situation.

As a generalisation, the large diaphragm condenser’s reduced high frequency capture and accompanying slower transient response means it tends to sound mellower and potentially smoother than a small diameter condenser, but with inferior off-axis response. Regardless of the diameter, however, the condenser microphone’s diaphragm is always going to be significantly lighter than a similarly-sized dynamic microphone’s diaphragm/coil assembly and can therefore move much faster, giving it excellent high frequency performance and transient response in comparison.

EQUIVALENT NOISE LEVEL

In ribbon and dynamic microphones the primary source of noise is thermal noise, as explained in the previous two instalments of this series (1) (2). This form of noise is rarely mentioned in their specifications because it is very low and will be insignificant compared to the noise from the preamplifiers they are connected to – which tend to be used at higher gains than they would be with condensers due to the lower outputs of ribbons and dynamics.

In condenser microphones the term equivalent noise level usually refers to the combination of the diaphragm’s self-noise and the noise from the microphone’s electronic circuitry. Self-noise refers to noise caused by air particles randomly striking the surface of the diaphragm. This form of noise is not a problem with dynamic microphones because their considerably heavier diaphragms are less sensitive to the random impacts of air molecules. It’s also not a problem with ribbon microphones because their ribbon elements are not held under enough tension to make them sufficiently responsive to random impacts of air molecules.

Self-noise and equivalent noise level are explained in detail in the following instalment of this series. For now, the important thing to know is that with all other parameters being equal, a large diaphragm will have less self-noise than a small diaphragm. It will also have greater sensitivity than the small diaphragm (meaning it provides a higher output level from the same sound source) but with a lower maximum SPL. So if you need to make a very quiet recording or if you need to record something that has very low volume, a large diaphragm condenser is a better choice than a small diaphragm condenser; it is quieter due to its lower self-noise, and its higher sensitivity requires less preamp gain and associated noise. If you need to record something very loud, a small diaphragm is a better choice; it can handle higher SPLs with ease, its lower sensitivity means less chance of overloading your preamp, and its higher self-noise won’t be an issue with a loud sound source. There’s more discussion about this kind of decision making in a later instalment of this series…

The noise from the electronic circuitry inside the microphone also needs to be considered. All active electronic circuits create noise – especially when that circuit has to work with the very small signals coming from a microphone capsule. (For many years this form of noise was unique to condenser microphones, but nowadays it also exists with active ribbon mics, active dynamic mics, USB mics and any other mic that has electronic circuitry in it.)

A presentation by Neumann’s Martin Schneider compared the frequency spectrum and amplitude of the diaphragm’s self-noise and the internal electronics’ noise for a large diaphragm condenser with a very low equivalent noise level. When averaged across the audible spectrum from 20Hz to 20kHz, the noise from the microphone’s internal electronics was approximately 3dB lower than the diaphragm’s self-noise. The large diaphragm microphone used in this example has an equivalent noise level of only 7dBA, therefore the noise from the microphone’s internal electronics (being on average just 3dB lower than the diaphragm’s self-noise) could be considered significant. Microphones with very small diaphragms (i.e. 6mm or less), such as Earthworks’ QTC30 or DPA’s 4060, have equivalent noise levels of 20dBA or more, most of which is self-noise. In these examples the noise of the microphone’s internal electronics is probably insignificant.

RESONANCE

Because the diaphragm is usually a circular membrane held under tension, it behaves like a tiny drum (imagine a miniature roto tom) with a strong primary resonant frequency, or note, of its own. Any sound that contains frequencies at or near the diaphragm’s resonant frequency will cause the diaphragm to resonate, emphasizing those particular frequencies above others and thereby adding to the microphone’s characteristic tonality.

The condenser microphone designer tweaks the diaphragm’s diameter, mass and tension to determine its resonant frequency – which typically ends up somewhere between 5kHz and 9kHz – and then decides how much damping should be applied to control the resonance. Increasing the damping reduces the strength of the resonance (which is equivalent to lowering the amount of boost on a peak/dip equaliser), but also reduces the diaphragm’s overall sensitivity (which is like lowering the fader). There is always a compromise between resonance and sensitivity, hence the market is full of condenser microphones with a characteristic boost in the upper midrange or high frequencies – often engineered and marketed as a feature to provide enhanced detail or to compensate for the loss of high frequencies through the air when distant miking. This balance of resonance and damping is another factor that determines how the condenser mic affects the tonality of the captured sound.

Most condenser diaphragms are ‘edge terminated’ which means the electrical connections (or ‘terminations’) to the diaphragm and backplate are made at the edges, leaving the diaphragm free to move. Some are ‘centre terminated’, which means the electrical connection is made to a terminal in the centre of the diaphragm – as seen in Neumann’s classic U47 and U67 mics respectively. This is like lightly placing your finger in the centre of a drum skin and listening to how it affects the resonances and therefore the tonality; it changes the balance of the diaphragm’s resonant modes, but in the case of the U47 and U67 it’s just one of many things that affect or determine their tonality. Another is, of course, their tube circuitry…

INTERNAL ELECTRONICS

Every circuit that an audio signal passes through imparts its own sonic fingerprint on that signal (typically as a combination of its inherent harmonic and intermodulation distortions), and every condenser microphone includes an impedance converter circuit that the signal must pass through in the process of becoming useable. That circuit will contain numerous electronic components, but the primary component will be an amplifying device of some kind (tube, FET, transistor, etc.) that imparts its own sonic fingerprint, contributing to the microphone’s overall tonality.

As with ribbon and dynamic mics, for many years a transformer was used to give the condenser microphone a balanced differential output suitable for sending a signal down a microphone cable and into a preamp. Contemporary designs use a solid state electronic circuit and don’t require an output transformer, although many still use one. The acronym ‘TLM’, used in the model names for many of Neumann’s contemporary microphones, stands for ‘Transformer Less Microphone’. Similarly, when DPA made an updated version of their classic 4006, ‘TL’ was appended to the model number to indicate it was ‘Transformer Less’. According to DPA’s documentation, removing the transformer allowed the 4006TL’s low frequency response to be extended downwards another octave from 20Hz to 10Hz.

Some condenser microphone manufacturers have returned to using transformers primarily for their ‘old school’ tonal properties, or simply to take advantage of the transformer’s relative simplicity for providing a balanced differential output. When Audio-Technica designed the 5047 – a premium version of their 5040 – they added a transformer output to maintain a ‘constant load output impedance’ and provide a ‘smooth sonic character’. [The effect of a varying load impedance was discussed in the second instalment of this series; it is the reason why the tonality of a passive ribbon microphone can be affected by the preamp it is connected to.]

DYNAMIC TRANSDUCTION

Like the ribbon microphones described in the previous instalment, dynamic microphones use magnetic induction to convert a sound into a signal. Instead of using a corrugated ribbon of aluminium, however, a coil of wire is attached to the back of a diaphragm (a circular membrane suspended in the air). The diaphragm is dome-shaped which, among other things, provides the structural integrity needed to support the coil. As with a ribbon element, the coil is immersed in a magnetic field. Sound vibrations cause the diaphragm to vibrate, moving the coil within the magnetic field and inducing an electrical current into it. For this reason, dynamic microphones are sometimes referred to as moving coil microphones. Every winding in the coil has a current induced into it, which collectively adds up to a higher induced current than could be created with a ribbon element in the same magnetic field (a ribbon is essentially just one winding of a coil).

Theoretically, the dynamic microphone doesn’t need a transformer because the diaphragm’s coil is fundamentally the same as the output of a transformer and can be connected directly to an XLR socket to provide a balanced differential output. However, many dynamic microphones rely on a transformer nonetheless for similar reasons as the ribbon microphone: to increase the signal level, provide a more appropriate output impedance, and provide a balanced differential output that’s suitable for driving the signal down a microphone cable and into a preamp. Electro-Voice’s RE20 and Shure’s SM58 both use transformers, while Electro-Voice’s RE320 and Shure’s Beta 58 do not. (Interestingly, Shure went back to using a transformer in the Beta 58A.)

One popular modification for Shure’s SM57 is to remove the transformer, which reportedly smooths out the frequency response and improves the high frequencies – at the expense of losing the 10dB of voltage gain provided by the transformer. There is plenty of ‘how to’ information online for this mod, and also for replacing the transformer with alternative versions, but be aware that these mods will void the microphone’s warranty and also change its characteristic tonality – which was probably what you bought it for in the first place. An in-line booster (see below) might be a better option…

FACTORS AFFECTING TONALITY

Although the dynamic microphone uses the same principle of magnetic induction as the ribbon microphone, its considerably heavier coil/diaphragm assembly results in reduced sensitivity to high frequencies and slower response to transients. Unlike the gentle high frequency roll-off of ribbon microphones or the extended high frequency response of condenser microphones, dynamic microphones roll-off rapidly – rarely reaching 20kHz at any notable level.

RESONATORS & DAMPERS

The dynamic microphone has a number of significant resonances; in fact, its entire frequency response could be considered as a series of controlled resonances. The first is the resonance of the diaphragm itself, which is often illustrated as being centred somewhere around 800Hz, depending on the mass, diameter and suspension. This resonance creates a large peak in the frequency response that resembles an inverted ‘V’, and is far from the straight horizontal line that’s considered ideal. Acoustic damping is applied to reduce this peak considerably. Although it is possible to damp the diaphragm’s resonance down to a flat response, increasing the damping decreases the diaphragm’s sensitivity and results in lower output so it’s always a compromise.

Other resonating systems and dampers are added as a form of passive EQ to extend the overall frequency response. One of these is a narrow tube that connects the air chamber behind the diaphragm to a larger acoustic chamber inside the mic body, forming a Helmholtz resonator (similar to a ported speaker enclosure) that boosts the low frequencies and balances them against the diaphragm’s damped resonant peak. Likewise, a resonating cavity is built into the dome-shaped space inside the diaphragm to boost the high frequencies and balance them against the low and midrange frequencies. The result of these three resonating systems – the diaphragm, the Helmholtz resonator and the resonating cavity behind the diaphragm – provides reasonably good bandwidth.

Additional resonators and dampers may be added to further enhance the performance and/or give the microphone its characteristic tonality. For example, beneath the wire mesh and internal foam wind filter of Shure’s SM57 is a small plastic disc with holes around the perimeter. It sits immediately above the diaphragm, assumedly to protect it from the foam. The underside of this disc, however, has a concave dip in the centre with a curvature that follows the top of the diaphragm; this creates a small resonating chamber immediately above the diaphragm that’s tuned to extend the high frequency performance. This is known as a resonator cap and is commonly used in dynamic microphones – look for a small dome of some kind sitting immediately above the diaphragm.

The result of these controlled resonances working together is a far better frequency response than the diaphragm alone can provide. Although the quoted frequency response for a typical dynamic microphone extends higher than that of a typical ribbon microphone, once the dynamic microphone reaches its quoted upper limit the high frequency roll-off is usually steeper. Fortunately, dynamic microphone designers are usually able to place the high frequency roll-off somewhere above 12kHz, which is high enough to avoid terms like ‘dull’ and ‘muffled’ – descriptions that would certainly be used if the same steep roll-off occurred just a bit lower.

The inside of a dynamic microphone is a work of miniature acoustic engineering art. A look inside Sennheiser’s 421 quickly explains its size and shape, with a bass pre-emphasis pipe from the back of the capsule feeding into a large acoustic chamber shaped to form a broadband resonator, along with four rear vents/ports, a resonator cap and numerous damping systems. There’s also a humbucking coil, a transformer and a five-position bass roll-off control. Even Shure’s visually simple SM57 contains numerous resonating cavities and damping materials, including the cavity inside the handle itself – as Granelli Audio Labs discovered while prototyping their G5790 (a modified Shure SM57 with a right-angle bend to make it easier for close-miking drums). They had to re-work their initial design to make sure it didn’t alter the original tonality of the SM57 – otherwise they wouldn’t be able to market it as “The mic you’ve always loved, the mod you’ve always needed.”

DIAPHRAGM PARAMETERS

Diaphragm diameter also plays a role in the tonality of a dynamic microphone, especially when considered with the size/weight of the coil. A larger diaphragm can support a larger coil which potentially means a higher induced current, but with reduced high frequency sensitivity due to the increased weight. The smaller and lighter the diaphragm/coil assembly is, the faster it can move – resulting in better high frequency performance and transient response. BeyerDynamic’s M201 is a relatively small dynamic microphone (outer dimensions measure 160mm x 24mm), presumably containing a relatively small diaphragm – or, at least, a very light one. Often recommended for snare and acoustic guitar, the M201 offers high frequency performance and transient response approaching that of a large diaphragm condenser.

You can’t always judge the diaphragm size by looking at the microphone itself, however. A dynamic microphone’s physical size is usually due to the resonating chambers and other acoustic work going on inside it, and is not an indication of the diaphragm size itself. This is especially the case with microphones designed for capturing low frequencies (e.g. kick drums) where large resonating acoustic chambers are used inside the microphone to extend and enhance its low frequency performance.

A look inside a physically large dynamic microphone such as Shure’s Beta 52A kick drum mic or AKG’s D12 series (D12, D12E, D112, etc.) reveals a much smaller diaphragm than the microphone’s physical size would suggest, but there’s a lot of other things going on inside to support it acoustically, electronically and physically. Many people are surprised to see that the capsule in Shure’s Beta 52A is approximately the same size as the capsule in Shure’s Beta 57A handheld vocal mic, while the capsule in AKG’s D12 is marginally larger but still significantly smaller than its housing would suggest. The myth that you need a large diaphragm to capture low frequencies is explored in more detail below (see ‘Subkick microphones’).

Most dynamic microphones use diaphragms between 12mm and 18mm diameter. Going larger than 18mm presents engineering challenges to keep the diaphragm/coil assembly light enough to do its job without flexing or collapsing under its own weight. Heil Sound’s PR series feature very large 38mm diaphragms with varying thicknesses to keep them as light as possible while providing strength where necessary; other dynamic microphone manufacturers use similar approaches when making large diameter diaphragms.

NOISE & DISTORTION

As with ribbon microphones, the dynamic microphone’s transducer has no inherent source of noise apart from the thermal noise (aka Johnson noise) that exists in all electrical circuits, which is due to the effect of temperature on the microphone’s electrical circuitry. The current induced into the dynamic microphone’s coil is considerably higher than the current induced into a ribbon element, resulting in a greater signal level (from the same source) that makes the thermal noise less significant than it is in a ribbon microphone. Also, dynamic microphones are generally used for close-miking relatively loud sound sources (drums, guitar amps, live vocals, etc.), resulting in a higher induced current that further reduces the significance of the thermal noise. In any sensible dynamic microphone application, the dominant noise should be from the chosen preamplifier.

The main forms of distortion in dynamic microphones are due to the diaphragm/coil assembly being pushed to extremes so it is no longer behaving in a linear manner; flexing the diaphragm or even reaching the end of its excursion and ‘bottoming out’ – however, it would take a very high SPL to push a dynamic microphone to audible extremes, and even more to damage it. There is also low-order harmonic distortion introduced by the transformer – particularly due to saturation from transients and high levels of low frequencies. The harmonic distortion of the transformer may be a contributor to the microphone’s overall tonality – particularly with transients and higher signal levels.

SPECIALISATION

The engineering challenges contained within the dynamic microphone’s transducer make it difficult to create a theoretically perfect microphone, but they’re also a convenient justification for making specialised microphones that exploit the transducer’s strengths and avoid its weaknesses. By juggling the diameter and mass of the diaphragm, the number of windings on the coil, the size and strength of the magnet, and the use of resonators, dampers and acoustic chambers, the dynamic microphone designer can create all kinds of specialised microphones – from large mics with mellow tonalities and rich low frequency responses suited for kick drums and basses, to small mics with bright tonalities and fast transient responses for snares and acoustic guitars. From a tonal point of view, some dynamic microphones rival condenser microphones when used in their specialised applications.

ADVANCES & ODDITIES

Although the dynamic microphone as we know it today can be traced back to the 1930s there have been many advances and improvements, along with one or two oddities…

RARE EARTH MAGNETS

As with ribbon microphones, the use of rare earth magnetic materials such as neodymium and samarium-cobalt – with their considerably stronger magnetic fields – offers the dynamic microphone designer numerous options.

Electro-Voice was the first microphone manufacturer to make a feature of neodymium magnets; their N/DYM series of dynamic microphones, released in 1986, had surprisingly higher output signals and better high frequency performance than most other dynamic microphones on the market at the time.

Shure’s Beta series, released in 1989, began life as neodymium versions of their most popular dynamic microphones (wisely released as entirely new mics rather than replacements), with higher outputs and better high frequency performance. For example, the Beta 58A offers 3dB higher output than the SM58 and its quoted frequency response extends to 16kHz rather than the SM58’s 15kHz.

OVERCOMING PROXIMITY EFFECT

Electro-Voice have been tackling the proximity effect in directional microphones for decades, dating back to their 664 dynamic cardioid from the mid 1950s. Initially intended for sound reinforcement applications, it was the first microphone to feature their ‘Variable D’ (‘D’ for ‘distance’) technology which conceptually uses multiple frequency-selective rear ports (rather than a single port as is commonly used) to create a cardioid response. The result is a cardioid microphone with almost no proximity effect; there is no significant bass boost or bass loss when moving closer or further from the microphone, meaning an instrument on stage could be very closely miked without the exaggerated bass boost and related low frequency feedback issues.

The Variable D technology continued with the 665 and 666, both intended for the broadcast market. The 667 followed and came supplied with its own transistorized preamplifier that offered numerous frequency response curves via two rotary switches: a four-position rotary switch for the low frequency response and a five-position rotary switch for the high frequency response. Together, these switches offered 20 different frequency response options, including flat, and numerous options for boosting and cutting the low, high-mid and high frequencies. In a simple yet practical design move, the different response options were displayed on the front panel so it was easy to ‘dial up’ the desired choice.

The most recognised of the Variable D microphones would be the RE20 and its derivatives. Its lack of any significant proximity effect has made it popular with radio announcers, television presenters, podcasters and others who need to move around while sitting and talking – rather than talking directly into the microphone from a fixed distance at all times. It also has excellent high frequency extension and a very consistent polar response across a wide range of frequencies – two distinctions it holds with few other dynamic microphones (Sennheiser’s MD441 comes to mind). No wonder Electro-Voice nicknamed it ‘the condenser killer’…  

HUMBUCKING

Although humbucking or ‘hum compensation’ coils are generally associated with electric guitar pickups, their first use in sound applications dates back to Electro-Voice’s V1 ribbon microphone from 1934 (see previous instalment), a year before the first patent for the humbucking guitar pickup. Humbucking coils are used in many of the most popular dynamic microphones currently on the market, and are usually listed among the microphone’s features.

The basic concept is simple: add an extra coil with the same characteristics as the coil that’s attached to the diaphragm, put it on the same axis as the diaphragm’s coil but don’t attach it to the diaphragm so it does not move through the magnetic field and therefore does not get the microphone signal induced into it. Any electrical interference (hum, lighting buzz, etc.) should be induced equally into both coils. Invert the polarity of the new coil, and then add the signals from the two coils together. The interference (which is in both coils but with inverted polarity in one of them) will cancel out, while the signal (which is only in one coil) will pass through to the output.

TWO-WAY MICROPHONES

As seen earlier, one of the problems with dynamic microphones is achieving an extended high frequency response and fast transient response with their relatively heavy diaphragm/coil assemblies. AKG addressed this problem with the use of two dynamic diaphragm/coil assemblies working together – one optimised for low frequencies and one optimised for high frequencies – with a filtering circuit merging the two signals together to create a two-way microphone. Discontinued long ago, there were three mics in the series – the D200E, the D202E and the D224E – with the most memorable being the D202E ‘sound rocket’, which dates back to 1966 and was ultimately replaced by the sleeker-looking D222E. Crossing over at 500Hz, the D202E offered a very flat response and remarkably consistent rejection to sounds arriving from the rear.

AudioTechnica took a similar approach with their AE2500 ‘dual element’ kick drum mic, which combines a dynamic capsule and an electret condenser capsule in the same microphone, but with a separate output for each capsule so they can be blended as desired. Sony’s C-100 uses two capsules in a two-way configuration with a single output, but they’re both condenser capsules. The C-100 is discussed further in the next instalment of this series.

PRINTED RIBBONS

An interesting but short-lived development in the design of dynamic microphones can be found in Fostex’s Regulated Phase (RP) series. These microphones used a flat circular diaphragm (as used in condenser microphones) with the ‘coil’ etched into a layer of foil bonded to the diaphragm itself – similar to the tracks on a printed circuit board – earning them the description of ‘printed ribbon’. The diaphragm was placed within the field of two circular magnets, and magnetic induction created the signal. The result was a series of microphones with the ruggedness of dynamic microphones, a transient response approaching that of condenser diaphragms, and a mellow tonality similar to a ribbon microphone. Among aficionados, Fostex’s M88RP remains an affordable collector’s item. Discontinued long before the ribbon microphone revival, it is worth speculating how successful the RP series would be if re-released today and taking advantage of later technologies such as rare earth magnets.

ACTIVE DYNAMICS

As with active ribbons, an active dynamic microphone includes internal electronic circuitry to buffer the signal from the voice coil, provide a stronger output signal, and remove the effect of the preamp on the microphone’s tonality. Blue were early adopters and perhaps pioneers of this idea with their Ball and Kickball studio microphones; their Encore 200 is a phantom powered handheld dynamic for live vocal applications. Lewitt Audio’s MTP 840 DM and AKG’s D12VR are both dynamic mics with active circuitry that can be turned on if desired, allowing them to be used as passive or active dynamics. Aston’s Stealth goes a step further: it’s a dynamic mic with a built-in Class A microphone preamplifier, again switchable in and out of circuit, that provides four switchable tonalities and a massive 50dB of gain.

USB MICROPHONES

An extension of the active microphone idea, the USB mic builds a preamp, converter and USB interface into the microphone so it can be directly connected to and powered by the USB input on a personal computer, laptop or mobile device. Many include a built-in headphone socket to allow latency-free monitoring while also working around the lack of headphone sockets on many mobile devices. USB mics eliminate the need for an interface for users who only need one microphone (solo musicians, podcasters, vloggers, etc.) and simplify the ‘plug and play’ recording concept considerably. It will be interesting to see if USB mic manufacturers adopt 32-bit gain-staging technology [as used in the AES42 digital microphones (see the following instalment) and in field recorders like Zoom’s F6 and Sound Devices’ MixPre series] to eliminate the need for gain control during recording.

The dynamic transducer is well-suited to the USB microphone because it does not require the high polarising voltage of the condenser microphone, has a higher output than a ribbon microphone, and is considerably more rugged than both. It’s also a popular choice for spoken voice applications that benefit from the simplicity of a direct-to-USB connection – such as podcasting and vlogging. Røde’s Podcaster is a good example; it uses a 28mm dynamic capsule, offers 18-bit resolution at sampling rates from 8k to 48k, and includes a 3.5mm stereo headphone output with volume control.

IN-LINE BOOSTERS

Although dynamic microphones don’t need in-line boosters to the same extent that vintage and passive ribbon microphones do, they can still benefit from them for the same reasons to preserve their characteristic tonality (see previous instalment). Any of the in-line boosters marketed for ribbon microphones should also work well with dynamic microphones, including Royer Lab’s dBooster, Triton Audio’s FetHead, Cloud Microphone’s CloudLifter, Radial Engineering’s McBoost and Klark Teknik’s Mic Booster CT1. It’s not unusual to see podcasters using in-line boosters with Shure’s SM7B, Electro-Voice’s RE20 and similar announcer/presenter microphones.

SUBKICK MICROPHONES

A moving coil loudspeaker is essentially a dynamic microphone operating in reverse, so it is conceptually simple to use one as a microphone by placing it in front of a sound source and connecting its terminals to a preamp input. The original subkick microphone was a DIY device made by placing a speaker in front a kick drum – something studio engineers have been doing for decades (Geoff Emerick reportedly used a speaker cabinet as a subkick mic on The Beatles’ ‘Paperback Writer’). In later years it became common to see the LF driver from Yamaha’s NS10 monitors clamped to a mic stand and being used as a subkick mic (this was convenient because every professional studio kept spare NS10 LF drivers on hand for emergency repairs).

Yamaha’s SKRM100 went considerably further than the DIY approach. In a design motivated by drummer Russ Miller, Yamaha improved and commercialised the subkick idea by mounting a 6.5 inch low frequency driver (white cone, of course!) into a standard 10-inch 7-ply birch/mahogany drum shell, fitting it with mesh heads and supplying it with a sturdy stand made from drum hardware. The drum shell, mesh heads and under-damped speaker cone all resonated in response to the energy from the kick drum and added their own tonalities to the sound captured from the kick drum, thereby creating a unique device for enhancing the low frequency reproduction of the kick drum (some well-known session drummers carry their own SKRM100 fitted with vintage hoops, claiming it extends the tone of the mesh heads). When mixed with a traditional kick mic placed inside the kick’s shell to capture the impact of the beater, the result can be impressive – particularly when given a small boost around 30Hz, which is an octave below the SKRM100’s significant 60Hz peak. Yamaha showed good judgement by describing the SKRM100 as a ‘low frequency capture device’ rather than a microphone. It has since been discontinued, but similar devices such as Solomon’s LoFReQ and DW’s Moon Mic are still on the market.

A Google search reveals plenty of DIY information for those interested in building their own subkick microphone, but be warned: the DIY subkick world is full of pseudo-science. While it is true that a large cone area is required to reproduce low frequencies, it is not required to capture them. A quick look at DPA’s 4060 lavalier microphone, with its tiny 5.4mm diaphragm that measures flat down to 30Hz before dropping to -2dB at 20Hz, puts a convincing end to that myth – as does comparing the capsules in Shure’s Beta 52A kick drum mic and Beta 57A handheld vocal mic, which are virtually the same size. The DIY subkick’s speaker cone does not capture any more low frequencies than any normal mic would capture in the same position; it simply captures less high frequencies and has very poor transient response. Blending this with a traditional kick drum mic placed inside the kick drum obviously results in more low frequency energy overall.

RIBBON TRANSDUCTION

Ribbon microphones use the principle of magnetic induction, which states that passing a conductor through a magnetic field will cause an electrical current to be induced into the conductor. [A conductor is any material that an electrical current can flow through unimpeded.]

In a ribbon microphone the conductor is a thin corrugated strip, or ‘ribbon’, of aluminium (or similar non-magnetic conducting material, such as titanium) that is suspended within a powerful magnetic field. The vibrating air particles caused by sound energy make the ribbon move back and forth within the magnetic field, inducing a current into it.

The induced current is proportional to the sound energy that created it and could be considered as the electrical signal, except that it’s not suitable for connecting directly to a microphone preamplifier – it is too small and the ribbon element’s impedance is too low to provide a useable output signal voltage. To solve these problems the induced current from the ribbon is passed through a transformer that transforms it into a useable signal voltage. Happily, the transformer also provides a balanced differential output suitable for running the signal through a microphone cable and into a preamp.

[Impedance, transformers, balanced differential outputs and more are discussed in a later instalment of this series. For now, consider impedance as being anything that opposes the flow of a signal (the higher the impedance in Ohms, the more it opposes the signal flow), a transformer as a device capable of changing signal levels and impedances, and balanced differential outputs as being very useful when transferring microphone signals down microphone cables.]

PASSIVE RIBBON MICROPHONE

The description given above is for a passive ribbon microphone, where ‘passive’ means it does not require any source of electrical power to operate; for example, it does not require phantom power. All vintage ribbon mics and many contemporary ribbon mics are passive, and have three important characteristics to be aware of:

• They can be damaged by phantom power, particularly if the microphone cable is faulty or the mic output is being patched between tie lines, preamps or console inputs with TRS-style plugs and sockets (as used in professional patchbays). Vintage models with centre-tapped transformers are the most prone to this type of damage.
• They have a low output signal and therefore require a quiet microphone preamp with lots of ‘clean’ (i.e. quiet) gain.
• Due to their relatively high and varying output impedance, their tonality is easily influenced by the input impedance of the preamps they are connected to.

Solutions to these problems, including in-line boosters and active ribbon microphones, are discussed below.

FACTORS AFFECTING TONALITY

Although the ribbon microphone uses the same principle of magnetic induction as the dynamic microphone (discussed in the next instalment), its considerably lighter ribbon assembly results in higher sensitivity to high frequencies and faster response to transients – although this is not always reflected in the published specifications.

HIGH FREQUENCY ROLL-OFF

The most prominent contributor to the tonality of a ribbon microphone is its inherent high frequency roll-off, which starts gently before falling rapidly to the first of a series of nulls (i.e. dips caused by cancellation) – all determined by the shortest distance that sound can travel from the front to the back of the ribbon element via the magnet assembly on the side. The first null occurs at the frequency with a wavelength equal to that distance. At this frequency the sound energy creates equal but opposite pressure on either side of the ribbon, so the pressure on one side of the ribbon cancels out the pressure on the other side. The net result is no movement of the ribbon and therefore no output signal, creating the first null in the frequency response. We’ll call this null the termination frequency and abbreviate it to ft. The high frequency roll-off begins at 0.5 x ft and will be -3dB at 0.625 x ft, -6dB at 0.75 x ft, and a complete null at ft. Further nulls occur at all whole number multiples of ft.

You can calculate ft with the following formula:

ft = 344 / d

Where 344 is the velocity of sound propagation in air (in metres per second, at a room temperature of 21°C), and d is the shortest distance from the front to the back of the ribbon (in metres).

As an example, consider a ribbon microphone where the distance from the front to the back of the ribbon (around the side of the magnet) is 30mm. We must first convert d to metres, making it 0.03m. Putting 0.03m into the formula shows us that ft will be 11.47kHz (i.e. 344 / 0.03). The roll-off begins at 5.73kHz (i.e. 0.5 x 11.47kHz), drops to -3dB at 7.17kHz (i.e. 0.625 x 11.47kHz) and -6dB at 8.6kHz (i.e. 0.75 x 11.47kHz) before falling to a complete null at 11.47kHz.

Halving d to 15mm (0.015m) raises ft to 22.93kHz. The roll-off now begins at 11.47kHz, drops to -3dB at 14.3kHz, -6dB at 17.2kHz, and reaches a complete null at 22.93kHz.

Obviously, reducing the distance from the front to the back of the ribbon improves the microphone’s high frequency response. Some manufacturers use a magnet assembly that gets narrower at the centre, like an hourglass, making this distance as short as possible to raise ft and the -3dB point.

Published frequency responses are usually defined within a specified deviation either side of 0dB, e.g. ±3dB or ±6dB. The ribbon microphone’s published frequency response will never extend as high as ft because the signal level at ft falls below the maximum allowed deviation (it represents complete cancellation, so there is no signal). The frequency spectrum captured by the ribbon element extends considerably beyond ft, but with a series of nulls at whole number multiples of ft. Due to this, a ribbon microphone can reproduce frequencies higher than ft, but the frequency response will not be linear and therefore the reproduction won’t be accurate.

TRANSIENT RESPONSE

A microphone’s transient response describes its ability to accurately capture transients, like the attack of a snare drum or the pluck of an acoustic guitar. Transients contain energy that extends up to very high frequencies, therefore a microphone requires good high frequency performance and extended bandwidth to capture a transient accurately. This can cause confusion when discussing ribbon microphones because their published frequency response curves rarely extend beyond the first null, therefore showing a reduced bandwidth and early high frequency roll-off that is counter-productive to the requirements for a good transient response.

However, a ribbon microphone’s transient response and bandwidth are determined by two unrelated mechanisms: the transient response is determined by the weight of the ribbon, while the upper limit of the published frequency response is determined by the distance from the front to the back of the ribbon element. The ribbon mic’s ability to track and reproduce transients makes it sound brighter and clearer than its quoted frequency response would suggest – assuming its transformer and any associated circuitry allows frequencies above ft to pass through to the output.

TRANSFORMERS

All ribbon mics use transformers, and these have an impact on the mic’s tonality – directly by how they load the ribbon element, and indirectly by the impedance they present to the preamplifier. The transformer also places limits on the microphone’s frequency response and transient response, and, if poorly designed, can introduce distortions if driven too hard.

Most ribbon mics use a step-up transformer with a turns ratio of 1:37, which means it amplifies the signal voltage from the ribbon element by 37 times (x 37). This is known as voltage gain; a voltage gain of 37 is like adding 31dB of gain with a preamp. However, the 1:37 turns ratio also increases the ribbon mic’s output impedance by 37 squared (37 x 37), which is 1369 times. Increasing the ribbon mic’s output impedance places greater demands on the preamp, requiring it to have a higher input impedance to avoid affecting the tonality of the microphone.

Any step-up transformer with a turns ratio of 1:37 will provide +31dB of voltage gain and multiply the impedance by 1369 times. That’s the easy part of transformer design. Making a transformer that does not saturate and go into harmonic distortion on big transients or high levels of low frequency energy is not so easy, especially when trying to keep it small enough to fit into a microphone. Neither is making a transformer with good LF response – especially if the manufacturer chooses to use a ribbon with higher impedance. Making the ribbon longer, narrower or thinner all offer performance benefits as described below, but at an increased impedance that might require a more expensive transformer.

The market is flooded with cheap passive ribbon microphones, and the internal transformer is one of the areas where the manufacturer makes significant cost savings. One common mod for cheap passive ribbon mics is to replace the internal transformer with one from an established transformer manufacturer, such as Lundahl’s LL2912, Samar’s RT series or similar from one of the DIY ribbon microphone suppliers found on-line.

ELEMENT DIMENSIONS

Three important factors that affect the ribbon microphone’s tonality are the ribbon element’s length, width and thickness.

Making the ribbon longer results in higher induced current and therefore a higher output signal, which is good. On the downside the extra length results in a narrower polar response in the vertical plane at high frequencies, meaning a duller tonality for sounds arriving in the vertical plane than for those arriving in the horizontal plane. Making the ribbon longer also results in increased impedance and therefore increased potential for the chosen preamp to affect the tonality, and may cause reduced transient response due to the increased weight of the ribbon. Making the ribbon shorter has the opposite effects: lower impedance, potentially faster transient response and better high frequency polar response in the vertical plane, but with a lower output (meaning greater reliance on a quiet preamp with lots of gain).

Increasing the ribbon’s width lowers the high frequency cut-off (ft) and makes the ribbon heavier, potentially reducing the transient response but lowering the impedance. Reducing the ribbon’s width makes it possible to reduce the distance from the front to the back of the ribbon, which raises the microphone’s high frequency cut-off (ft) as explained above and therefore extends the usable bandwidth. It also makes the ribbon lighter, which improves its transient response. On the downside, it makes the ribbon more fragile and increases its impedance.

Using thinner material for the ribbon makes it lighter and therefore potentially improves its transient response, but also makes it more fragile. In addition, as the ribbon material gets thinner its output signal level increases due to its lower mass but its impedance increases due to the smaller cross-sectional conducting area of the ribbon material. Using thicker material for the ribbon makes it heavier and potentially reduces its transient response, but also makes it less fragile and lowers the impedance.

Any of the above-mentioned variations in the ribbon’s length, width or thickness that increase its impedance may also require the use of a different (and potentially more expensive) transformer design, especially if good low frequency response is required.

CORRUGATIONS & RESONANCE

The corrugations in the ribbon element provide elasticity and therefore suspension, allowing it to move freely in accordance with the sound energy. The type and spacing of the corrugations contribute to how much tension can be applied to the ribbon – which is a major contributor to the ribbon mic’s audio performance. The tension determines the ribbon element’s primary resonant frequency, which is typically tuned to somewhere between 15Hz and 45Hz depending on the manufacturer’s design goals. This low resonant frequency is one of the contributors to the ribbon microphone’s characteristic ‘warm and natural’ sound. Compared to the much higher resonant frequencies of condenser microphones (typically between 5kHz and 9kHz), this low resonant frequency is often touted as the reason why the sound from a ribbon microphone can handle a lot of high frequency EQ boosting without sounding harsh – unlike the sound from typical small diaphragm condenser microphones when the same EQ boosts are applied.

LOW FREQUENCY RESPONSE

Resonance and corrugations aside, the ribbon’s low frequency response is ultimately determined by two unavoidable facts. Firstly, it is a pressure gradient microphone; its output level is determined by the atmospheric pressure difference (caused by sound) between the front and back of the ribbon element. As the frequency gets lower, the wavelength gets longer and therefore the pressure difference between the front and back of the ribbon element gets smaller – resulting in less output. [This is explained in detail in a later instalment of this series.] Secondly, the ribbon microphone requires a transformer, and transformers impose their own low frequency limitations on the signal.

NOISE & DISTORTION

Due to the simple passive nature of the ribbon microphone’s transducer, there is no source of noise apart from the thermal noise (aka Johnson noise) that exists in all electrical circuits due to the effect of temperature. This would normally be considered insignificant except that traditional vintage ribbon microphones generate a very small signal that requires considerable gain – enough to bring the thermal noise into significance along with any noise from the preamp – leading to the oft-repeated statement that ribbon mics are noisy and need good preamps. Does this noise affect the tonality of the microphone? If it’s audible and is different between different microphone models, it can be considered part of a microphone’s tonality.

The main forms of distortion in ribbon microphones are due to the ribbon element being pushed to extremes so it is no longer behaving in a linear manner, and low-order harmonic distortion introduced by the transformer – particularly due to saturation from transient peaks and high levels of low frequencies.

PREAMP LOADING

And finally, for those using passive ribbon microphones there is the impact of the preamp on the tonality of the ribbon mic. To prevent the preamp from affecting the tonality of the microphone, the rule-of-thumb is that the preamp’s input impedance should be at least five times greater than the ribbon microphone’s output impedance. The important thing to understand about impedance is that it is frequency dependent; therefore the impedance at one frequency may not be the same at another frequency. With most ribbon microphones having an output impedance of around 300 ohms throughout the midrange frequencies, a preamp with an input impedance of 1500 ohms (5 x 300 ohms) seems reasonable and is easily affordable. However, at the ribbon mic’s resonant frequency its impedance could exceed 1000 ohms, requiring a preamp with at least 5000 ohms input impedance to avoid affecting the ribbon mic’s tonality in the low frequency range (where the resonance occurs). Suddenly the range of suitable and affordable preamps reduces significantly, and investing in an in-line booster (discussed below) makes sense.

ADVANCES

After decades in obscurity, ribbon microphones have been busy catching up to – and perhaps even overtaking – dynamic and condenser microphones. Let’s look at some of the factors behind the revival, along with a couple of evolutionary dead ends…

RARE EARTH MAGNETS

The ribbon microphone is inherently simple and the basic components haven’t changed since the beginning, but there has been considerable improvements with the materials those components are made with. Most significantly, the use of rare earth magnetic materials such as neodymium and samarium-cobalt allow stronger magnetic fields to be created from the same size magnet. For any given ribbon element, a stronger magnetic field means a higher induced current. Designers have used these materials in a number of ways to create new ribbon microphones with higher outputs, better high frequency performance and smaller size.

ACTIVE RIBBONS

Pioneered by Royer Labs with the R122 (introduced in 2002), active ribbon microphones solve many of the limitations of passive ribbon microphones by building a buffering and output circuit into the microphone itself, and provide output levels similar to condenser microphones. They require phantom power to operate and are therefore not damaged by it. The input impedance of the microphone preamp should not affect the tonality of an active ribbon mic any more than it would affect the tonality of a condenser mic.

IN-LINE BOOSTERS

These external devices aim to solve the three problems of passive ribbons (mentioned earlier) by providing a buffer and output circuit that is inserted between the ribbon mic and the preamp – a bit like taking the inside circuitry of an active ribbon mic and putting it in an external box. Apart from preserving the tonality of the ribbon mic by making it independent of the preamp it is connected to, in-line boosters provide output levels similar to condenser mics while also protecting the ribbon element from phantom power – in fact, they need phantom power to operate. Examples include Royer Lab’s dBooster, Triton Audio’s FetHead, Cloud Microphone’s CloudLifter, Radial Engineering’s McBoost and Klark Teknik’s Mic Booster CT1.

ACOUSTIC NANOFILM

In most ribbon microphones the ribbon element is made from ultra-thin aluminium foil, typically between 1.5 and 4 microns thick (0.0015mm to 0.004mm). It can be easily damaged by a small burst of wind or a momentary application of phantom power. In June 2008 ribbon mic manufacturer Crowley & Tripp released a video showing a ribbon element being continually blasted with phantom power, forcing it out of shape in a way that should destroy it. However, when the power was removed the ribbon element returned to its proper corrugated shape, undamaged. Made from an acoustic nanofilm cleverly called ‘Roswellite’, it was not long before Shure – whose reputation is built on making rugged microphones – acquired the company and released two rugged ribbon microphones using Roswellite ribbon elements: the KSM313 and KSM353. The model numbers hark back to Shure’s 300 series of ribbon microphones from the ‘50s and ‘60s.

IMMERSIVE AUDIO

The show’s audience area fills an elliptical space in among the four aircraft. Buchan quickly determined that the show would best be enjoyed when the audience are on their feet. The show is brief enough to not be fatiguing for most people and it means the young and the short of stature can move about to be guaranteed of good sightlines.

But to truly immerse the audience, the show needed a truly immersive audio system.

Buchan asked Design Stage to lead the audio design and procurement, while Josh Wilkinson took care of the sound design.

“The brief was for a 360° surround sound system that encompassed the audience viewing area, as well as overhead speakers for special effects as well as general public address duties,” reports Design Stage boss, Phil Viney.

After investigating a number of premium loudspeakers Phil Viney and the team decided on Meyer Sound.

“One of the key reasons behind the decision is the Meyer’s use of low-voltage power. Meyer pioneered the use of powered loudspeakers in live sound. A Meyer PA would use one cable to run power and audio. But rather than running 240V cabling plus audio, which can give you some segregation issues and interference, they came up with a system where they run a 48V DC power rail and balanced audio down the same cable.”

Meyer’s MPS-488HP IntelligentDC Speaker PSUs provide 48V DC power and full range signal down a single cable, which addresses any safety concerns by running low voltage cables in public areas — an issue Phil Viney was alive to.

Some eight speaker positions mark the perimeter of the viewing area, comprising a Meyer UP-4XP full-range loudspeaker and a MM-10XP sub. The two loudspeakers are housed in a custom-designed tailfin-like chassis.

Overhead, Phil Viney spec’ed four pairs of Meyer Sound UPJuniors. “They sound huge during the flyover effects,” observes Phil Viney. “They’re an amazing-sounding loudspeaker.”

A pair of Meyer’s Galaxy processors provide all the DSP and matrixing. Meyer’s new Spacemap Go immersive mixing tool wasn’t quite ready in time for this project. Instead, Josh (the sound designer) and Dan (the control programmer) ‘baked in’ the audio aspects of the show. That said, the Galaxy processors are only a firmware upgrade away from being ready to use with Spacemap Go on a show.

“The design allows for the use of the new Milan AVB protocol,” explains Phil Viney. “It’s early days, and the MOTU AVB interface isn’t quite on the same page as the Milan version of AVB, so the show is currently using analogue lines but that will change. The MOTU 16A interface provides 16 additional inputs into the system which we’ve made available on a patchbay. It’s designed for visiting crew to use their own audio mixer.”

MORE ON THE SOUND DESIGN

Josh Wilkinson reflects on the job of designing an arresting sound track in one of the world’s most unusual immersive audio arenas.

Josh Wilkinson: “From the onset Buchan made the soundtrack a priority. Which isn’t always the case and made the project a real pleasure.

“My brief was to create something that would really blow people away… that they wouldn’t expect.”

Josh built the tracks in Reaper, with an Ambisonic panning plug-in, mixing down key elements like the voiceover, aircraft sounds, music and ambience into subgroups.

But given the bespoke nature of the immersive audio setup, there was no way to have the mix ready for the space without being in the space. Josh spent a number of solitary nights sitting in the middle of the audience area mixing his work for the system.

“There was no substitute for being in the space. For example, the overhead speakers are a key element but they track the length of the roof span, which is 70m or so long. The furthest pair of Meyer UPJuniors are 50m from the listening position! It’s very hard to predict what a pan might sound like until you’re there hearing it. It turns out that you can let the natural delay do most of the work for you.”

The audio focus of the presentation changes when the projections move from the 747 to the other aircraft. In fact, it’s the panning of the voice over audio that shifts the audience focus as they turn and face the aircraft that’s the focus of the commentary.

Everyone on the project noted Josh’s talent and there’s little doubt this project is a feather in his cap.