GOLDILOCKS & THE dBs

The term ‘gain’ is often used in discussions about microphones and preamplifiers. ‘Gain’ is essentially another word for ‘amplification’; in the context of microphones, it tells us how much amplification is being applied by the preamplifier. If the signal coming out of the preamplifier is 10x bigger than the signal going into it we could say the preamplifier has a gain of 10x. That’s easy to understand, so why is gain always specified in decibels (dB) rather than as a multiplication value?

Microphones create very small signals that require very large amounts of amplification, typically measured in hundreds or even thousands. To avoid writing huge numbers, gain (like many other measurements related to audio signal levels) is usually expressed in decibels. We’ll discuss decibels in detail in a later installment of this series; the important thing to understand for now is that they allow us to use very small numbers to represent very large numbers. For example, the maximum gain on some high quality preamplifiers is +70dB, which is equivalent to x3162. What would you rather see written next to the gain knob on your microphone preamplifier: ‘+70dB’ or ‘x3162’?

The problem with gain is that it brings the noise up with it. Generally speaking, more gain means more noise – although what’s really happening is that the preamplifier is amplifying the noise that comes from its input circuit plus the noise that comes from the microphone, so the more gain you use the more audible the overall noise becomes. [We look further into that when we look at EIN in the ‘Noise 2’ installment of this series…]

Most contemporary microphone preamplifier designs are capable of very low noise performance, but they all exhibit some noise. As the gain is increased this noise reaches a point where it becomes apparent but is still acceptable, e.g. it’s noticeable when solo’d but isn’t noticeable in the mix. Beyond the point of ‘acceptable’, the noise becomes excessive and is considered ‘unacceptable’.

As we’ll see in the forthcoming installments, the noise from the microphone preamplifier is usually only of concern when using passive microphones (passive dynamics and passive ribbons) due to their low Sensitivity and the associated high gain required to reach a useful level. The noise from the microphone preamplifier is not very significant when using active microphones (condensers, active dynamics and active ribbons) because their higher Sensitivity requires less gain, and the noise generated by the microphone itself is considerably higher than the noise from the preamplifier.

In any case, the mic preamplifier on a budget bus-powered interface is likely to reach an unacceptable level of noise at a lower gain setting than it would on an expensive mains-powered rack-mounting microphone preamplifier, simply because it has more noise to start with and therefore requires less gain before its noise becomes audible or objectionable.

Minimising noise from the microphone preamplifier starts by choosing a microphone that can deliver a healthy signal level: high enough that it does not require so much gain from the preamplifier that the noise becomes audible, but not so high that it overloads the input circuit of the microphone preamplifier or downstream devices and causes distortion.

If you’ve got the gain turned all the way down to minimum and still have a high signal level then there’s a risk of distortion; if the sound is not already obviously distorting, it’s possible that the incoming signal level is on the borderline where distortion does not occur during the sound check or rehearsal, but does occur during the excitement of the performance when levels are a dB or so higher.

The right microphone should deliver a signal that puts the preamplifier’s gain into the ‘Goldilocks Zone’; the microphone signal’s level should be high enough to avoid requiring too much gain (thereby making any noise more apparent), but low enough to avoid risking distortion. This brings us to the first microphone specification…

SENSITIVITY

A microphone’s Sensitivity refers to how much voltage (electrical signal) will be present at its output when a known amount of acoustic energy (sound) is present at the diaphragm or ribbon element. If a microphone with a high Sensitivity was placed alongside a microphone with a low Sensitivity and both were focused on the same sound source from the same distance, the higher Sensitivity microphone will give a higher output.

Sensitivity In mV/Pa

Sensitivity is usually specified in ‘mV/Pa’, which means ‘milliVolts per Pascal’. One milliVolt is one thousandth of a Volt (in the same way that a ‘millimetre’ is a thousandth of a metre), and a Volt is a measurement of the amount of electrical pressure contained in the signal that represents the sound. The higher the voltage, the higher the signal level.

A Pascal is a measurement of atmospheric pressure and is used here to represent the sound energy in the air. As a matter of interest, the softest sound that human beings can hear is 0.00002 Pascals (20 microPascals or 20uPa), and is also known as the ‘threshold of audibility’.

One Pascal (1Pa) is equivalent to a Sound Pressure Level (SPL) of 94dB, which is about the same as hearing a steel string acoustic guitar being played at full volume from about 2m away. The table below contains some typical maximum SPLs of numerous musical instruments and distances to provide a point of reference. Note that every doubling of distance causes a 6dB decrease in SPL, and every halving of distance causes a 6dB increase of SPL.

A 1kHz sine wave tone at a level of 1Pa or 94dB SPL (at the diaphragm or ribbon) is the standard that microphone manufacturers use when measuring Sensitivity, making it easy to compare the Sensitivities of microphones from different manufacturers.

Sensitivity In dBV

Some manufacturers prefer to use a dBV value for Sensitivity, in which case they are specifying it relative to a reference where 0dB = 1Pa = 1V (unless otherwise specified). It should have a ‘V’ at the end to indicate that the reference is 1 Volt, giving us the ‘dBV’ terminology, although some manufacturers neglect to add the ‘V’ which means we cannot be certain what their reference is. They could be intentionally using a lower value than 1V as a reference to create a higher Sensitivity value and we’d never know because that’s how the numbers game works. For now, however, we’ll assume they’re all doing the right thing and using 1V or otherwise specifying their reference…

A Sensitivity value of -20dBV means the microphone’s output voltage is 20dB lower than 0dB, which means it is 20dB lower than 1 Volt, which is 0.1 Volts. How does that work? Read on… (If logarithms and powers of 10 make your eyes glaze over, skip down to the next subheading, ‘Meaningful Sensitivity’.)

For comparison purposes, the following formula converts Sensitivity from dBV into mV/Pa:

mV/Pa = 10^{(dBV / 20)} x 1000

Be sure to include the minus sign when entering the dBV value if the specified Sensitivity value has a minus sign in front of it. Also note that the formula multiplies the answer by 1000 to provide the answer in mV rather than volts.

The next formula reverses the process, converting mV/Pa values into dBV values:

dBV = 20 x log(mV/Pa / 1000)

This formula assumes you are entering the voltage as milliVolts, and divides that value by 1000 to convert it to volts before finding the logarithm.

Let’s check how these formulae work by converting a Sensitivity value from mV/PA to dBV and then back to mV/Pa. We’ll start by converting a Sensitivity of 15mV/Pa to dBV:

dBV = 20 x log(15 / 1000)

dBV = 20 x log(0.015)

dBV = 20 x -1.8239

dBV = -36.48dB

So a Sensitivity of 15mV/Pa is equivalent to -36.48dBV. Let’s check that calculation by going back the other way, hopefully converting -36.48dBV back into 15mV/Pa:

mV/Pa = 10^{(-36.48 / 20)} x 1000

mV/Pa = 10 ^-1.8239 x 1000

mV/Pa = 0.015 x 1000

mV/Pa = 15mV/Pa

Bingo…

Meaningful Sensitivity

To be meaningful, a Sensitivity specification should be given in mV/Pa or dBV. If it is given in mV/Pa, the ‘Pa’ tells us that 1Pa (one Pascal, equivalent to 94dB SPL) is the reference. If it is given in dBV the ‘V’ tells us that 1 Volt is the reference. In both cases, if we know the reference then we can make meaningful comparisons between specifications from different manufacturers that use the same reference. And, with the help of the formulae above, we can also make comparisons between specifications that quote mV/Pa and those that quote dBV.

Generally speaking, condenser microphones have the highest Sensitivity of all the different microphone types, while passive ribbon microphones have the lowest. Likewise, with all other factors being equal, a larger diaphragm offers higher Sensitivity than a smaller diaphragm (the reasons for this are explained in the next installment, when we look at Self-Noise).

Sensitivity & Mic Choice

So how does Sensitivity influence our choice of microphones? In the tradition of matching the microphone to the sound source, it makes sense to use high Sensitivity mics for soft sounds and low Sensitivity mics for loud sounds. Using a high Sensitivity mic on a soft sound means less gain is required from the preamp, resulting in less noise. Conversely, using a low Sensitivity mic on a loud sound minimises the possibility of distortion due to overloading the preamp or downstream devices. Here are some examples:

Neumann’s U87 large dual-diaphragm condenser is rated at 28mV/Pa in cardioid mode, while their KM184 small single-diaphragm cardioid condenser is rated considerably lower at 15mV/Pa. Audio-Technica’s AT5040 knocks them both out of the park with a whopping 56.2mV/Pa. It’s twice the output of the U87 and almost four times the output of the KM184, and is enough to overload a padless mic preamp when capturing a moderately loud signal. (The later-released premium version of this microphone, the AT5047, offers a lower Sensitivity of 35.5mV/Pa. Despite the addition of an output transformer that could’ve allowed an even higher output, Audio-Technica obviously decided to bring it down a bit.) At the other extreme, DPA’s miniature 4062 has a Sensitivity of 1mV/Pa, which is very low for any microphone but is in keeping with its intended application of being designed for capturing very loud sounds – a situation where low Sensitivity can be an advantage to avoid overloading the preamp and/or downstream devices (this is especially important considering that miniature microphones like the 4062 are often used with portable battery-powered wireless transmitters which can be easily overloaded).

To put these widely varying figures into perspective, if the AT5040, AT5047, U87, KM184 and 4062 were all placed the same distance from the same sound source, the AT5047 would require 3.96dB more gain to match the AT5040’s level, while the U87 would require 6.02dB more gain, the KM184 would require 11.44dB more gain, and the 4062 would require 34.99dB more gain.

If close-miking a snare drum from the edge of the rim, the high output level of the AT5040 would drive some circuits into clipping (even at minimum gain) and the signal would be unusable. Meanwhile the 4062 would require 4.08dB more gain than a Shure SM57 (an industry standard for this application, and therefore a good reference) placed in the same spot. This is not the sort of application that a high Sensitivity mic like the AT5040 is intended for, and a low Sensitivity mic like the 4062 would be a better choice in terms of landing the signal in the Goldilocks Zone – although it’s possible that a hard hit on the snare could exceed the Maximum SPL that either mic can handle without distorting significantly (we’ll explore that problem in a later installment of this series).

What about miking gentle finger-picking on a nylon string guitar from 30cm in front of the point where the neck joins the body? The AT5040 would deliver a signal that most preamps would be happy to work with, while the 4062 would require a preamp with very low noise and high gain to match it. This is not the sort of application that a low Sensitivity microphone like the 4062 is intended for.

Sennheiser’s MD421 II dynamic microphone has a Sensitivity of 2mV/Pa, Electro-Voices’ RE20 comes in at 1.5mV/Pa, and Shure’s SM57 and SM58 both sit at 1.6mV/Pa. If all four mics were placed at the same distance from the same sound source, the RE20 would require 2.5dB more gain to match the MD421, while the SM57 and SM58 would each require 1.94dB more gain. With such small differences between their Sensitivities, all four mics would be equally suitable or equally unsuitable for any given application. From a Sensitivity point of view they’d all be suitable for close-miking a snare drum, but they’d all be unsuitable for miking a gently finger-picked nylon string guitar from 30cm.

Royer’s R121 passive ribbon has a Sensitivity of 4.4mV/Pa, while its active counterpart, the R122, offers 16mV/Pa; that’s an 11.21dB increase in output level, which means 11.21dB less gain is required from the preamp to bring the R122 up to the same signal level as the R121. Similarly, AEA’s R44C (a re-creation of RCA’s classic 44BX) has a Sensitivity of 2.25mV/Pa, while its active counterpart, the A440, offers a higher Sensitivity of 30mV/Pa and therefore requires 22.5dB less gain on the same signal at the same distance. Similarly, the R44C requires 5.83dB more gain to match the R121, while the R122 requires 5.44dB more gain to match the A440.

The R122 and A440 are better choices than the R121 or R44C for capturing finger-picking on a nylon string guitar at 30cm due to their higher Sensitivities, but the opposite would apply for capturing the sound of a Marshall stack pumping out distorted power chords for heavy metal. As far as their Sensitivities are concerned they’d all be good choices for close-miking a snare, but a loud rimshot (the kind of transient ambush that jazz drummers like to sneak in between brush work) might drive some of them into distortion by exceeding their respective Maximum SPL figures, as discussed in a later installment of this series.

SENSITIVITY MATCHING

The three microphone families – ribbons, dynamics and condensers – offer a huge range of Sensitivities covering just about any sound capturing application. Use high Sensitivity microphones for capturing quiet sounds, low Sensitivity mics for capturing loud sounds, and aim to keep your preamplifier’s gain in the Goldilocks Zone. Sounds easy, right? Unfortunately there’s more to it than that because we also have to factor in noise from the microphone and from the preamplifier, which is the subject of the next installment…

NEED TO KNOW BASIS

Press the icon underneath a channel’s meter on the touchscreen to open its input processing options. Above the gain slider is a meter with assists for dialling in an ideal input level – Low, Good and Over. You can engage a limiter and low cut filter here as well.

Zoom has understandably simplified EQ to a single slider which swings between ‘Bass’ and ‘Treble’. In use it quite clearly attenuates and boosts at either extreme, like a seesaw pivoting at 1k. Of course I would prefer a parametric EQ with sweepable mids but, again, Zoom deemed a single-control approach perfectly sufficient for its target market. The same uncomplicated philosophy applies to the Compressor/DeEsser where the Min/Max slider is labelled ‘Make your voice sound professional.’ Not gonna lie – it seems odd having the compressor directly linked to the de-esser. I assume the rationale is to negate sibilance as compression pronounces it but it’d be nice to have the option for one or the other when input sources aren’t quite so predictable. The Phone channel offers a two-band EQ with separate sliders for Bass and Treble. Despite my complaints, I have to admit the foolproof controls work fine the majority of the time. 

SHOWING UP

Your podcast guests are in, mics are setup, Skype call running, headphones donned. Now what?

First, make sure a fresh SD card is inserted and formatted. 

Then, you’ll need to decide who’s going to be recorded. If one or more guests will enter the show later, or are just there to listen in, un-click the ‘On Air’ button on their microphone channel. You’ll still hear them and they’ll hear you, but their mic output won’t be recorded to the SD card or play through the speakers/USB output. If you want to shut them up altogether, press Mute. 

Then press the big red button. The Mark button will drop markers into the file at different points in the recording. Using the super responsive sound pads mid-show is fun and seamless, as is live mixing on the smooth faders. Be aware of the loud click made by the On Air and Mute buttons; silent rubber buttons would’ve been preferable here. Recordings can be paused and resumed with the Play/Pause button whereas Stop will finalise the recorded file and place it in the library. 

POST PRODUCTION

PodTrak P8 shows its true colours after recording. Uniquely, the little mixer offers a measure of onboard audio editing capability. I was expecting no more than a top and tail cut function but was pleasantly surprised by how much useful editing can be done on that little screen. 

Pulling up a recorded file displays its full waveform along the bottom of the screen. With the row of tools above it you can easily trim start and edit points, place a cut point anywhere to split into multiple files, drop in a background music bed from a file on the SD card, optimise overall level with an automatic function, and even place fade ins and fade outs. Play and pause the recording with the physical button or on-screen, and use the Mark button to drop markers for referencing edit points. When you’re finished, export it as an MP3 file. Enter Transfer mode to copy it to your computer via USB. 

I flew through a quick cleanup and edit of a recording much quicker than expected. The only thing I missed was a scrub/jog wheel but you can’t have everything. In fact, I’d suggest Zoom has sufficiently tooled up the PodTrak P8’s edit functionality to allow most seasoned podcasters to only need a computer to upload the episode, unless some surgical or single track-level editing is required. 

The Zoom PodTrak P8 is for podcasters. Everything about it caters to the immediate demands of managing, recording and editing a podcast without being so technical it obstructs organic conversation or creative flow. For aspiring or experienced podcasters alike, the PodTrak P8 deserves a serious look.

THINKING AHEAD

COVID HIATUS

Just as Studio 52 in Collingwood closed in November 2019, so too was Empire opening its first stage of completion. Paul and Trevor convened staff, industry luminaries and friends to celebrate the newly-minted Empire Studios with a Christmas party in 2019, “It was a great night of live music and a great chance to showcase the new facility,” recalls Paul.

Construction continued in 2020 and the larger Studio 2 opened in March. Covid hit Empire at the worst possible time. “You know when the Melbourne Grand Prix got cancelled? That was our studio open day [March, 2020] the weekend before the first lockdown.”

Covid put a halt to the festivities. It would take almost another year before Empire could really stretch its legs again.

“On top of all our other work lost, the entire 2020 Kool Skools Project was also cancelled — 28 albums of original music by all these young people who missed the opportunity. I think young people have really copped the worst of it, missing so many opportunities that may never be replaced,” said Paul.

But 2021 is proving to be more auspicious: “2021 has got off to a good start and we have been extremely busy working with a huge range of great people in all genres,” continued Paul. “Again the snap lockdown in Melbourne in May and June has had a huge impact and came at a really bad time. We had a Sydney artist and producer in to record and they literally had to turn around and fly straight back out the same day to avoid getting stuck in Melbourne. So many bookings postponed and then postponed again, it feels a bit like Groundhog Day.”

RECLAIMED & RECYCLED

Given Paul and Trevor were building Empire while Studio 52 was still trading, there was a limit to how many materials they could repurpose. Heidelberg is a furniture manufacturing hotspot and Paul got to know some of the local businesses. “We have recycled a lot of timber that has been considered as seconds or slightly damaged by the local furniture factories.” Including the gorgeous diffuser at the rear of Studio 1’s control room. Not only that, but many of the acoustic panels are made from reclaimed materials. “We also have installed a 30kW Solar system on the roof, so it’s great to be doing our bit to save the planet while saving a huge amount on power bills.

There are some reminders of Collingwood, including the main console desks which have been modified, there was also a mixture of components taken out a small studio in Balaclava that Empire acquired when it was closing. “Much of the gear is from Studio 52 but there is also a lot of new stuff we purchased for Heidelberg,” explains Paul. There are also some special pieces that used to belong to Steve Lincoln-Smith, founder of Innovative Music and great friend of Paul’s, who sadly passed away during the build. “It’s great to have his personal Kurzweil and Hammond along with a few other pieces I purchased from his estate”, they are now actively in use at Empire — a fitting destination for synths owned by such a fine player.

You won’t find large-format analogue consoles at Empire. We don’t record through the consoles, they are just for monitoring purposes. The setup is inspired by how people are comfortable working at home, each control room’s centrepiece is a keyboard and mouse, flanked by racks of preamps, EQs and compressors. Call the setup a ‘Frankenstein’ desk, if you like. Each is stacked with outboard many will remember from Studio 52. Paul and Trevor’s love for all the Universal Audio preamps like the 4710, 610 and 6176; certain key TL Audio models, including the PA-1 and VP-1, Warm Audio 1073s and many other sonic colours and flavours.

“You don’t have to patch anything. It’s all ready to roll so you don’t lose any setup time,” enthuses Paul. Everything in the control rooms is built for stability. “A 96-channel Yamaha DM2000 may not be the latest thing but we just use that for monitor mixes and playback control.” Similarly, racks of preamps may not have the sex appeal of an SSL K series, but they offer more sonic variety and you can rip out a channel and replace it instantly if needs be. Almost all the gear is hard-wired as ‘pure path’ with very little patching. You want the 6176? It’s on Channel 9. Recording guitar direct ? Straight from the Kemper, through tubes, into the RME converters.

Empire Music Studios
Address: 9 Northern Rd, Heidelberg West VIC 3081
Email: paul@studio52.com.au
Phone: 0412 686 252 (Paul Higgins)
Web: www.studio52.com.au

VOICE TOOLS

Røde Connect offers a handful of audio processing options suitably tailored to enhance spoken word. Click on the Mic icon above a fader to reveal the tools which, apart from a gain level setting, include a noise gate, compressor, and Aphex’s Exciter and Big Bottom. 

The noise gate is a necessity in my opinion. The NT-USB Mini’s condenser capsule and relatively wide cardioid pickup pattern make it prone to picking up unwanted sounds in a podcasting context. Leaving the NT-USB Mini on your desk 20-30cm from your mouth will inevitably result in room reflections bleeding into the mic and potentially cheapening your sound. Add in a few extra presenters in the same room and spill becomes a glaring issue. Circumvent this in two ways: one, maximise separation by getting the NT-USB Mini closer to your mouth (the yoke mount has a mic stand thread underneath, perfect for a boom arm); or two, engage the noise gate in Røde Connect to silence all but the voice in front of the mic (this won’t diminish room reflections but will give you a cleaner recorded signal). For best results, do both, and opt to record in a quiet or treated room.

The one-button compressor adds a tasteful punch to a voice while taming peaks although more control in the form of a threshold setting would be nice. I love what the Aphex Exciter and Big Bottom do. The former adds non-harsh ‘air’ to a voice while the latter provides a warm, inviting bulge in the low end which works even better with proximity effect when you’re up against the mic.

VIRTUAL VISITORS

Røde Connect supports video call applications like Skype and Zoom so you can easily integrate remote participants into a podcast. Open the Setup Assistant, click Next, and drag the Virtual channel into the remaining spot on the mixer. To bring audio from a call into this channel, select the ‘Røde Connect Virtual’ input and output on the video call application (note: by default the audio will otherwise appear in the System channel). The caller will automatically receive the full output of the Røde Connect mixer minus their own voice. I tested this out with zero speed bumps, apart from the audio quality occasionally turning sketchy at the other end if I started talking over music – video call lines aren’t designed to carry broadband audio. Having said that, I appreciated how easy the recording was to set up and especially being able to have separate faders for System audio and Virtual audio from a caller.

Another way of using Røde Connect is for live streaming/Twitching. Set the game audio output to System, chat app output to Virtual, and select the Røde Connect Streaming playback engine as the input device in a live streaming application like OBS for a full mix of your audio sources to hit the airwaves. 

POST SHOW

All recorded podcasts are saved into a list in the Library. Here, you’re able to colour code individual files, scrub through a podcast, jump to markers inserted while recording, and export the podcast in a file format of your choice (WAV 44.1k/48k, MP3 up to 320kbps). Røde has thrown in a feature that’ll automatically optimise a recording for a particular platform. Multitrack recordings, when exported, will save individual files and a stereo downmix to a destination folder where you then can easily drop them into a DAW for editing. Unfortunately, you can’t edit recorded podcasts within Connect.

Røde released a v1.1 firmware update while writing this review, which introduces sound pads to the software for triggering audio clips mid-show, and solo buttons on all channel strips.

CONNECT 4

Røde has done it again and created yet another product we didn’t know we needed. If you’re wondering how to man a podcast on your computer with minimal fuss, buy a few NT-USB Minis and download Røde Connect. You’ll be pleasantly surprised. It’s clean, elegant, and enormously less complicated than learning or managing a DAW. Targeted features like the Aphex effects, separate System and Virtual inputs, and quick export options offer a streamlined workflow without cutting corners on quality. I can’t think of a simpler way to record a podcast on a budget and a laptop than a few NT-USB Minis and Røde Connect.

Story: Joe Matera