Room Sound and Various Mics

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Room Sound and Various Mics

Post by Ariosto »

I have noticed that it appears that some of my mics have a lot less noise than others, although all are reasonable.

I've made several recordings of room sound in a very quiet time with the gain on each mic set roughly the same on the meters on my recorder. i.e. To peak at the same levels. It appears that my dynamic and ribbon mics (passive) are the quietist giving a noise floor of -64.9 dB and my AKG C414 xls set on cardioid is -51.8 dB. (AKG specs say the self noise is -6dB). This is a difference of -13dB.

Am I correct in saying that passive mics (dynamic and ribbon) - Beyer M201 and Royer R101 - are noticeably quieter than active mics and especially ones like the AKG?

I dare say I will be told that my test was not very scientific, but is this discovery reasonable? ( I do narration which is close miked in a small room with some basic room treatment and I have no trouble usually with noise floor problems).
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Re: Room Sound and Various Mics

Post by Hugh Robjohns »

Ariosto wrote: Fri Jul 30, 2021 8:45 amAm I correct in saying that passive mics (dynamic and ribbon) - Beyer M201 and Royer R101 - are noticeably quieter than active mics and especially ones like the AKG?

You might be right in relative terms for your particular mic collection, but the big picture is a lot more complicated.

The only source of electronic noise in a passive mic is the electrical resistance of the electrical 'generator' -- the ribbon or moving coil itself and/or the output transformer (if present).

Of course, there other noise source is acoustic noise, and I'll come to in a moment.

If the total electrical resistance of the generator is, say, 150 Ohms then the noise voltage due to electrons bumping into each other inside the circuitry (the Thermal or Johnson noise) is theoretically -130.9dBu. Let's call it -131dBu as we're all friends here!

If the mic's total impedance is closer to 30 Ohms (as some vintage ribbons are) then the theoretical Johnson noise voltage drops to -137.9dBu. So lower source impedance is better from a noise perspective, but in practice it often also means less sensitivity, and thus more preamp gain is needed, and that raises the noise again... so it's complicated! :think:

(Both these Johnson noise figures assume an ambient temperature of 20C and a measuring bandwidth of 20Hz to 20kHz -- change either and you skew the numbers).

In active mics like capacitor mics, you have a pile of electronic circuitry to consider. A DC-biased capacitor or electret mic always requires an impedance conversion circuit and the active devices involved -- transistors and valves -- all introduce noise of their own called self-noise.

The usual way of specifying this self-noise is to replace the mic's capsule with an equivalent value of capacitor (thus removing any acoustic noise as a source), measure the output voltage (which is now only electronic noise), and relate that to the equivalent level of acoustic noise that would have needed to reach the mic's capsule to generate the same output signal level. A really good impedance converter might add a few decibels of noise equating to around 5 or 6dB SPL.

However, in real life the electrical circuitry isn't the only source of noise. And in most cases, it isn't the most significant source of noise! Air molecules in a warm room are constantly banging into each other and into the microphone diaphragm, generating some noise voltage at the output.

So naturally the size of the diaphragm also makes a big difference to the amount of acoustic noise being collected. However, contrary to what you might be thinking, a small diaphragm (pencil) mic is inherently significantly noisier than a large diaphragm mic, and the difference is typically between 12 and 20dB. There are a variety of reasons why this is the case, related to diaphragm tension (affected by the polar pattern, of course), and the surface area actually sampling the sound wave which determines the signal-noise ratio, amongst other aspects.

A very good large diaphragm mic might have a self-noise figure of, say, 8dB SPL, whereas a very good small-diaphragm mic is more likely to be around 18dB SPL and most are in the low twenties dB SPL. Many manufacturers only measure the impedance converter's electronic noise, because it's easy to do, and ignore the acoustic noise altogether. However, the better companies (like DPA and Neumann) usually measure the actual acoustic and electronic noise together by placing the mic in an anechoic chamber. The resulting figure will relate to real life much more accurately than the electronics-only figure, of course!

Moreover, the frequency response of the mic affects how much acoustic noise is captured and -- often more importantly -- its spectrum, which affects its audibility. This is another reason why dynamic mics often appear (perceptually) less noisy than capacitor mics -- they generally have a curtailed HF response.

One other factor to consider is externally coupled noise. Electrodynamic mics are inherently sensitive to magnetic interference such as hum fields from nearby mains transformers or mains wiring. And electrostatic mics are sensitive to external electrostatic fields and RF interference etc. If present, these external factors will increase the noise floor. Also, mechanical noise coupled via the cable or mounting arrangement, or even turbulent air flows, could also add to the noise floor -- and subsonic or ultrasonic noises may well worsen a measurement without being audible, of course.

On top of all this you have the mic preamp itself and the noise that its electronics generates. This noise is broadly proportional to the amount of gain being applied, but (usually) not in a linear way. Many preamps are effectively quieter at higher gains than they are at lower or middle gains -- although you don't usually notice because the reason you use a low gain is because the mic is putting out a stronger signal anyway!

Mic preamp noise is quoted as an EIN figure -- equivalent input noise -- and it's measured by connecting a 150 Ohm across the inputs, applying a known amount of gain (usually 60dB), and measuring the output noise level. Whatever that level is (say -65dBu), we then subtract 60dB (the gain being applied) and that's the EIN figure: -125dBu in this example.

As I mentioned above, a perfect 150 Ohm resistor on its own generates -131dBu (at 20C and between 20Hz and 20kHz), so that figure is the Holy Grail of mic preamp designers... but it is impossible to reach in practice of course because a preamp has a lot more electronics in it than a single 150 Ohm resistor. Most preamps add 3 or 4dB of self noise, so the EIN figure is usually around -127dBu. The best will do better, but only by a decibel or two, and the worst will be a several decibels worse.

One way of making the EIN number appear better than it really is, is to reduce the test resistance applied at the preamp's input and some manufacturers do that with 50 Ohms or even a short circuit (0 Ohms). A short will put the Johnson input noise down around -155dBu, leaving a lot more room for electronic noise before the EIN numbers look bad! Watch out for that on the spec sheet -- if they don't specify the source resistance (and ideally the temp and bandwidth too) don't trust the number!

And lastly, the other significant factor in a mic's apparent noise performance is the sensitivity of the mic itself. In general, passive mics have a much lower sensitivity than capacitor mics, so they output a smaller electrical signal for a given acoustic SPL. That means more electronic gain is required to raise the signal to a usable level, and thus potentially more noise from the preamp too.

So the big picture is complicated with lots of interacting factors involved.

Returning to your example, I'm a little surprised your AKG mic is as noisy as you claim, although much will depend on the quality of your preamp at different gain settings. However, I'm not surprised that your dynamic mics appear very quiet too.

But, of course, we don't just choose mics for their low noise, do we?
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Re: Room Sound and Various Mics

Post by Tim Gillett »

Ariosto wrote: Fri Jul 30, 2021 8:45 am Am I correct in saying that passive mics (dynamic and ribbon) - Beyer M201 and Royer R101 - are noticeably quieter than active mics and especially ones like the AKG?


Relative to the room noise, an LDC like the 414 should normally result in lower noise from the preamp after it, compared to a typical dynamic or ribbon.

Have a look at the mic type suspended at some distance above an orchestra. In my observation it's usually been some type of quality LDC like your 414. Its low noise will be less likely to add noise problems in very quiet passages such as a solo flute, violin etc.
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Re: Room Sound and Various Mics

Post by Bob Bickerton »

Actually SDCs and MDCs are commonly used in the main array for orchestral recording.

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Re: Room Sound and Various Mics

Post by Tim Gillett »

Bob Bickerton wrote: Fri Jul 30, 2021 12:07 pm Actually SDCs and MDCs are commonly used in the main array for orchestral recording.

Bob

If noise turned out to be an issue in quiet passages, all things being equal which type of mic would be an improvement?
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Re: Room Sound and Various Mics

Post by forumuser840717 »

Tim Gillett wrote: Fri Jul 30, 2021 1:10 pm
Bob Bickerton wrote: Fri Jul 30, 2021 12:07 pm Actually SDCs and MDCs are commonly used in the main array for orchestral recording.

Bob

If noise turned out to be an issue in quiet passages, all things being equal which type of mic would be an improvement?

It's very unlikely to because, in all but the very quietest rooms, and assuming half decent equipment, the room's background noise will far outweigh the noise of the recording chain. And if the room noise doesn't then the noise of 70+ players breathing, shuffling, fidgeting and rustling their clothes will. I don't think I've ever wanted to change a mic because it was too noisy. (Not since I gave away some Earthworks omnis to someone who inexplicably liked them and wasn't bothered by the noise, which was audible in even quite a noisy room!)

(I usually use a mixture of SDC, MDC, LDC, ribbons and even the odd moving coil mic on orchestras, and other things. And occasionally a crystal, carbon, or controlled reluctance mic though theyre not really general purpose toys.)
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Re: Room Sound and Various Mics

Post by MOF »

If noise turned out to be an issue in quiet passages, all things being equal which type of mic would be an improvement?

The larger diaphragm microphones, because for a given SPL there’s more sound wave energy being received so the preamplifier has less amplifying to do.
Hugh said it in his reply
So naturally the size of the diaphragm also makes a big difference to the amount of acoustic noise being collected. However, contrary to what you might be thinking, a small diaphragm (pencil) mic is inherently significantly noisier than a large diaphragm mic, and the difference is typically between 12 and 20dB. There are a variety of reasons why this is the case, related to diaphragm tension (affected by the polar pattern, of course), and the surface area actually sampling the sound wave which determines the signal-noise ratio, amongst other aspects

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Re: Room Sound and Various Mics

Post by Tim Gillett »

MOF wrote: Fri Jul 30, 2021 1:31 pm
If noise turned out to be an issue in quiet passages, all things being equal which type of mic would be an improvement?

The larger diaphragm microphones, because for a given SPL there’s more sound wave energy being received so the preamplifier has less amplifying to do.
Hugh said it in his reply
So naturally the size of the diaphragm also makes a big difference to the amount of acoustic noise being collected. However, contrary to what you might be thinking, a small diaphragm (pencil) mic is inherently significantly noisier than a large diaphragm mic, and the difference is typically between 12 and 20dB. There are a variety of reasons why this is the case, related to diaphragm tension (affected by the polar pattern, of course), and the surface area actually sampling the sound wave which determines the signal-noise ratio, amongst other aspects


Quite. Thanks.
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Re: Room Sound and Various Mics

Post by Bob Bickerton »

Tim Gillett wrote: Fri Jul 30, 2021 1:10 pm
Bob Bickerton wrote: Fri Jul 30, 2021 12:07 pm Actually SDCs and MDCs are commonly used in the main array for orchestral recording.

Bob

If noise turned out to be an issue in quiet passages, all things being equal which type of mic would be an improvement?

As has been said, noise is not an issue in orchestral settings and so LDCs will be selected for reasons other than self-noise performance.

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Re: Room Sound and Various Mics

Post by Hugh Robjohns »

Tim Gillett wrote: Fri Jul 30, 2021 1:10 pm If noise turned out to be an issue in quiet passages, all things being equal which type of mic would be an improvement?

A closer one? :lol:

In all seriousness, I'm sensing an unwelcome return to argumentative, point-scoring and general trolling in some of your recent posts again. Let's not go back there, Eh?
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Re: Room Sound and Various Mics

Post by Ariosto »

Many thanks for the long and detailed and expert answers to my question. As I expected, I am way out of my depth here. The main message though appears to be not to worry about noise from mics unless it becomes apparent.

One reason I asked is because some platforms for the recorded voice (Audible etc) ask for a noise floor of -60dB or better. (Not that I do much on Audible!) In any case a lot of the silences between words or sentences have slight breathing sounds, which is often considered normal, and so the noise floor is covered anyway.

Sorry to be so late coming back, but our Internet connection has been down all day due to maintenance improvements.
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Re: Room Sound and Various Mics

Post by Hugh Robjohns »

Ariosto wrote: Fri Jul 30, 2021 8:45 amIt appears that my dynamic and ribbon mics (passive) are the quietist giving a noise floor of -64.9 dB and my AKG C414 xls set on cardioid is -51.8 dB. (AKG specs say the self noise is -6dB). This is a difference of -13dB.

As I mentioned earlier, I was a bit surprised at your AKG noise figure and so I've done a bit more digging.

The figure you quoted is this one:

Equivalent noise level to IEC 60268-4 (A-weighted): 6 dB-A (0 dB pre-attenuation)

But in the same spec sheet it also quotes this rather different one:

Equivalent noise level to IEC 60268-4: 20 dB (0 dB pre-attenuation)

14dB is a much larger difference that I would have expected when comparing flat and A-wtd measurements of what would normally be something akin to white noise, and the implies quite a lot of noise energy being carried at the very high and low frequencies.

But whatever... the point is that your DAW meters would be showing a flat response measurement of the noise floor, not an A-weighted one -- hence a higher reading than the spec might suggest.

That said, I've not heard any complaints of current C414XLS being noisy and I don't recall any issues with the ones I've reviewed and used, either...
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Re: Room Sound and Various Mics

Post by Ariosto »

The AKG C414 xLS is a nice mic and I certainly would not have any complaints.

On another point, recording voice using two mics on separate tracks and mixing both down to mono, would this add any appreciable noise?
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Re: Room Sound and Various Mics

Post by Hugh Robjohns »

Ariosto wrote: Fri Jul 30, 2021 4:30 pmOn another point, recording voice using two mics on separate tracks and mixing both down to mono, would this add any appreciable noise?

Yes, it would add noise, but it would also add more wanted signal.

If the two mics were placed coincidentally, so there was no arrival time difference, and they had similar frequency responses, then you would have two lots of wanted, coherent signal which would sum to be +6dB louder than either mic on its own.

The noise from each mic is essentially a random signal and thus incoherent so that when summed the noise level would rise by, roughly, +3dB.

So, if the wanted signal goes up by 6dB but the noise only goes up by 3dB, you've gained 3dB of signal-noise ratio. If you adjust the gains so that the peak level of the summed signal is the same as either mic individually, the noise floor will appear to be roughly 3dB lower. This is a technique that's used in all sorts of systems to improve the SNR and signal 'quality'.

However, in practice, you're not going to be able to mount the two mics coincidentally, especially if using LDCs, and the inevitable spacing between capsules will typically cause time-of arrival cancellations at high frequencies, resulting in a potentially nasty HF response.

Experiment with it by all means, but it's probably more trouble than it's worth...
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