Mic self noise question

[anton]

Hi Folks,

I have a question about mic self noise. Does it increase as you turn up the gain on the preamp? So a mic with a self noise spec of 16db would get closer to that figure the more gain you apply to the mic. Or is it a constant once you plug in the mic and power it on?

thanks

Anton

[Bob Womack]

The self-noise is a constant at the mic. It is typically measured at a fixed acoustic level, typically one pascal. However, when you turn up the gain on a preamp, both the signal and the noise are raised by the same figure. So where does this figure in? If you present a quiet acoustic sound to the mic, you will need to gain up to a useful level. The noise will come up with the signal. If you present a loud noise to the mic, you don't have to gain up as far to get a useful level and thus the noise isn't as amplified. Examples: Put a noisy mic in front of a large wound-out guitar amp and you don't hear much noise. Put it in front of an acoustic guitar and you'll get a lot more noise. Here is a good brief on it:

NEUMANN ON SELF NOISE

Incidentally, of all the fine and expensive production mics in the world, the production mic with the lowest self noise is the inexpensive Rode NT1 at 4dba.

Bob

[rick-slo]

Originally Posted by anton

I have a question about mic self noise. Does it increase as you turn up the gain on the preamp?

Mike self noise does not increase with preamp gain increase. Brownian motion of air molecules contributes to mike
self noise as does thermal noise in wiring. Therefore record in a cold room.

[Ty Ford]

Sorry!

Buzz.

Self noise is generated at the mic. When you increase the preamp gain it also increases the self noise.

Regards,

Ty Ford

[rick-slo]

Originally Posted by Ty Ford

Sorry!

Buzz.

Self noise is generated at the mic. When you increase the preamp gain it also increases the self noise.

Regards,

Ty Ford

In one sense that is obvious - self noise increases as does the volume of the recording overall. In another other sense it is obvious what is being referred to is the self noise coming from the mike itself - the percentages.

[Ty Ford]

Derek,

Wth all good humor intended, I have no I deal what your second sentence means.



Regards,

Ty Ford

[rick-slo]

Gotta go to work right now. Maybe Bob will explain it to you.

[Ty Ford]

bob already did....

"The self-noise is a constant at the mic. It is typically measured at a fixed acoustic level, typically one pascal. However, when you turn up the gain on a preamp, both the signal and the noise are raised by the same figure."

You turn up the mic, you turn up the self noise. Done.

The same is true for analog tape noise. It's noise at the source. If you raise the gain, the noise increases.

Regards,

Ty Ford

[runamuck]

Originally Posted by Ty Ford

bob already did....

"The self-noise is a constant at the mic. It is typically measured at a fixed acoustic level, typically one pascal. However, when you turn up the gain on a preamp, both the signal and the noise are raised by the same figure."

You turn up the mic, you turn up the self noise. Done.

The same is true for analog tape noise. It's noise at the source. If you raise the gain, the noise increases.

Regards,

Ty Ford

As the gain goes up the noise increases but so does the signal. So doesn't the ratio of noise to signal stay the same?

I think it does and that's what's meant by a mic's self-noise..

[Ty Ford]

Not exactly.

Preamps are designed to sound best at a certain part of their operating curve.

If you're above that sweet spot, noise increases more than when in the sweet spot. Usually any more than 60dB preamp gain and you're preamp noise will increase non-linearly (relative to the linearity across the sweet spot.)

Regards,

Ty Ford

[rick-slo]

Originally Posted by Ty Ford

Not exactly.

Preamps are designed to sound best at a certain part of their operating curve.

If you're above that sweet spot, noise increases more than when in the sweet spot. Usually any more than 60dB preamp gain and you're preamp noise will increase non-linearly (relative to the linearity across the sweet spot.)

Regards,

Ty Ford

Question was about microphone self noise. Throwing bananas, grapes, and peach pits into the apple basket obfuscates rather than clarifies. Also for what it's worth a decent preamp has significantly less self noise than a microphone, even out side the sweet spot.

[Bob Womack]

Originally Posted by runamuck

As the gain goes up the noise increases but so does the signal. So doesn't the ratio of noise to signal stay the same?

I think it does and that's what's meant by a mic's self-noise..

The variable figure in this situation is the sound level of the source you are recording. If the source is loud you use less gain at the preamp and as a result you jack up the noise floor less. If the sound source is quiet, you are required to jack up the preamp gain to get a signal that is usable. This is why sound effects recordists require the quietest mics: Sometimes their sounds are nothing more than the extremely quiet swish of clothing. If you jack up that sound to usable level, a noisy mic will present lots of noise in the final recording while a quiet mic, with its broader signal to noise ratio, will present far less. Same with a harp, a very quiet instrument. If I put up an AKG C451B at 18db self-noise and -41dbv sensitivity versus a Rode NT1 at 4db self-noise and -34dbv sensitivity, I'll be introducing an extra 14db of noise when both are brought up to working level based upon self noise only. Now add in the 7dbv difference in sensitivity and you are looking at 21db of extra noise from the noisier C451B given equal recording levels.

The point here being that signal to noise is only fixed for absolute specification demonstration. Once you introduce the mic into real-world operations, the acoustic source levels rise and fall above and below one pascal, forcing the signal-to-noise ratio to vary. The amount of noise from the mic is fixed, but the amount of signal varies per the sound pressure level presented to the microphone, thus varying the ratio.

Does that make sense?

Bob

[rick-slo]

More briefly the signal to noise ratio of a given mike for a given sound does not change with preamp gain changes. For example start out 10dB mike self noise versus 100db signal (say trumpet), the 1 to 10 ratio stays the same with preamp gain changes.

[ChuckS]

Originally Posted by rick-slo

More briefly the signal to noise ratio of a given mike for a given sound does not change with preamp gain changes. For example start out 10dB mike self noise versus 100db signal (say trumpet), the 1 to 10 ratio stays the same with preamp gain changes.

I agree with the intent of your post and everything up until "the 1 to 10 ratio stays the same". As the preamp gain is changed, the ratio of signal to noise does stay the same, but in this example the difference remains at 90 dB which is way way more than a 10:1 ratio (each 10dB difference is ratio of 10:1)

[ChuckS]

Originally Posted by Bob Womack

The variable figure in this situation is the sound level of the source you are recording. If the source is loud you use less gain at the preamp and as a result you jack up the noise floor less. If the sound source is quiet, you are required to jack up the preamp gain to get a signal that is usable. This is why sound effects recordists require the quietest mics: Sometimes their sounds are nothing more than the extremely quiet swish of clothing. If you jack up that sound to usable level, a noisy mic will present lots of noise in the final recording while a quiet mic, with its broader signal to noise ratio, will present far less. Same with a harp, a very quiet instrument. If I put up an AKG C451B at 18db self-noise and -41dbv sensitivity versus a Rode NT1 at 4db self-noise and -34dbv sensitivity, I'll be introducing an extra 14db of noise when both are brought up to working level based upon self noise only. Now add in the 7dbv difference in sensitivity and you are looking at 21db of extra noise from the noisier C451B given equal recording levels.

The point here being that signal to noise is only fixed for absolute specification demonstration. Once you introduce the mic into real-world operations, the acoustic source levels rise and fall above and below one pascal, forcing the signal-to-noise ratio to vary. The amount of noise from the mic is fixed, but the amount of signal varies per the sound pressure level presented to the microphone, thus varying the ratio.

Does that make sense?

Bob

i agree with everything you say except where you added in the 7dB for sensitivity differences ( in your example). The way mic noise levels are spec'd, (in dB) relative to the mics level at 1 pascal already factors in the sensitivity differences. I'm not saying sensitivity is not meaningful, I'm just saying you don't add it inti e differences between mic noise spec levels.