How are Engineers measuring the Noise Floor of a Microphone?

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Paul678

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Mar 3, 2013
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What is the procedure, and the equipment needed to do this properly?

Thanks for any info.....

 
Paul678 said:
What is the procedure, and the equipment needed to do this properly?

Thanks for any info.....

You will need to use a "dummy" capsule if it is a condenser microphone, or "dummy" resistor for a ribbon.
The easiest (and cheapest) is to use software.
 
Best, M
 
Measuring self noise is quite complicated. First you need to measure the mics sensitivity. Then you measure the noise level without sound exposure. Some manufacturers use dummy heads with a cap replacing the condenser capusle. But this often gives a slighty lower figure than the acutal mic. A better way to do it is to measure the mic inside an isolation chamber; in German those are called "Rauschbombe" - noise shell. They're usually made of a metal cylinder inside another, separated by isolation material. The outer shell is shockmounted in a frame. Knowing the sensitivity and the amplification factor of your preamp, you can now calculate the self-noise. As self-noise is usually given in dB-A you need to apply a weighting filter.

For DIY and non-commercial stuff it's enough to compare to a known microphone from a serious manufacturer. Match levels carefully. If your room is quiet enough it's easy enough to estimate a ballpark figure and/or determine if a mic is too noisy for its type.
 
Rossi said:
Measuring self noise is quite complicated. First you need to measure the mics sensitivity. Then you measure the noise level without sound exposure. Some manufacturers use dummy heads with a cap replacing the condenser capusle. But this often gives a slighty lower figure than the acutal mic. A better way to do it is to measure the mic inside an isolation chamber; in German those are called "Rauschbombe" - noise shell. They're usually made of a metal cylinder inside another, separated by isolation material. The outer shell is shockmounted in a frame. Knowing the sensitivity and the amplification factor of your preamp, you can now calculate the self-noise. As self-noise is usually given in dB-A you need to apply a weighting filter.

For DIY and non-commercial stuff it's enough to compare to a known microphone from a serious manufacturer. Match levels carefully. If your room is quiet enough it's easy enough to estimate a ballpark figure and/or determine if a mic is too noisy for its type.

In other words, the actual noise in usage will depend on the gain or sensitivity of the mic,
because with lower sensitivity, the preamp will need to be turned up, and the noise floor
will be higher.

Ok, but what sort of noise measuring test equipment is used?  Is it typically measured as dBm/Hz in the
microphone world?

 
Rossi said:
Measuring self noise is quite complicated. First you need to measure the mics sensitivity. Then you measure the noise level without sound exposure. Some manufacturers use dummy heads with a cap replacing the condenser capusle. But this often gives a slighty lower figure than the acutal mic. A better way to do it is to measure the mic inside an isolation chamber; in German those are called "Rauschbombe" - noise shell.

Indeed, the higher noise can be observed in smaller diaphragm (esp. omni) capsules due to Johnson noise of trapped air in the back chamber. For all practical purposes, the noise of electronic circuit in use with LDC capsules (which somehow I assumed what OP uses) is dominating. I discussed this issue with a few (well known) manufacturers at AES shows and that was an agreement...

I do agree, for DIY purposes a comparison with a good known quality mic with careful level matching is more than enough for noise evaluation. The only problem to arise is if one of the mics has better low end response and there is some low end rumble (for example from cars, or subway) not to confuse it with higher self noise. IOW, you want to listen to the top end noise, i.e. hiss...

Best, M
 
Paul, when you use condenser mics, the mic preamp amplification plays almost no part in the noise figure (with any decent mic pre) because the output noise of the mic dominates. So the noise you hear is the amplified mic noise.

The usual way to express mic noise is the self noise figure, A-weighted in dB. Equipment varies vastly between manufacturers. Usually they use dedicated measurement preamps that are calibrated and offer exact amplification factors.

Keep in mind that noise figures do not give much information about the noise spectrum. A-weighting helps, but higher frequency noise is usually more annoying than lower frequency noise.

@ Marik: Saw your ribbon mic today at Musikmesse - looks very neat! Wish I could have talked to you in person after so many years of internet communication.
 
Rossi said:
@ Marik: Saw your ribbon mic today at Musikmesse - looks very neat! Wish I could have talked to you in person after so many years of internet communication.

That would be great! This year I could not come because of some personal things, but I am planning to be at Musikmesse next year.

Also, in fall I should be in Hamburg teaching piano masterclasses and having recitals there and in Kassel, so at some point we'll have some beer together and a good chat!

Where are you located?

Best, M
 
I'm located in the southwest, near the French border - which is not a very popular region for visitors, unfortunately. But I've been at the Musikmesse for at least ten years in a row, so chances are good, I'll be there next year as well. :)



 
Hi.

Here's a method that guys has used in local mic lab.

At first all measuring happens in anechoic (down to 100Hz) room.
With test signal that has known SPL 94dB@1m the mic under test is connected to mic pre and then to Sennheiser UPM-550 level meter which has build in A and CCIR filters. Next the meter is adjusted to show 0dB. Then mic goes in sound proof isolation chamber and Sennheiser is used to read the level with A or CCIR filter on whichever is preferred. For example my MK-7 was measured at -75dBA refered to 0dB (94dB spl) At last actual noise floor is calculated by subtracting measured level from known SPL (94dB spl - 75dBA = 19dBA)

Lab guys have told me this is common method in mic industry.

Hope this helps.

-Paavo
 
One thing I have done:
Hook the DUT(s) up to your DAW in a quite room with some additional reference mic's that are maybe similar or of better known quantity.

Check your pre-gain so you get as close as possible to the same level on gtr/vox/whatever.

Record silence all mic's as discrete tracks for a number of hours....

Look, See, and don't forget to Listen....

The idea is to record silence, perhaps boost the gain quite a bit so you get a waveform you can see.

I did this with a couple tube and FET mic's I built and compared to commercial SDC's of a KM184 type. Lots a noises and things on the 184's... Much less on the DIY mic's...

This may not provide you with a number like "9db self noise A weighted" but will provide some further knowledge regarding character of the noise floor.


Hope that helps!

Cheers,
jb
 
Nah, that won't give you much useful information. You really need an isolation chamber; a quiet room is not quiet enough for measurements. Also, different mic designs pick up different noises. What you see on your waveform display is mostly very low frequency stuff that's barely audible but produces quite some energy. A mic whose amplifier goes very low (such as the KM184) will look worse than it really is. That's why noise is usually measured with A-weighting.

IMO, if you can't measure properly, you better go with a simple listening test. For instance, record acoustic guitar with two mics side by side; play a chord and let it fade completely. Match levels in the DAW using the phase cancellation test and then listen at which point noise becomes noticeable as the chord fades.
 
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