acoustic resistor to upshift an electret microphone maximum SPL

[njiruk]

Hello,
At the moment I'm using an (obviously expensive) B&K microphone with a dynamic threshold at 162dB, to be able to measure the acoustic pressure inside a clarinet mouthpiece.

As I'm considering to move on to a commercial application I'm trying to find a way to do the same with a cheap microphone.
Electret microphones have the required properties, except of the very much lower dynamic threshold (maximum 125dB).

So I wax thinking, wouldn't it be possible to simply design an acoustic resistance by placing the electret in a small capsule with one opening that is covered with a small stiff plate. That plate would have it's first modal resonance far above the highest frequency of interest (10kHz for me), so that the stiffness is reigning under that frequency. It's displacement will be proportional to the pressure in front of the plate and thus, inside the capsule a proportional but much weaker pressure variation will occur...
Or am I overlooking something?
Of course, the dynamic range will be the same as the electret, so small pressures won't be measurable, but for that I might use an array of those electrets perhaps.

Alternatively, I'm also interested if anyone would know of another cheap microphone type that could do the job. I considered MEMS microphones, but it seems to me that their threshold is not higher then electrets and their low frequency response seems to be worse...

Thanks!
K

 

[GregLocock]

"dynamic threshold " is what? I'm guessing this a is a google translation of maximum SPL for the mic.

So, as you say, cheap mics won't go that high, and off hand I can't think of a reason why they would, so the next alternative is an attenuator. In general these do not have flat frequency responses, so you may need to compensate for that.

One method would be to drill a hole behind the diaphragm, for example if you opened it up to half the area of the diaphragm then you would halve the sensitivity of the mic.

However your approach would probably work.

Cheers

Greg Locock


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[Strong]

Take a look at ICP-type dynamic pressure sensors at

http://www.pcb.com.

They are not cheap, but have very good performance.

Walt

 

[IRstuff]

I'm unclear what you are concerned about. The typical electret microphone already has a poorer sensitivity compared to a high-end microphone, so muffling it will make that worse. What is your concern? An electret is unlikely to be damaged by the sound pressure inside a mouthpiece. What makes an electret less sensitive is what makes it less prone to pressure damage, which is the fact that the diaphragm is much stiffer, i.e., thicker than a conventional microphone's diaphragm. If it's that much cheaper, why not just try it?

http://blog.shure.com/top-8-microphone-myths-exposed/

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[njiruk]

@GregLocock
I should have used the standard term instead of "dynamic threshold" indeed. But yes, I mean maximum SPL.
It is just strange that:
-I don't seem to find cases of people who tried/are looking for the same...
-It shouldn't be too complicated to manufacture such an electret so that an extra attenuator wouldn't be needed.
I have considered the "hole behind the diaphragm technique".
I would say that this transforms the electret in a differential microphone. No matter the size of the hole (unless very small) the pressure on both sides would be close to equal... On the other hand, it may be interesting to use a light attenuator on one of the sides so that the pressure difference lies in the required dynamic range.

@IRstuff
Sensitivity doesn't make the maximum SPL... Electrets start saturating above about 125dB, that doesn't damage the mic, but I would like a proper measurement...

 

[IRstuff]

"It shouldn't be too complicated to manufacture such an electret so that an extra attenuator wouldn't be needed."

by definition, the attenuator reduces the signal at all points, so whatever you do to increase the maximum means the minimum is likewise increased, so your sensitivity will go down. Which is why no one does that, since almost everyone else is interested in getting the best sensitivity.

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[amanuensis]

Why do you say that acoustic pressure is proportional to the plate displacement ?

Plate Velocity = j * omega * Displacement
From Acoustic point of view, Air Velocity is proportional to Delta Pressure / Delta X.
Since there is equality between Acoustic Velocity and plate Velocity, we get that : Displacement is proportional to (1/j*omega)* Delta pressure / Delta X.
The assumption which says that the plate is very rigid, is given by Plate Displacement = F/K (Force divided by Stiffness).
Could you tell me how you get your equality ? Just for the fun.

Now what I say : Smaller plates are more efficient radiators than larger ones and increasing stiffness increases the radiation of sound energy, due to the discontinuity of motion at the edges.

It's not an easy problem to model!

 

[IRstuff]

All I was trying to get across is that reducing the top end will most likely hurt your sensitivity.

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[njiruk]

@amanuensis
You should take into account the acoustics of the small capsule, it will act as a compliance C so that its impedance Z = P/U = P/jωX = 1/jωC explains that displacement is proportional to pressure.

@IRstuff
Indeed, that's what I meant with "the dynamic range will be the same as the electret, so small pressures won't be measurable", but you're right and I should have used the word sensitivity indeed.

 

[GregLocock]

A neater solution is to use a sort of untuned helmholtz chamber. This is a posh word for a tight fitting cap over the microphone with a small hole in it. The approximate attenuation will be in the ratio of the hole area to the mic diaphragm area, but there are frequency effects which can be tuned by changing the dead volume inside the cap, and the length of the small hole.

I don't think the manufacturer of cheap mics has any great interest in providing extra dynamic range, you're probably working between electronic noise at the bottom, and mechanical or magnetic clipping at the top. So as IRstuff says, with a physical attenuator you can shift the max SPL, but the minimum measurable will shift with it.

I suspect it would improve the conversation enormously if you could give parameters fro frequency response and S/N ratio etc. Even better, of course, would be data.

Cheers

Greg Locock


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