Hello together,

I'm new here and my english isn't the best, but I hope you can still help me on this:

So far I used ECM with preamplifier which is usualy mounted in-vehicle ceilings for hands free phone use.
Its sensitivity is 230mV/Pa (-12.76dB)
The problem is, that the frequency response drasticly decreases below 80 Hz and I need to measure lower frequencies.
At 40 Hz it should have still no less then -3dB.
I don't want to use measurment microphones from B&K or similar for a couple hundrets of euro.
Do you know any microphone which is capable of measureing such low frequencies?

I know capsules like this: https://www.mouser.de/datasheet/2/334/POM-2730L-HD-R-1219352.pdf
without preamplifier.
But I don't realy trust their frequency response in the datassheet and probaply would have to build a preamplier circuit
for my 3m long wire.
Or is it possible to evaluate signals from a microphone with -30 dB sensitivity over a 3m long wire?

Thank you for your help.

Rafael

 

One alternative is to use filtering to raise low freq response, not the preferred way
as you will G up both signal and noise.

Like hearing aids today correcting for an individuals freq response to make it flat.
Boost the lossy ranges the person has.


Regards, Dana.

 

My guess is that the problem is in your preamp circuit. I trust their data sheet over your preamp circuit which you have not shared with us. Are you using the microphone at the link in post #1, or something different?

This is an electronics forum. Do you really think you will get good advice about an audio circuit without showing us the schematic?

ak

 

Hello,

Have a look at this project:
http://sound.whsites.net/project93.htm

Bertus

 

Of course the low frequencies are cut by low value coupling capacitors in the preamp circuit. Of course the low frequencies are cut because the mic is used with voices for a telephone in a noisy car (not a symphony orchestra in a concert hall).

Your 3m "wire" should be a shielded audio cable.

 

Low frequency microphone

Rafael K: For excellent response to low audio and subsonic freqs I say just use 18" _woofer_ as directional Mic with direct coupled _instrumentation amp_ as preamp! With that exact setup I was able 2 make vry high quality recording of _partridge drumming_ (which is centered around 7Hz) buried 30db below audible noise floor!

Now for ppl saying _human hearing range only goes down to abt 15Hz_ all I can say is you're right! Lowest 'notes' are more like _felt_ than heard! So anyhow anyone who has ever _heard_ partridge drumming or vry distant thunder or explosion (like from ice breaking in spring) knows exactly what I'm saying!

 

While we're waiting for the TS to provide useful information ...

The examples you give are impact sounds, and the Fourier transform of an impact event shows that it contains a ton of higher frequency energy that is well within the range of human hearing. Note that the term for thunder is "a clap of thunder", not "a boom of thunder". The sharp clap reduces to a diffuse boom as the source is more and more distant due to two things: low pass filtering and multipath distortion (called multimode distortion by the fiber optics people; same fizix). We hear the harmonics, and the brain reconstructs the fundamental.

All perception is deception.

ak

 

A loudspeaker used as a microphone produces a very exaggerated resonant boooom sound. An electret microphone has a very flat and tight (no resonance) low frequency response. The preamp cuts the low frequencies because it is used in a telephone system that cuts frequencies below 300Hz. Simply increase the value of the coupling capacitors that are cutting the low frequencies.

The only time I heard about a partridge was "a partridge in a pear tree" that my true love sent to me. I looked it up and a woodpecker is different.

 

Re-reading the question, I see where you are inferring from the mention of cell phone use that the preamp has severe low frequency roll off. Good guess. With clear photos of the board we probably can figure which capacitor to change.

ak

 

The only time I heard about a partridge was "a partridge in a pear tree"

FWIW I believe @Aleph(0) is referring to a 'ruffled grouse'? --- Having heard it 'in the wild' myself - it's an eerie sound! -- A diminishing 'Thump-thump-thump-thumpthumpthumpthump' -- à la an 'accelerating' pile-driver operating at seemingly 'ethereal distance' from the listener...

Granting @AnalogKid 's spot-on comments as Re: harmonic content of non-sinusoidal vibration - I must confess that the perceived sound is so 'deep' that one tends to ascribe same to that arising from their own vasculature -- And who will deny that an external 'partridge' is a far kinder 'diagnosis' than audible bruits?!

But then maybe @Aleph(0)'s reference is in regard to the 'percussion man' (Danny Bonaduce?) of a mercifully retro 'band'

Best regards and TTFN
HP

 

Saludos.
Could this fit your needs ?
---->

----> http://www.bgmicro.com/GeophoneKit-2.aspx

----> https://www.egl-geo.com/product/geo...MIrKng6frj4AIVCJ7ACh2tqwCKEAMYASAAEgKOpPD_BwE

----> https://www.ebay.com/sch/i.html?_nkw=geophone+sensors&ul_noapp=true

----> https://www.google.com/search?q=geo...AUIDygC&biw=1366&bih=604#imgrc=ePykywlKLYz58M:

 

Granting @AnalogKid 's spot-on comments as Re: harmonic content of non-sinusoidal vibration - I must confess that the perceived sound is so 'deep' that one tends to ascribe same to that arising from their own vasculature

Close. Loud deep bass sounds, especially those near or below the bottom end of the range of normal hearing, physically vibrate the adrenal gland, causing secretion. "Moving to the beat" isn't just a phrase, it is a biochemical reaction.

ak

 

My guess is that the problem is in your preamp circuit. I trust their data sheet over your preamp circuit which you have not shared with us. Are you using the microphone at the link in post #1, or something different?

This is an electronics forum. Do you really think you will get good advice about an audio circuit without showing us the schematic?

ak

its not "my" preamp circuit. It's a finished product where i do not have any data except the measured frequency response. I don't know which capsule is used and how the preamp curcuit looks like. I just see part of the board with the capsule so i can't show you how the schematic looks like (without wrecking it). And the parts I see are SMD smaller 1mm so guess I won't be able to replace parts of it

One alternative is to use filtering to raise low freq response, not the preferred way
as you will G up both signal and noise.

Like hearing aids today correcting for an individuals freq response to make it flat.
Boost the lossy ranges the person has.


Regards, Dana.

as you say that's not the prefered way because of amplifier signal and noise.
But thanks

Hello,

Have a look at this project:
http://sound.whsites.net/project93.htm

Bertus

This porject sounds interesting. I'm gonna take a look at this.

Rafael K: For excellent response to low audio and subsonic freqs I say just use 18" _woofer_ as directional Mic with direct coupled _instrumentation amp_ as preamp! With that exact setup I was able 2 make vry high quality recording of _partridge drumming_ (which is centered around 7Hz) buried 30db below audible noise floor!

Now for ppl saying _human hearing range only goes down to abt 15Hz_ all I can say is you're right! Lowest 'notes' are more like _felt_ than heard! So anyhow anyone who has ever _heard_ partridge drumming or vry distant thunder or explosion (like from ice breaking in spring) knows exactly what I'm saying!

i don't have the space for using 18" woofers.

 

its not "my" preamp circuit. It's a finished product where i do not have any data except the measured frequency response.

In that case, are you looking for a recommendation for a different microphone-and-preamp module, or do you want to build one from scratch?

ak

 

Preferred is a different micro-preamp-module. I guess that wasn't that clear in my question.
If there is no way around i have to build one up from scratch. But that's just the second option.

 

I guess that wasn't that clear in my question.

That's not the only thing.

What is this for?
What frequency range do you need?
Is this to be located indoors or outdoors? Car = outdoors.
What power sources are available?
What output signal level?
Automatic gain control allowed/forbidden?
Any other location, packaging, environmental conditions?

ak

 

I'm quite happy with the mic I already have, except its low frequency behaviour.
so:
Frequency Range: 40Hz - 5kHz (the 5kHz are quite common for handsfree mics)
Location is in car cabins.
Power source is 12V batterie (generator), which will be regulated to the necessary volatage
Sensitvity including preamp: ~ 200mV/Pa
Automatic gain control is forbidden.
Packaging: I thought of something like cube with 30x30x30mm or similar
It is for measureing interior noise in a vehicle cabin and do some online analysis

 

You do not need the balanced outputs produced by two opamps. Use just one opamp for the mic preamp like this:

 

Rafael K: For excellent response to low audio and subsonic freqs I say just use 18" _woofer_ as directional Mic with direct coupled _instrumentation amp_ as preamp! With that exact setup I was able 2 make vry high quality recording of _partridge drumming_ (which is centered around 7Hz) buried 30db below audible noise floor!

Now for ppl saying _human hearing range only goes down to abt 15Hz_ all I can say is you're right! Lowest 'notes' are more like _felt_ than heard! So anyhow anyone who has ever _heard_ partridge drumming or vry distant thunder or explosion (like from ice breaking in spring) knows exactly what I'm saying!

I once re-coned an 18" bass driver in one of our Peavy P.A speakers and for some reason I cannot now remember, I connected a 'scope to the terminals and was amazed at the amount of "noise" it was picking up. In particular was one very large waveform at around 7 Hz that we traced to a ventilation fan in an office several hundred yards away.
One of my engineers made a portable subsonic detector using a 4" car door speaker and an amplifier with a meter to indicate levels. I think the roll off frequency could be varied up to about 100 Hz with several switched passband/stopband frequencies.
Whilst I was an apprentice, the company I worked for had a dept. that made audio measuring instruments, and to calibrate them, they had an Anechoic chamber with an incredible level of sound and vibration insulation. If you sat still for a while inside, you could hear the blood flowing through your arteries to your head.

 

I am an old geezer (73) with normal-for-my-age high frequency hearing loss. My hearing aids make my hearing young again. For hearing tests they have an anechoic chamber where I hear my blood moving around.
The new lenses in my eyes fixed the cataracts and made my vision better than new.
The few medications after my heart attack make me strong and energetic again.