Microphone for low frequency recording use

[Hopup]

I would like to measure very low intensity and also low frequency soundwaves emanating from surfaces. I would need microphone design which is capable of recording these soundwaves. Frequency range would be around 1-20Hz and the sound would be recorded near the surface of the object.

If you have some idea how to do it, it would be useful.

 

[Chemelec]

Even if you have such a Microphone, Most devices won't Record them.
Also unlikely you could hear those Low frequencies.
And Most speaker won't reproduce them with any amount of loudness.

 

[Hopup]

Even if you have such a Microphone, Most devices won't Record them.
Also unlikely you could hear those Low frequencies.
And Most speaker won't reproduce them with any amount of loudness.

No, I can't hear them, would take much output power to hear them. I'm only interested about the data what I would get by measuring them. I don't want to reproduce them.

I think it is certainly possible to do with simple parts and design, but I don't know exactly how. Recording them is not problem since I can easily amplify the signal and use it after I just get it.

 

[Chemelec]

A Larger Speaker has a Lower Frequency response than most Microphones and using this Circuit it can make the Speaker act like a Microphone.
http://chemelec.com/Projects/Spkr-Mic/Spkr-mic.htm

 

[(*steve*)]

A speaker in a sealed enclosure could be used. However a small vent is required unless you also want to measure barometric pressure!

In any conventional microphone, if both sides of the diaphragm are open, low frequency signals are lost due to the fact that the place of the signal arriving behind the diaphragm is almost the same as that arriving at the front.

A similar feature also adds the directionality of a microphone where the size of the diaphragm becomes a significant fraction of the wavelength of the sound (in this case where both high and low pressure regions exist on the same side of the diaphragm). In your case, with low frequencies, you'll lose directionality - but it lets you get away with a larger diaphragm.

The other thing you need to worry about is that any microphone which produces an output proportional to the rate at which it moves (and a speaker is one of these) will produce a smaller output at lower frequencies for the same excursion distance.

 

[Chemelec]

A Long Throw Voice Coil type Speaker is probably also Better

 

[Hopup]

Thanks for your ideas. Idea of using speaker is fine, but I don't think it will work with such low power sound waves.

The measuring device must be quite small, about 3-5cm diameter for accurate measurement. Length is not problem, but it does limit the surface area used for registering the vibrations of air.

I agree Steve that It needs some kind of enclosure in any case for proper operation.

 

[Hopup]

Is it possible to build some structure front of the element to amplify 1-20 Hz frequencies?

 

[Chemelec]

Possibly Cut some Tubes (Pipes) to Various Lengths, to Resonate at Frequencies between 1 and 20 Hz, with your microphone on one end of the tubes.

 

[Hopup]

Ok, I figured the design out, I can use Helmholtz resonator. Would this kind of microphone work if I attach it in very light plane of plastic speaker cone?

http://www.farnell.com/datasheets/2305668.pdf

It has only 8Hz, but does that mean it won't work at all with lower frequencies?

 

[duke37]

Earthquake detectors have to respond to very low amplitude and frequencies. These are done by measuring the distance on a freely suspended weight from the earth.
Use LVDT or capacitance?

 

[Hopup]

Earthquake detectors have to respond to very low amplitude and frequencies. These are done by measuring the distance on a freely suspended weight from the earth.
Use LVDT or capacitance?

But that kind of design would not work when the device is not parallel to earth?

 

[chad786]

I was thinking of doing something similar, detecting sub audible sounds in the atmosphere.
My idea was to use one of these:
http://www.te.com/commerce/Document...DS_DT_Series_without_Leads_A1.pdfCAT-PFS0004
against a membrane in a sealed box except for a small vent. After reading this thread an old speaker would be better.
This outfit has a large number of sensors that you may find useful.
http://www.te.com/usa-en/products/brands/meas.html/?tab=pgp-story

 

[Audioguru]

A speaker has a resonant frequency where it is very sensitive which makes it sound "boomy" as a microphone.
An electret mic does not show a resonant frequency and probably produces frequencies very low (lower than audio).

 

[(*steve*)]

probably produces frequencies very low

Before you say that, you should probably read this.

When the microphone is exposed to air to receive a sound wave, a small amount of acoustic energy reaches the diaphragm whose threshold sound pressure level is about 40 dB. Most electret condenser microphones cannot faithfully capture the signal when the frequency of the acoustic wave is lower than 20 Hz. This is because when the corresponding sound pressure level is lower than 40 dB, the acoustic wave cannot provide enough force to actuate the microphone’s diaphragm.​

They also compare electret microphones against small speakers (with favourable results).

@Hopup, Maybe an electret mic with a horn attached to it could be an option. Do you know how loud the signals are that you're trying to pick up?

The attached article also suggests the (in retrospect) obvious answer of using an accelerometer which is attached to some surface.

 

[Audioguru]

A horn to concentrate very low frequencies must be enormous.

 

[davenn]

Earthquake detectors have to respond to very low amplitude and frequencies. These are done by measuring the distance on a freely suspended weight from the earth.
Use LVDT or capacitance?

Yeah, I was going to eventually bring this up
A geophone resonant around 10 Hz will easily handle 1- 20 Hz

The geophones I use in my seismic recording system are centred on 4.5 Hz but easily record 0.1 - 25 Hz

from my www page …..
http://www.sydneystormcity.com/g_phones.htm
This is a view of the Geo-Space, 4.5Hz, 4000 Ohm coil, geophone seismometers that I use for my E/W, N/S and VERT. Short Period system

around 1.5 inch diameter and 2 inches high


Dave

 

[Hopup]

I will do prototype using Helmholtz resonator chamber which is located in front of the some sort of cone, preferably made with super light material if that would help picking up the sound waves. This way I can amplify the sound waves I want in very small enclosure, I think.

Dave's geophone is very interesting, but I can't use it in my device because of its size. I need to operate the device in many surfaces which are not parallel to earth.

Steve's idea of using accelerometer is probably good one, I might use accelerometer or some sort of piezo mic which is attached to the cone.