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I'm creating an application where I want to output a constant frequency through a speaker to test the frequency range of a microphone device I've developed. The problem is, my application measures very low frequencies in the range of 2Hz to around 200Hz, where most speakers start at 200Hz!

I've looked into subwoofers but they still seem to begin around the 20Hz range whereas I need it a bit lower.

I'm using MATLAB to output the constant frequency using the built in functionality there. I need a speaker which I can output a very low frequency sound through the 3.5mm port in a laptop/sound card to a speaker which can accurately produce these frequencies.

Is this possible? If so, any recommendations on a speaker which can produce these ranges or anything I need to think about to get as accurate results as possible? Needs to be fairly cheap (~<£50), the cheaper the better.

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  • \$\begingroup\$ You're probably not going to get a speaker large enough for that price. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 27 '14 at 12:07
  • \$\begingroup\$ Why not use a small dc motor, an eccentric and a diaphragm. OK it means a bit of a mechanical build but it would work at very low frequencies and if you can get the motor to rev up to 12,000 RPM it'll cover your bandwidth requirement. \$\endgroup\$ – Andy aka Jan 27 '14 at 12:11
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    \$\begingroup\$ "Accurate" in what way? There are plenty of speakers that work down to 2 Hz (like, nearly all of them). Sometimes the low frequencies are attenuated, but they do make some sound, and I'm not aware of speakers that significantly distort the frequency. Or do you need the reproduction to be accurate in some other respect? Or, are you also requiring an amplifier? \$\endgroup\$ – Phil Frost Jan 27 '14 at 12:12
  • \$\begingroup\$ You won't find a speaker that has a flat response in the low bass area plus the room resonance will influence the results, so "accurate" doesn't seem to apply. Do you have a reference mic with a known response in order to compare it with your own mic? \$\endgroup\$ – alexan_e Jan 27 '14 at 12:21
  • \$\begingroup\$ Thanks for all the responses. I seem to be getting the response I was expecting that I could do it with any speaker, but responses will vary and would make calculating my microphones ability harder. I had considered @Andyaka suggestion of the motor based frequency system, but was a little deterred but what was involved to do it, however I think I've found a similar system at work which could produce a similar job. \$\endgroup\$ – ritchie888 Jan 27 '14 at 12:52
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Every speaker works at every frequency, just that the efficiency falls off outside the rated range. You can make the cone of a subwoofer move in and out over its maximum travel distance at 2 Hz easily enough. The problem is that the displacement required for the same sound power goes up with decreasing frequency.

You can make more use of the sound power you do get out of the speaker by tighter mechanical coupling to the microphone. Mount the speaker on one end of a box pointing inward, and the microphone in the middle of the opposite end, which stays open. The wavelength for 2 Hz is about 165 m, so a box a few feet accross won't cause interference fringes off the walls, like it would at over a kHz or so.

Even if the speaker is out in the open, moving in and out 1/2 inch, and the microphone is a couple feet away, you have to ask yourself what you expect this microphone to pick up for real that would be louder than that. If it can't pick up this infrasound with 1/2 inch displacement, what is it going to pick up in the real world?

Another issue is that you will probably need to design your own power amp to driver the speaker. Normal audio amps have high pass filters that should attenuate 2 Hz severely.

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