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Comment by wl | Hacker News Reader

I wouldn't recommend the Earthworks M50 for this application. Earthworks' design approach for these microphones is to maximize time domain performance by having excessive bandwidth, minimal phase distortion, and flat frequency response. In order to achieve this, they have to use a tiny condenser element, which has relatively low sensitivity and thus relatively high noise.

If you're looking to measure things near or below the limits of human hearing, I'd recommend looking at microphones designed for infrasound rather than low sensitivity microphones designed to go into the ultrasonic range. PCB and Brüel & Kjær are the big names here.

Measuring infrasound is pretty difficult, unfortunately. Most audio gear has a high pass filter, which makes sense for most uses but not this. Also, pseudo-noise from air currents is often a huge source of error.

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Since you seem fairly knowledgeable about this subject, perhaps you could answer a related question I've been wondering: Would an effective low-cost way of measuring low frequencies (let's say ~20hz) be to use a subwoofer speaker (used as a microphone) hooked up to something that would amplify and recording of the signal? My reasoning is that they're naturally designed to work with low frequencies to begin with (more so than a typical microphone anyway). I would also reason that you could also do a frequency response test over a range of known frequencies and power levels for the specific subwoofer to begin with, and then write a specific software filter to normalize whatever you record from it to get the 'true' sound. Does that make sense?

Edit: Also, note that I mean doing hardware amplification of the electrical signal in the wire coming directly from the motion of the magnet of the speaker.

Yes, sub-woofer loudspeakers make excellent low-frequency microphones when used with a suitable pre-Amp. And using a PC with Spectrogram software is a great way to record and visualise the sounds.

However, please read my post above on what I actually found.

You're right that you can use a speaker as a microphone, because they're pretty much dynamic microphones run in reverse. However, speakers are exceptionally low sensitivity when used as microphones. You could amplify them and get a usable signal level out of them, but the noise would probably be much worse than a purpose built microphone.

Microphone size doesn't really matter that much for low frequency sound. The Earthworks M50 suggested above? I wouldn't recommend it for this application because if you're going to spend a lot of money, it might as well be on something well-suited to the application. However, the approach it takes works and can be done for much less money. Cheap omnidirectional electrets go down pretty far[1], and you don't need the ruler flat response of the M50. You just need amplification circuitry that doesn't filter out the low frequency stuff you want.

DSP is commonly used to correct the frequency response of microphones. However, measuring frequency response is difficult and expensive.

[1] http://volcanomodels.sr.unh.edu/jbj/MICROPHONES/microphone_l...

Cool, thank you for the link. To measure and correct for the frequency response of the microphone, can't you just play a range of known frequencies that each have known power levels, and then measure the gain/loss in power level that gets recorded by the mic, and then solve for a filter that gives you unity gain on the mic? I assume that you'd have peaks in the frequency response around the natural resonance and harmonics of the mic, and you'd basically just be adjusting for those.

That's how the process works, but it's more complicated than you'd think. The room a speaker is in changes the overall frequency response. Speakers aren't flat, either. There are ways to get past these obstacles, but it can be difficult and expensive to get right. Going to the the world of infrasound just increases the complexity.

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