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It's been a while since I've done this, but I have an acoustic sensor meant to detect low frequency sound waves attached to a 5/32" ID hose that leads out to open air. I seem to recall from my grad school days that there's a relationship between the length of the hose and the effective pass band (low- or high-pass, I don't recall) of the air column leading to the sensor, but I can't find references online.

I'm looking for that relationship. Basically what length hose do I need if I want a cutoff frequency of, e.g., 20 Hz? 50 Hz? Or more generally what is the equation? Or am I just misremembering, in which case I'll delete the post.

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You are looking at the organ pipe resonance, in which the maximum sensitivity will be when the length of the hose is 1/4 of a wavelength. However it will also respond to other frequencies, particularly 3/4 5/4 and so on. c (m/s)=L (m)*f(Hz). c=300 m/s, f=50 hence wavelength at 50 Hz is 6m, you need a 1.5m tube.

If you want a low pass filter you need to use an expansion box. That is small pipe ->big pipe-> small pipe, where the length of the big pipe determines the cutoff frequency, and the ratio of areas determines the attenuation.

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