Detecting very low level low freq?

Thread Starter


Joined Feb 25, 2008
so, in dead of night when outside has low ambient noise, from inside i can hear a low level (faint) hum, in the range of 250Hz. any ideas to build a simple directional detector that 1) can detect the frequency, and 2) allow me to get an idea as to the direction the sound is emanating from. i suspect the sound is coming from a nearby pool pump, but would be neat to build a detector that can help solve the mystery (and stop annoying me).

since i am working another project that may use piezo element, how about mounting a bare piezo element to a 1/2" thick vertical AL plate or wood with heavy base, couple that to a high gain opamp? just not sure if 250Hz would couple well to a piezo element, might get ran over by noise in the gain. i guess one side of piezo should be on heavy mass while the other side couple to a "fork" that has resonance down low near 250Hz?

your thoughts?
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Joined Nov 23, 2012
Use two microphones back-to-back (one on each side of a 3/4" piece of plywood (to block/deaden sound). Then connect to differential amplifier. You will need to zero the signals with a pot on one of the microphones to zero the delta in a silent room. Then rotate and find the biggest response to the + input on your op amp or instrumentation amp.

Have fun.


Joined Nov 30, 2010
(one on each side of a 3/4" piece of plywood
Styrofoam works, too.

ps, 250 Hz isn't consider, "Low" in some frames of reference, like power line frequency or musical instruments. 250 Hz is near the fifth string on a guitar.
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Joined Nov 30, 2010
if I would have had Styrofoam, I would have suggested that.
I have a piece of styrofoam in the laundry room...left over packing from a garbage disposer. That's why I thought of it. Besides that, styrofoam heads are used for testing headphones, so styrofoam probably works in the audio range.