I want to count and log the BPM of a rather large bass speaker (250-400W RMS range i assume).
BUT there is a mostly wooden/stone wall (~40cm) between me and the speaker + a 50cm air gap.
I managed to detect the speaker with a normal microphone, but that gets confused once there are other noises like talking or foot steps in the room.
So my idea was to build a large coil around some graphite rod to build a sensor for electromagnetism. Then connect that to a audio amp and feed the result into a audio card microphone input.
Would that maybe work? Any guesses how far the magnetic field of a bass speaker is detectable? How much wire should i use for the coil?
Edit:
So i found this video https://www.youtube.com/watch?v=83edokt3K5c of someone using a giant coil to pick up a very small speaker from a few feet away, so i assume its 100% possible, and the speaker i am trying to pick up has like 200-300x the power of what he is picking up in the video.
If you just want the count and not the strength or waveform, I wonder if the magnetic field from the voice coil of the speaker would be strong enough to trigger a reed switch at that distance. A common reed switch has a top switching speed between 100Hz to 500Hz, which is within the 20Hz to 250Hz of human hearing bass.
Reed switches aren’t that sensitive. The magnet usually has to be within a centimeter or so from the switch to operate it.