Magnetic bass detection?

Submitted ⁨⁨4⁩ ⁨weeks⁩ ago⁩ by ⁨einfach_orangensaft@sh.itjust.works⁩ to ⁨electronics@discuss.tchncs.de⁩

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 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.

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vk3kri@mastodon.radio ⁨4⁩ ⁨weeks⁩ ago
@einfach_orangensaft
How about a magnetic telephone pickup coil?
Not recommending this one in particular - just an example that wasn't from amazon.
https://www.elliottelectronicsupply.com/telephone-pickup-coil-suction-cup.html
TBH I'm not sure if there's enough magnetic leakage from the voice coil in a speaker for this to work.
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- einfach_orangensaft@sh.itjust.works ⁨4⁩ ⁨weeks⁩ ago
  Interesting, i didnt knew those existed at all, but thx for the suggestion i will try one of those!
  
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OwOarchist@pawb.social ⁨4⁩ ⁨weeks⁩ ago
Maybe just stick with audio recording, but use a more directional microphone? (And use foam to isolate it from any vibrations in the walls/floor.)

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- einfach_orangensaft@sh.itjust.works ⁨4⁩ ⁨weeks⁩ ago
  Directional gets kinda tricky in the low frequency range :/
  
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  - OwOarchist@pawb.social ⁨4⁩ ⁨weeks⁩ ago
    Interferometry, maybe?
    
    Two microphones, side by side. Signals that come from directly in front (or directly behind, unfortunately) will hit both microphones simultaneously. But sounds coming from the side or any other angle will hit each microphone at slightly different times. If you then have a hardware or software solution to filter out any sound waves that don’t match, you’ve got a highly directional microphone that should work well in any frequency range. (Can also use this setup with 3 microphones if it needs to be 3D and filter out noise from above and below as well.)
    
    There are commercially available setups that do this, but in theory it shouldn’t be too hard to build your own, either. I mean, in theory, you could just feed the outputs of both mics through an and-gate transistor into a single output, and that very simple hardware would do the filtering for you – you’ll only get the outputs from when both mics pic up the same sound at the same time.
    
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prenatal_confusion@feddit.org ⁨4⁩ ⁨weeks⁩ ago
Noisy neighbors?

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- einfach_orangensaft@sh.itjust.works ⁨4⁩ ⁨weeks⁩ ago
  Yeah very very much so, for years, they only shut up when i bang the sealing, they love loud EDM at 3am in the morning and i kinda wana automate the sealing banging part now.
  
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  - prenatal_confusion@feddit.org ⁨4⁩ ⁨weeks⁩ ago
    Warum brauchst du den bpm part? Reicht das nicht ein accelerometer an einem esp um das zu erfassen? Oder Mikro oder vergleichbares
    
    Das klopfen dann über was separates mit Motor und Relais
    
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partial_accumen@lemmy.world ⁨4⁩ ⁨weeks⁩ ago
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.

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- cmnybo@discuss.tchncs.de ⁨4⁩ ⁨weeks⁩ ago
  Reed switches aren’t that sensitive. The magnet usually has to be within a centimeter or so from the switch to operate it.
  
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alzymologist@sopuli.xyz ⁨4⁩ ⁨weeks⁩ ago
Could be doable with just a hall sensor chip, you have one in your phone, just check it out quickly. Large coil will probably catch slow vibrations too. Also try just filtering audio signal, like get only sustained bass sound, footsteps and voice have short lived low frequency components compared to most music except for percussion I suppose.

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- einfach_orangensaft@sh.itjust.works ⁨4⁩ ⁨weeks⁩ ago
  I tryed with the magnetometer in my phone before and sadly it didnt pick up anything, i think the polling rate may be to low too. But ima search around for some more sensitive hall effect sensors
  
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  - alzymologist@sopuli.xyz ⁨4⁩ ⁨weeks⁩ ago
    Ok, I’ve done experiment myself; it was a dumb idea. Sure I can see the bass beat on smarthone hall sensor as sharp periodic spikes… from few cm away of the coil.
    
    Thing is, any measurable signal outside of coupled system is essentially power loss, which industrial made speakers are, obviously, optimized against. The field loops back into the magnetic drive through the shortest achievable path. Then, it’s not your typical radiative EM system, it’s low frequency near field transmission - field drops exponentially as you move away from the coil.
    
    Most probably, you’d need to measure long periodic signal, do lots of oversampling, etc. to gen anything. Or build a high fidelity resonator to steal the field from the speaker - essentially, same thing, except the resonator will probably start singing, too.
    
    So I’d give up on magnetic approach; why go for counteroptimized signal when you have optimized acoustic field? Just analyze it - look for sustained harmonic vibrations, fat envelopes so different from human voice barking and foorsteps; EDM are full of those nice sounds.
    
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