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Simple thought, this is more for experimenting than anything else. I am well aware the end result may sound horrible, so please don't presume to preach to me here.

I play music. I have a battery powered amplifier / speaker which I modded with a bigger battery and power jacks for my small mixer and DI box. The sound balance is pretty shitty, but it's for fun and gets the job done for outdoor gigs.

I have a couple of battery powered omnidirectional collar mics that I want to use. The problem is, they're pretty sensitive and I can only crank them up so far before feedback starts, at a few different frequencies. The microphone inputs on the amp have no tone adjustment.

Microphone specs:

  • 20Hz-20kHz response, omnidirectional

  • 1kohm impedance

Simply put, I want to open up the microphone jack and solder a couple of resistors and caps in there to make a band-pass filter solely for the human vocal range. Now I am completely aware this could mess up the sound pretty badly, but I'm not using this system to record anything, ever. I just want to eliminate the feedback by isolating/passing only the human vocal range with a passive filter. If it sounds nasty (which, again, it probably will) it's a simple matter of desoldering stuff and putting the original wire back.

So, the question:

What range of frequencies best describes the human voice, and for the electrically-inclined, could you help me determine the resistor/capacitor values that will likely work best without killing the sound of my voice?

  • you'd probably get better results trying to get the mics closer to source than you could obtain through any kind of filtering - inverse square is your friend in this scenario, I'd say. I use 2 omnis [along with others] to mic drum kits, which other engineers always look askance at, until they hear the results. – Tetsujin Aug 7 '16 at 17:43
  • @Tetsujin Not really an option, lol. But I've solved my issue, see my comment on Richard Crowley's answer. – Boloar Aug 18 '16 at 6:39
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Traditional voice range was 300 Hz to 3000 Hz. But that was for telephones where only basic communication was the objective. For singing, you need to include at least a couple octaves of overtones. You can look up the musical scale of various voices and you can look up the frequencies from the names of the notes.

Ref: https://en.wikipedia.org/wiki/Voice_classification_in_non-classical_music Ref: https://en.wikipedia.org/wiki/Piano_key_frequencies

You would need at least two octaves above the upper limit of your voice range, and perhaps more.

Note that making a passive bandpass filter (actually a high-pass filter plus a low-pass filter) is pretty tricky, and especially at the very low microphone levels.

Remember also that those feedback frequencies you are trying to eliminate are probably right within that bandpass that you are trying to filter for your microphone. For that reason, it is more common to use notch-filters to knock down the feedback frequencies without affecting the rest of the spectrum.

  • You're right, of course. Notch filters not really an option, haha. I basically ended up hacking together an adjustable high-pass filter on the mic input. The amp has a couple of effects that I literally never use, controlled by pots, so I disconnected and then wired one of those (50k) potentiometers up to the mic with a 100nF cap. Boom! Low frequency feedback completely gone, and high frequency feedback only when indoors. Thanks! – Boloar Aug 18 '16 at 6:37
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Utilise a software-based frequency shifter. Shift the mic signal up by a few Hz. This should eliminate feedback, at the cost of confusing your vocalist slightly.

  • Not really an option, haha. But it's a thought. – Boloar Aug 18 '16 at 6:42

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