So, just like the title suggests, my college professor and I are trying to suppress certain overtones of an instrument's sound. I have a microphone hooked up to an oscilloscope, and I'm using the oscilloscope to perform an FFT analysis of the sound. I am trying to come up with a way to pass the signal through a hardware device to suppress certain overtones in the FFT analysis, and then record the result so we can hear what it sounds like when all you have are just the odd or just the even overtones, or even just the fundamental frequency.

I'm pretty sure I've heard somewhere that you can implement this in hardware, but I'm not entirely sure where to begin. Oh and this isn't a homework assignment. My professor and I are doing this just for the hell of it.

  • 2
    \$\begingroup\$ Record the sound, convert to an FFT, modify the FFT in software, reproduce the new sound. \$\endgroup\$ – Andy aka May 5 '17 at 18:00
  • \$\begingroup\$ Some search terms for you: "notch filter" and "comb filter". \$\endgroup\$ – Hearth May 5 '17 at 18:18
  • \$\begingroup\$ Are you trying to do this "live"? If not, are you trying to do this on recordings that have a sequence of tones, or just recordings of a single note? \$\endgroup\$ – user49628 May 5 '17 at 19:02
  • \$\begingroup\$ @user49628 Yes and no. I'm trying to pass a "live" sound through the device, and then take the output of that device and record it on a laptop and play the sound back from the laptop. \$\endgroup\$ – Eric May 5 '17 at 21:33

That piece of hardware is a bandstop filter. It removes one overtone and is usable maximally at one octave pitch range if it's not a digital signal processor with an adaptive filtering program, but a fixed filter circuit.

For that 1 octave range the filter attenuates one octave in the overtone range. Accurate results need quite complex filter (=high order transfer function).

A bandstop filter is common in mixing desks as parametric eq or graphic eq. which can be used to attenuate some frequency band. In modern desks they really are sharp enough for attenuating a predefined overtone, but they are actually programmed dsp devices, not filter circuits.

Those equalizers are also available as rack devices both as dsp and filter circuits

Premium processing software for musicians exist in wide varieties. Their capablities go far beyond any fixed filtering circuits. The software is available as well for realtime performances as for postprocessing the recordings.

ADDENDUM: for demonstration purposes you should consider to play only recordings to be compared. one unprocessed and one filtered, because

  • you cannot practically remove the overtones from the direct acoustic sound of the instrument, only from the electric signal.
  • no recording can have the richness and fidelity of the original acoustic sound in proper acoustics

Nearly all pieces of Audio workstation software have the needed filter included and more is available as plugins. Premium filter plugins such as FabFilter Pro-Q are well worth to check to see what is available and possible to use by the musicians, not mathematicians. Free 1 to 4 week trials are generally available.

  • \$\begingroup\$ Awesome! Thank you very much for your answer! I was having trouble looking for examples because I didn't know what this kind of device/filter was called. Thank you! \$\endgroup\$ – Eric May 5 '17 at 21:31

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