Avoid comb filtering effects

Discussion in 'Microphones (live or studio)' started by AUD10, Oct 6, 2018.

  1. AUD10

    AUD10 Active Member

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    Mar 20, 2005
    How do you avoid comb filtering effects where there are singers or instruments with microphones spaced closely together? Often in situations with limited performance space, it is difficult to adhere to the 3:1 rule.
     
  2. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    You see frequencies moves in different wave lengths in the air and therefor they travel at different speed. A cancellation occurs when using 2 or more mics and the sound hits the mics at different time which results in inverted polarity of a frequency.
    upload_2018-10-6_8-51-10.png
    The closer your mics are should result in less cancellation
    One way is to put the source at equal distance of the microphones. A X/Y technic is easy to do and give great results. (better done with 2 identical mic/preamp)
    upload_2018-10-6_8-56-5.png
     
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  3. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    Get the mics closer to the sources. Exploit polar patterns to enhance isolation.
     
  4. Boswell

    Boswell Moderator Distinguished Member

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    The comb filtering you refer to is due to sound from one source being picked up by a second microphone at a different distance from the source. The sound arrives later at the the further microphone, and the resulting time delay can create destructive interference in the mix. For there to be audible comb filtering, the relative amplitudes of the two signals should be less than about 10dB, a figure that is the justification of the crude 3:1 distance rule, as the sound intensity level falls off with the square of the distance.

    It's usually not possible to avoid some bleed between microphones set up for different performers in the same performance space. The best way of dealing with it is not to avoid it at all costs, it's to keep it to an acceptable level through careful choice of microphone type, pickup pattern and positioning.

    The wavelength of a sound wave is proportional to the reciprocal of its frequency. Ignoring dispersion and other effects, the velocity of sound in air is independent of frequency.
     
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  5. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    Sometimes I see the 3:1 rule of thumb described using the distance between the mics. That's not quite right (though it usually gets you close enough) so I made this illustration to make it clear that it's all about relative distance from source to mic.
    Three%20to%20One%20Rule%20of%20Thumb.jpg
     
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  6. kmetal

    kmetal Kyle P. Gushue Well-Known Member

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    There is also combination filtering that can result from acoustical reflections from hard/reflective surfaces within the room. Absorbative materials like moving blankets and acoustical baffles help this type of comb filtering.
     
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