5 ms·

Because it's not really periodic enough. I don't think any natural sound source short of a musical instrument (and even for them, it's doubtful) is so perfectly

by dri_ft 10y ago

Because it's not really periodic enough. I don't think any natural sound source short of a musical instrument (and even for them, it's doubtful) is so perfectly periodic that an out of phase version would stay in opposition (i.e. in the inverted/opposite phase of the cycle) for long enough for this to be of any use. But there are (FFT-based?) noise removal tools that can work for this sort of thing. edit: also, the weird phasing artefacts this would introduce would be much more prominent and noticeable, even if it succeeded in reducing the overall volume. So much so that doing basically this ('flanging') is a standard musical effect for making parts sound psychedelic.

x1798DE 10y ago
Follow-up question, what if you put a separate microphone further away from the speaker to get a synchronized sample of the background noise to subtract away? I suspect the logical end to that sort of thing is to use multiple microphones to add "depth" to your sound recording, then filter out sounds coming from sufficiently far away.
- worik 10y ago
  Like the way the Grateful Dead used a pair of microphones to cancel out feedback from the P.A. It worked for them and would work, so long as the person you are recording keeps their head very still. Any movement would change the parts of the sound emerging from their throat that gets cancelled by the cancelling mic.
  - deleted 10y ago
    [deleted]
  - scarecrowbob 10y ago
    While it is indeed a cool idea, it really does create more problems than it solves-- there are reasons why it never caught on in live sound reinforcement.
- riprowan 10y ago
  That's how noise cancelling headphones work but it introduces audio artifacts that are often just as bad as the thing you're trying to remove. The thing most people don't consider is that white noise (hiss, a fan) is readily tuned out by the brain - the brain is brilliant at reducing background noise - but artifacts introduced by artificial noise reduction are correlated to the signal and the brain interprets it as a distortion of the signal, not background noise. That's why audiophile audio engineers fix the noise instead of using FFT noise reduction - at least whenever possible.
  - b1daly 10y ago
    This is true that the brain will filter out steady state noise, but this effect is much less strong once a signal has been recorded. Basically, on playback, the steady state noise sounds much louder than it did when you are actually present in the room. The same holds for the reverberation in a room, it will sound much louder on a recording than on location. For example, if you record someone speaking in a reverberant space like a church, it will sound like they are in a very reverberant space. Which they are, when you are listening live, your brain filters out the "non-direct" sounds so you notice just the voice. I'm pretty sure these noise filtering processes are related to the high priority the brain puts on understanding speech.
    - riprowan 10y ago
      All true. However FM radio always has white noise, so adding a little bit to it is probably better than adding any correlated noise from signal processing. FFT noise reduction definitely has its place and I do use it from time to time. I just highly doubt "live on-air signal processing" is a good application given that the noiseprint is constantly changing.
    - tacos 10y ago
      These broad generalizations about psychoacoustics are simply not accurate. It sounds like what every novice engineer experiences before honing his or her craft. If you're surprised by what you hear after recording something, either you weren't listening properly while recording it, you haven't learned how to record it properly yet, or your recording/listening environment itself is flawed. Of course we notice different things about an object after taking a picture of it. But you've got to be very careful about cause/effect/bias.

by dri_ft 10y ago

x1798DE 10y ago

Follow-up question, what if you put a separate microphone further away from the speaker to get a synchronized sample of the background noise to subtract away? I suspect the logical end to that sort of thing is to use multiple microphones to add "depth" to your sound recording, then filter out sounds coming from sufficiently far away.

worik 10y ago
Like the way the Grateful Dead used a pair of microphones to cancel out feedback from the P.A. It worked for them and would work, so long as the person you are recording keeps their head very still. Any movement would change the parts of the sound emerging from their throat that gets cancelled by the cancelling mic.
- deleted 10y ago
  [deleted]
- scarecrowbob 10y ago
  While it is indeed a cool idea, it really does create more problems than it solves-- there are reasons why it never caught on in live sound reinforcement.
riprowan 10y ago
That's how noise cancelling headphones work but it introduces audio artifacts that are often just as bad as the thing you're trying to remove. The thing most people don't consider is that white noise (hiss, a fan) is readily tuned out by the brain - the brain is brilliant at reducing background noise - but artifacts introduced by artificial noise reduction are correlated to the signal and the brain interprets it as a distortion of the signal, not background noise. That's why audiophile audio engineers fix the noise instead of using FFT noise reduction - at least whenever possible.
- b1daly 10y ago
  This is true that the brain will filter out steady state noise, but this effect is much less strong once a signal has been recorded. Basically, on playback, the steady state noise sounds much louder than it did when you are actually present in the room. The same holds for the reverberation in a room, it will sound much louder on a recording than on location. For example, if you record someone speaking in a reverberant space like a church, it will sound like they are in a very reverberant space. Which they are, when you are listening live, your brain filters out the "non-direct" sounds so you notice just the voice. I'm pretty sure these noise filtering processes are related to the high priority the brain puts on understanding speech.
  - riprowan 10y ago
    All true. However FM radio always has white noise, so adding a little bit to it is probably better than adding any correlated noise from signal processing. FFT noise reduction definitely has its place and I do use it from time to time. I just highly doubt "live on-air signal processing" is a good application given that the noiseprint is constantly changing.
  - tacos 10y ago
    These broad generalizations about psychoacoustics are simply not accurate. It sounds like what every novice engineer experiences before honing his or her craft. If you're surprised by what you hear after recording something, either you weren't listening properly while recording it, you haven't learned how to record it properly yet, or your recording/listening environment itself is flawed. Of course we notice different things about an object after taking a picture of it. But you've got to be very careful about cause/effect/bias.