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Author Topic: question for the pro's on Comb Filtering  (Read 14431 times)

John Roberts {JR}

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Re: question for the pro's on Comb Filtering
« Reply #20 on: September 10, 2012, 07:09:12 pm »

Taking it one step further, after listening to comb filtering with pink noise or music. Use a repetitive broad spectrum noise impulse signal, like a metronome. Not only do you hear the onset of comb filtering but also, when enough delay is added, echo's. Which turn out to be frequency dependent. Quite contrary to most literature that says that echo's start around 60 ms, regardless of frequency. I found this quite revealing. Apparently it's more likely to state that echo's occur at 24 wavelengths of delay or more. AFAIK credit for this little experiment, in my case, goes to Bob McCarthy.
If really interested look at the early work by Haas, and IIRC Madsen on fusion of multiple sound arrivals into one (Haas Fusion region). This was all figured out several decades ago, and well known to sound designers.

JR
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Merlijn van Veen

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question for the pro's on Comb Filtering
« Reply #21 on: September 11, 2012, 03:28:07 am »

Thanks for clearing that up, in my case it was Bob McCarthy who first let me hear that echo's are frequency dependent.

Adam Black

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Re: question for the pro's on Comb Filtering
« Reply #22 on: September 11, 2012, 09:28:48 am »

...would a series of .wav files be able to teach what to listen for?

Here is a video from awhile back of us playing with comb filters.

-A
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John Roberts {JR}

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Re: question for the pro's on Comb Filtering
« Reply #23 on: September 11, 2012, 10:05:58 am »

If you have access to a digital console that has delay on the inputs-it is real easy to do.

Just take a single input and assign it to 2 channels.  Set the gain the same.

Turn up one channel and listen to the main output.  Now turn up the other.  THe level should increase 6dB (if all things are set the same).

Now add a little bit of delay (say 1ms) to one of the inputs.  Hear the sound change?  THAT is combfiltering.  Now play with the delay time and see how it changes.

If you only have delay on the outputs-then take 2 outputs and sum them together in a DSP or other mixer.  You could even return the 2 outputs to 2 inputs on the console-assign those 2 inputs to a DIFFERENT output (say an aux) and listen to the aux.
While this may be too much information, if you decide to do your own experiments with summing together a delayed signal with itself, the amount of delay will affect the sound character. 1mSec delay would create the lowest notch or comb at 1 kHz, and multiple notches above that (2kHz, 3 kHz, etc).  This will be similar to the studio effect called flanging, while the studio effect is made more dramatic by sweeping the delay time shorter and longer. A flanger running at constant delay sounds like um... combing.  8)

Since our hearing is more sensitive to changing comb filters, one old trick to better hear combing is to move your head. If this changes the relative path delay you will hear the combing change slightly.

Very short delays only cause combing at higher frequencies so are less audible. Longer delays cause combs that start lower in frequency and are spaced closer together, while a long enough delay sounds like a repeat or a second different event.

JR
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Patrick Tracy

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Re: question for the pro's on Comb Filtering
« Reply #24 on: September 11, 2012, 11:21:22 am »

1mSec delay would create the lowest notch or comb at 1 kHz, and multiple notches above that (2kHz, 3 kHz, etc).

Wouldn't 1ms cancel at 500Hz and up rather than 1k and up? Like 500, 1.5k, 2.5k etc.?
« Last Edit: September 11, 2012, 11:25:00 am by Patrick Tracy »
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Ivan Beaver

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Re: question for the pro's on Comb Filtering
« Reply #25 on: September 11, 2012, 11:56:21 am »

Wouldn't 1ms cancel at 500Hz and up rather than 1k and up? Like 500, 1.5k, 2.5k etc.?
Yes the first cancellation notch is one half the time and spaced at the time intervals.

So 1ms would be around 500Hz with the notches at 1500-2500-3500 etc (give or take that 1000Hz is not exactly 1ms)
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Greg_Cameron

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Re: question for the pro's on Comb Filtering
« Reply #26 on: September 11, 2012, 11:58:18 am »

Some years back when Scovi was still using analog, he said he would not switch until there was bus compensation re: latency on a digital desk. Of course he has been with Digi (or whatever :-) since and it's not an issue.

I have to agree. Though I don't double bus, I always have the Venue's delay compensation turned on just on principle and "just in case." It seems like it should be par for the course.
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John Roberts {JR}

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Re: question for the pro's on Comb Filtering
« Reply #27 on: September 11, 2012, 12:45:22 pm »

Yes, apparently not too much information for this crew.  :-[

With 1 mSec delay the notches are spaced 1 kHz apart and for same polarity sum with lowest notch at 500Hz, for opposite polarity sum, 1 Khz lowest notch.

If I wanted to try to weasel out, I would say reflections off of wall boundaries would be inverted polarity, but no, I just posted too vague of an answer (I was wrong).
 [edit] thinking about it, I'm not so sure a simple reflection is inverted polarity, so lets just say I was wrong. [/edit]

Not to veer even further but understanding how delayed versions of signal combine is useful for understanding acoustic feedback. Delay time based on path length defines where the feedback nodes "can" occur. it is a "can" rathter than "does" occur, since it is the combination of loop gain, and potential to run away.   

JR
« Last Edit: September 11, 2012, 02:48:29 pm by John Roberts {JR} »
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Andre Vare

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Re: question for the pro's on Comb Filtering
« Reply #28 on: September 11, 2012, 01:08:30 pm »

If really interested look at the early work by Haas, and IIRC Madsen on fusion of multiple sound arrivals into one (Haas Fusion region). This was all figured out several decades ago, and well known to sound designers.

Expanding on this, Toole has done significant research on the time and level of multiple sounds and created a drawing showing the signifcance of time and level to the effect it has on the original sound perception.  The JPG has additional information on it, but the core information is is quite clear.

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Patrick Tracy

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Re: question for the pro's on Comb Filtering
« Reply #29 on: September 11, 2012, 04:49:15 pm »

Yes the first cancellation notch is one half the time and spaced at the time intervals.

So 1ms would be around 500Hz with the notches at 1500-2500-3500 etc (give or take that 1000Hz is not exactly 1ms)

1ms is the exact time interval for 1kHz but the wavelength is approximately 1 foot at the usual speed of sound.

ProSoundWeb Community

Re: question for the pro's on Comb Filtering
« Reply #29 on: September 11, 2012, 04:49:15 pm »


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