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[JeffreyHague]

Hey again all!
Sort of a long post - sorry but I'd love to hear your thoughts.

Recently I have been doing some informal training on live sound and realized it has been a long time since I have read any of the texts on the subject so I figured it was a good idea to refresh my memory just to be sure I haven't forgotten anything important.

I have been re-reading the Sound Reinforcement Handbook, 2nd edition - Gary Davis & Ralph Jones - and I have come across a couple statements that I either don't understand or that go against what I have always thought.
Please elaborate on the following:

1)
Page 188 discussing the use of 1/3 octave (or higher) eq to control feedback in monitor systems, the author makes this statement:

"(For those who are very technically inclined, consider that feedback is a wavelength - related phenomenon, so tuning a frequency - calibrated equalizer, no matter how stable, may not keep the feedback from drifting back in as the wavelengths change due to atmospheric conditions.)"

I know there have always been arguments back and forth as to whether temperature and humidity affect sound and I believe the common consensus is that they do but not in any dramatic way - certainly not to actually affect wavelength vs frequency. Nominal air pressure will alter wavelength vs frequency but again only to a small extent and generally the only thing that will sufficiently change air pressure is altitude - not something that is likely to change between soundcheck and performance.
My question is; what is meant by "feedback is a wavelength - related phenomenon"? The author seems to imply that feedback is not a frequency - related phenomenon.

2)
Page 190 discussing position of the FOH console:

"In fact, a location which is centered mid way between two speaker stacks is least desirable because of phase cancellations (and comb filtering) of sound from the two stacks. If one wishes to be near the middle of the audience (from left to right), then at least make sure the position is somewhat off center to reduce the phase problems."

This seems exactly the opposite of what I have learned. Midway between the 2 stacks is where there will be the least phase problems because the sound reaches that position at the same time from both stacks. Have I missed something truly fundamental?

Thanks!

[John Roberts {JR}]

Hey again all!
Sort of a long post - sorry but I'd love to hear your thoughts.

Recently I have been doing some informal training on live sound and realized it has been a long time since I have read any of the texts on the subject so I figured it was a good idea to refresh my memory just to be sure I haven't forgotten anything important.

I have been re-reading the Sound Reinforcement Handbook, 2nd edition - Gary Davis & Ralph Jones - and I have come across a couple statements that I either don't understand or that go against what I have always thought.
Please elaborate on the following:

1)
Page 188 discussing the use of 1/3 octave (or higher) eq to control feedback in monitor systems, the author makes this statement:

"(For those who are very technically inclined, consider that feedback is a wavelength - related phenomenon, so tuning a frequency - calibrated equalizer, no matter how stable, may not keep the feedback from drifting back in as the wavelengths change due to atmospheric conditions.)"

I know there have always been arguments back and forth as to whether temperature and humidity affect sound and I believe the common consensus is that they do but not in any dramatic way - certainly not to actually affect wavelength vs frequency. Nominal air pressure will alter wavelength vs frequency but again only to a small extent and generally the only thing that will sufficiently change air pressure is altitude - not something that is likely to change between soundcheck and performance.
My question is; what is meant by "feedback is a wavelength - related phenomenon"? The author seems to imply that feedback is not a frequency - related phenomenon.

That is a very old marketing handout, in fact IIRC the Yamaha version is a rewrite of an even older version from a different company. Over decades in print and glaring errors would have been corrected.

I have written at length over the years about feedback and won't give you the long version again (maybe do a search), the short version is that feedback is a phenomenon related to sound from speakers being picked up by microphones and fed back into the speakers. If they repeat with more than unity gain the signal amplitude builds until the electronic path saturates. The frequency or pitch of this feedback is related to the transit time between the speaker and microphone. =The transit time in the electronics is insignificant. Indeed heat and humidity affect the speed of sound in air so can easily affect the frequency of feedback.

The speed of sound is affected by temperature and humidity. Because it is less dense, sound passes through hot air faster than it passes through cold air.

2)
Page 190 discussing position of the FOH console:

"In fact, a location which is centered mid way between two speaker stacks is least desirable because of phase cancellations (and comb filtering) of sound from the two stacks. If one wishes to be near the middle of the audience (from left to right), then at least make sure the position is somewhat off center to reduce the phase problems."

This seems exactly the opposite of what I have learned. Midway between the 2 stacks is where there will be the least phase problems because the sound reaches that position at the same time from both stacks. Have I missed something truly fundamental?

Thanks!

In theory for mono sound sources, exactly centered would be fine. Most cancellation problems are with very low frequencies related to room modes, boundary reflections, and the like.

If you mix in the ideal world keep doing what you think is right.

JR

[Ivan Beaver]

Hey again all!

This seems exactly the opposite of what I have learned. Midway between the 2 stacks is where there will be the least phase problems because the sound reaches that position at the same time from both stacks. Have I missed something truly fundamental?

Thanks!

The EXACT center between stacks is OK, but even a foot off will create issues.  As if you stand at one end of a large console and move to the other.

This is because the level from both stacks is essentially the same, but the distance is a little bit off.

You get the most amount of combfiltering when the levels are the same.  The more different they are, the less the combfiltering will be (at least in depth of the notches).  The freq will stay the same regardless of level.

A 1 foot difference will start the combfiltering around 500Hz and up.  6" will be from around 1KHz and up.

So you better be (as Charlie Hughes likes to say) "Dead monkey nuts on".  Even a couple of inches off will give lots of combfiltering.

The difference in level will be small parts of a dB.  So if you move off center, the level differences become greater, so less combfilter depth (and the associated humps in between the dips as well)

[Scott Helmke]

If you want to mix so that it sounds best for most of the audience, then you want to be where most of the audience is located - off center.

[Patrick Tracy]

2)
Page 190 discussing position of the FOH console:

"In fact, a location which is centered mid way between two speaker stacks is least desirable because of phase cancellations (and comb filtering) of sound from the two stacks. If one wishes to be near the middle of the audience (from left to right), then at least make sure the position is somewhat off center to reduce the phase problems."

This seems exactly the opposite of what I have learned. Midway between the 2 stacks is where there will be the least phase problems because the sound reaches that position at the same time from both stacks. Have I missed something truly fundamental?

That depends on what frequencies you're most worried about. Even the "exact center" isn't going to be exact for both ears at once, there's technically going to be some kind of interference no matter what. But some kinds of comb filtering are actually decoded by our binaural hearing as directional cues, so I'd have no problem being dead center if it were close enough to put the two stacks at a sufficiently wide angle to perceive them as separate sources. I'd suspect even 30° would be sufficient. I'd be more concerned with placing myself to avoid a substantial LF null or node.

[David Morison]

My question is; what is meant by "feedback is a wavelength - related phenomenon"? The author seems to imply that feedback is not a frequency - related phenomenon.

No, because anything wavelength dependant is also frequency dependant - they're linked (inversely) by the speed of sound.

As to whether environmental conditions can cause enough drift to matter, that depends on how narrow your anti feedback notch filter is, and how close you were to the exact frequency in the first place. If you used a traditional 1/3 octave graphic, and the feedback happened to be exactly on the centre frequency of a band, then that drift may well not be enough to need to move to the next band.
However, if the feedback wasn't exactly in the centre of the 1/3 octave band, or if you were using a parametric EQ with the bandwidth cranked all the way down to the narrowest setting, then there is slightly more chance of having to retune it.

HTH,
David.

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