Feedback quandary

[Harpman]

You would think having compression on a vocal channel whould be used to eliminate feedback on a channel rather than cause it.  I ran into a scenerio last Saturday where I isolated feedback coming from one of the vocal channels and the only way I was able to quickly eliminate it was to DISABLE compression on the vocal channel.  Anyone care to explain that one?

[RoadRanger]

You would think having compression on a vocal channel would be used to eliminate feedback on a channel rather than cause it.

Nobody I know thinks that - quite the opposite as you demonstrated.

[Harpman]

I know, it was weird. I did my typical cutting surrounding frequencies in the 5kHz range to eliminate the mid-range FB and didn't do a thing. Disabled the compression and voila no more feedback. Goes against the "laws of sound" 

[RoadRanger]

I think you misunderstood what I said - compression DOES increase the likelihood of feedback.

[Fluddman]

+1 Compression does increase the chances of feedback.

That's why there are quite a few people asking Makie to make the aux sends switchable pre/post EQ and Dynamics. Currently they can be switched pre or post the channel fader but not pre or post EQ/Dynamics - I am not sure how they missed this one. (Please vote for it!)

Cheers

[Harpman]

I just reloaded the saved show (save sometime back) for venue we just played at and realized that I don't have gating or compression enabled for the vocal channels.  I must have inadvertently loaded another show or preset to turn compression (heavy at that) back on.  The threshold was set for -20dB, 4:1 with soft knee with fast attack/release.

[Fluddman]

Yeah that'd do it  .

I imagine once you start to build up lots of shows and scenes, its going to get tricky managing them.

Cheers

[Harpman]

My issue tends to be duplicate shows since I use one docked iPad (3rd gen) and one undocked iPad (4th gen).

[WK154]

"There you go again" by RR, that's Ronald Reagan not RK. Making uninformed and unsubstantiated statements like "I think you misunderstood what I said - compression DOES increase the likelihood of feedback." is how wife's tales get started and makes us FOH guys look bad. Compression does NOT increase the likelihood of feedback in fact it does the opposite as was intended. Let me give you a simple example in Audio 101. Let say that the feedback threshold for you gig is -7dB FS and your music rms is at -20 db that would leave you 13 dB of crest factor (the difference between the rms and the loudest peak in your music) before you trigger feedback. In my experience and most other Audio Engineers  a crest factor  of up to 12dB would be typical. There are exceptions mostly in bad music.  So with this setup  you have 1 dB headroom before triggering feedback. Now lets enable compression and use Harpman's settings of -20 dB threshold and 1:4 ratio. That would enable 13 x 4 = 52 dB more input before you reach feedback threshold. Other things would fall apart long before this would happen. Yes I know that it was only one channel that it would be diluted by the mix but for simplicities sake I'll consider it the main source. What Harpman forgot to mention in this post but did in another is that he had a gain setting of 7  dB in the compressor settings !!! Well an additional 7 dB would send this system screaming without the compressor, 6 dB is a 100% increase in loudness above the feedback threshold. The lesson here is that observation without understanding the basics or checking your settings can lead to the wrong conclusions. RR there is an old proverb "that if you want to be rich you don't hang out with the poor" so let me introduce you to Bob Katz. There is an excellent text by Bob Katz on this and many other aspects of audio in his book "Mastering Audio...". Heavy reading for most but if you pick up 0.001% you'll be light years ahead. Harpman your initial gut feeling was correct and yes the "Laws of Sound" are intact. Your compressor settings are another matter.

[Fluddman]

WK154, thanks for your great explanation and of course you are correct.

Given that this is a forum for the DL1608, not the mixer of choice for most audio professionals, I think RR's statement and my own are not all that dangerous.

Admittedly we should have said 'compressor' instead of 'compression'. 

After all it is very common advice for users with feedback problems to suggest they try turning off any compressors.

Cheers

[WK154]

You hit the nail on the head with "After all it is very common advice for users with feedback problems to suggest they try turning off any compressors."
It's the common bad  advise (other than a quick check for possible bad settings) that's the problem.  The word usage "compression, compressor " is fine and "dangerous" I would normally say that came out of left field but in this case it's from down under (pun intended). You'd also be surprised what audio pro's work with.
G'day mate

[JMc]

I was with Harpman when he was experiencing his feedback problem recently and thank him for posting the question here.  I had discovered that problem for myself with my own DL 1608 and band and suspected his problem was compression on the vocal channel.  As a general rule of thumb then, is it advised that compression only be used very sparingly, if at all, on vocal channels?  I've got ours completely disabled for the time being and we seem to be doing just fine without it. 

[RoadRanger]

As a general rule of thumb then, is it advised that compression only be used very sparingly, if at all, on vocal channels?

Yes.

[Greg C.]

The simplified way I've told people that using a compressor can increase the chance for feedback is only when applying makeup gain. Otherwise there is no change in feedback potential using a compressor since there is no change in channel or bus gain. It's pretty simple really. Gain is gain regardless of where it's applied in relation to mic level or speaker level changes. If you have a stable system and decide to turn on a vocal channel or bus compressor but don't add makeup gain, the stability will not change. Add some makeup gain, and things will change.

[WK154]

+1 ... and the same holds true for EQ's if you decide to boost a range of frequencies all hell can break loose if you don't compensate for the overall level increase, especially if you're close to your feedback limit.