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Feedback quandary

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Harpman:

--- Quote from: Greg C. on July 19, 2013, 07:29:48 PM ---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.

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Seems like that's what happened in my case Greg. Too much make-up gain and your right, gain is gain no matter how you look at it  :)

Harpman:

--- Quote from: JMc on July 19, 2013, 05:54:56 PM ---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.

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Jeff, thanks for chiming in here. When your at a live, your first goal is to isolate where the feedback is occurring (I start muting channels). My next step is cutting frequencies (typically mids around the 5K range), but in my case it wasn't doing anything. This is when Jeff suggested killing the compression on my GF's vocal channel based on what he had experienced. When I went back through my saved shows the next day, that's when I realized I had the wrong show loaded.  I name my shows based on band and venue. For example "BE@AR" is Barefoot Express @ Anthony's Ristorante. The issue with using two iPads at gigs it that you still end up with duplicate shows and I wasn't paying attention to the one I loaded.  I had 2 BE@AR and the older one had compression on on the vocals and the other didn't. I must have had this issue before and disabled compression on the vocal channels, but who remembers over 6 months ago  :).

Thanks all for you feedback here (no pun intended LOL). I tend to gravitate to EQ as being the feedback issue because 90% of the time, it is.  When faced with feedback issues, my goal is to isolate and remediate as quick as possible and analyze later. 

Harpman:

--- Quote from: WK154 on July 18, 2013, 06:38:13 PM ---"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.

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WK154, thanks for the support here.  What your saying and Greg is spot on with MG.  I can totally see that.  I'm pretty sure I have that book by Bob Katz. Very good reading!

Greg C.:

--- Quote from: WK154 on July 20, 2013, 01:59:53 AM ---+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.

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A lot of EQs have a makeup gain control, but I can't recall any situation where I've used it. And I almost never use an EQ boost on house or monitor graphics, cuts only. The one exception is a local historic theater I do sound design in for plays occasionally. The installed speaker system is a early 70's era Altec home brew box with HF horns that don't have anything above 4k. I've got a linear boost on the house graph starting at 3k gradually increasing on each band ending at +10dB at 20kHz in an attempt to get some sibilance and "air" out of the headset mics. Even that wasn't enough. I had to boost the highs on all the individual mic too. If I did that with any decent speaker system, the feedback with omni headset mics would take off in a jiffy. But not with this system.

WK154:
My own approach even on the old graphic EQ's is to set them in cut only selection if they had it. Never used boost myself other than to experiment. I find that fixing (or trying) mic position or technique is time better spend. The right mic for the job does wonders and changing acoustics in a venue you have no control over is of course impossible. Yes Divas are a PIA at times but some actually come around and learn something new. I use compressors for limiters only for such Murphy events as mics being knocked over (NT3 two month ago) or them being dropped.

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