Sound System Tweaking Suggestions?

Colm Chomicky · From: Kansas, (Prairie Village)

I occasionally sit in with a bluegrass band and one of the guys asked me a technical question.

The bluegrass band is experimenting with different sound setups and is trying to use a single condenser (cardioid) mic so that we can congregate around it rather than having each instrument and each vocal mic'd. We've tried this w/ conventional PA systems and recently w/ the Bose PAS system. We like the clean, true sound of the Bose, but still had some trouble with feedback (which the Bose system is supposed to eliminate). We also liked not having to deal w/ monitors w/ the Bose system, but you have to play w/ setup, gain, volume to try to avoid feedback. We also wrestle with feedback on the conventional PA systems. Somebody suggested using a figure 8 mic w/ the Bose. Somebody else said we're trying to defy physics. We're open to other suggestions or the benefit of other's experience.

Mike Wheeler · From: Delaware, Ohio, USA

I agree the Bose system is very natural and has much to offer a group such as yours. The feedback you are experiencing really IS a matter of physics and will, potentially, cause trouble with whichever type of "acoustic" or "single" micing technique you use. Whenever the "house" volume is loud enough to "feed" into the mic, it can be re-amplified "back" into the house...and this creates a cycle which continues (usually as a squeal) until something breaks or someone stops it.

The single mic setup IS very desireable for your situation, but you may need to compromise. Maybe using good pickup systems for the instruments and using the single mic for vocals only. (singers can crowd closer to the mic so it doesn't have to be turned up as much). I think this would work very well with the Bose system resulting in a pretty natural acoustic sound. (assuming everything goes into the Bose...no separate amps)

Good luck.

T. C. Furlong · From: Lake County, Illinois, USA

You are defying the laws of physics if you expect the Bose system to eliminate feedback in the situation you describe. To make any mic work with any loudspeaker without feeding back, something called critical distance must be respected. That is basically the idea that the loudspeaker must be far enough away from the mic and out of the mic's pickup pattern and the source must be close enough to the mic and in its pickup pattern relative to the position of the mic to the loudspeaker. The loudspeakers dispersion must also be taken into account. Put more simply, determine the patterns of the mic and loudspeaker and place them so that there is as litle interaction as possible. If you were planning on using the Bose cylindrical radiator as Bose intends, behind the band and as a monitor, it won't work with the single mic set up because of critical distance. And more of the sound of the loudspeaker will be picked up by the microphone causing something called comb filtering (nasty hollow sound, think talking through a cardboard tube). It's physics and not even the almighty Bose Corporation can change those laws.
I have not looked in the user's guide for that product. Do they address critical distance?
TC

Francis Chamberlain · From: Franklin, KY, USA (deceased)

I have a question on the subject of feedback.
Am I right when I say that a feedback can only occur by the sound being reproduced through an active microphone or an instrument being to close to an amplifier? In other words there must be an active microphone or some type of pickup for the sound to be reproduced to cause feedback.

Bryan Bradfield · From: Lethbridge, Alberta, Canada.

Francis - You are correct when you say that the feedback occurs through an "active" microphone, if by that you mean a microphone that is "on". Let's face it, the original sound came through the microphone, so if that microphone is in a position to pick up the broadcast sound, it will feed back. In addition, you suggest that an instrument being too close to an amplifier will also cause feedback. The instrument does not necessarily have to be too close. Some pickups will feed back at a larger distance if the instrument is directly in the line of fire of the amplifier.

Francis Chamberlain · From: Franklin, KY, USA (deceased)

Thanks Bryan for your reply. When I said
"Active" Microphone, yes I did mean a mic
that was "ON", since some microphones have
a "on" and "off" switch. I think I'm right
about an instrument pickup. When we think
of a mic or a pickup they serve the same
purpose and that is to pick up a signal,
right? When this signal is reproduced then
you have a feedback. Without a microphone
or a pickup, it is impossible to have
feedback, right? Thanks again Bryan.

Dave Grafe · From: Hudson River Valley NY

I applaude your friends for committing to the single mic technique, it's the best and most natural sound that you can get provided you learn to choreograph your moves to balance the sound of lead and backing voices, instruments, solos, etc.

As you are discovering, this method makes it extremely easy to induce feedback in a sound system because the increased distance between the desired signal (your band) and the microphone requires greatly increased sensitivity, also amplifying the ambient sound in the process. With proper equipment and technique, however, this method can be used to deliver great sound at pretty high levels. Here's how professionals do it:

(1) Forget about using the Bose stuff for this application - I know you like what it sounds like and it has its place in the world but it also has several inherent design flaws that make it completely unsuitable for this task.
(a) The dispersion pattern of the BOSE gear is far too wide, with virtually no directional control at all, thus sending much of the amplified sound bouncing off the walls of the room and joining the direct sound of the speakers, which arrive at the microphone with little or no attenuation at all. This generates a vast set of resonant nodes causing feedback that simply cannot be managed.
(b) the processor that the BOSE rig contains is clueless when it comes to this sort of feedback phenomena and thus makes all the wrong choices. Bottom line: You can not and will not win.

(2) Use a high-quality cardioid large-diaphragm condenser microphone - you need the cardioid pattern to reject sound from off stage and to support the proper blending effect as you gather around it, the lead vocal or solo instrumnent always directly in front of and on axis, the lesser voices arrayed to the sides and nothing ever more than 90 degrees off-axis (directly beside the mic).

(3) Most importantly --ACHTUNG BUCKO, THIS IS MANDATORY-- you must have a top-flight 1/3 octave equalizer in the circuit - I'm not talking about the $150 special that you can buy down at your local guitar mega-marketer, either, but a truly professional-grade filter set. Klark-Teknik and XTA make the high-priced spread, but for somewhat less money one can get excellent results from Ashly's GQ or GQX series and the better units from UREI and dbx are also quite usable. IMHO the very best bang for the buck is the old White Instruments 4400, with rotary knobs instead of linear faders, each filter having resolution accurate to 1/2 of a db and the nicest sounding and easiest working unit anywhere. Originally retailing for over $800 you can get a good used one on ebay for $100 or so these days if you're careful. If you get a dog unit, just find another one, at that price you can afford to sort through a few units for one that's really cherry.

(4) Now that you have the right tools for the job it's time to "ring out" the system. Set up the mic in the center of the stage, exactly where you will be using it, being sure to keep it BEHIND the (now properly directional) speaker arrays.

Set all the EQ filters on the console and the 1/3 octave EQ completely flat - this is usually the center of the range of each knob (unless you have a passive all-cut UREI EQ, in which case the faders should all be at the top of their travel).

Turn the mic up in the mains until you just begin to hear approaching feedback. Without touching the EQ on the console, find the filter on the 1/3 octave that MOST affects the ringing frequency (filters just above or below the one closest to the ringing frequency may give a false reading so check them all) and reduce THAT ONE FILTER CONTROL until the feedback stops. Turn the mic channel up some more until you begin to hear another (or perhaps more of the same) frequency ringing, find the filter that is most effective at this ringing frequency and turn it down just enough to eliminate that "hot spot." Keep turning the mic up and notching out the various problem areas until you have a multitude of frequencies all on the verge of feedback at the same time and the mic is at least 5b to 15db hotter than you expect to need it for the show. If you end up with most of the faders turned all the way down and still no joy then you need to flatten everything out and start from the beginning again - don't feel bad if it takes a while, this is not easy stuff for pros, never mind the unpracticed.

You will eventually reach a point of diminishing returns, remember that generally when the room fills with people it will behave much better than when you ring it out empty. It's OK if the room is noisy when you ring it out, this ambience actually helps to generate the undesirable resonances that you want to identify and control.

If you are also using a monitor system it is best to place a single wedge directly off-axis, that is centered and directly opposite the front of the microphone. You will need another 1/3 octave to ring out this monitor mix, using the same method described for ringing out the mains. If the mains are in stereo you will need an EQ for each side, ring them out separately first and then combine them for final adjustments.

That's it, once you get it working really well, write down the setting of every single knob, if you use the same sound system regularly you will find the you can dial in your documented settings (especially true with the White EQ) first thing and have very little tweaking to do after that. During the show you can use the console channel EQ to perform minor tweaks as desired. If another bad feedback node rears its ugly head kill it at the 1/3 octave.

NEVER apply boost to any filter when using this mic technique, no good can possibly come of it - besides, boosting filters by definition increases phase distortion, the primary thing you are trying to eliminate by using only one microphone in the first place.

One more note, besides being the way to make monitors sound their best in just about any situation, it's just a really great idea to ring out any sound sytem before letting the audience into the room. Besides controlling feedback, this practice ensures that the system itself is reproducing the desired program with a minimum of coloration.

Hope that helps some,

dg

[This message was edited by Dave Grafe on 14 January 2006 at 11:14 PM.]

T. C. Furlong · From: Lake County, Illinois, USA

Right on the money Dave. I wish more of the sound reinforcement "pros" that work at my company were as insightful and generous with their knowledge. We do have a brilliant guy on our team who has done extensive field work with the single mic technique for Bluegrass bands. He believes that a mid-size or small diaphram condensor mic actually works better for this application. I am starting to agree with him. He hand-builds some wonderful sounding and gorgeous looking microphones. He has even named one model "The Bluegrass Mic".
TC

Earnest Bovine · From: Los Angeles CA USA

By far the best sounding group at the Julian Bluegrass festival 2004 was the Reel Time Travelers, and they were the only group that used just one mike.

We had a blugrass rehearsal yesterday, just standing in a circle in the bass player's living room. It sounded better than it ever sounds on stage with all those mics.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Get that "cylindrical radiator" thing off the stage and away from the group. Put it off the stage to the side, or in a corner somewhere. In no case should you be using it as a monitor onstage near the band! They're fine for a solo performer playing "supper club" music, but they don't work well when a whole band is making noise nearby.

And yes, the best bluegrass sounds are done with one mike (and no monitors).

Colm Chomicky · From: Kansas, (Prairie Village)

Thanks, everyone for your great suggestions! I'll share this with the guys.

Chuck McGill · From: An hour from Memphis and 2 from Nashville, R.I.P.

Very well put Dave. I have used the Peavey
EQ with the Feedback locate for this procedure with good results many times. Very
important to start with the right PA box and
a good cardioid mic.

[This message was edited by Chuck McGill on 17 January 2006 at 04:23 AM.]

Ray Minich · From: Bradford, Pa. Frozen Tundra

Looks like Dave G. has been there and done that. Very nicely put Dave.

[This message was edited by Ray Minich on 17 January 2006 at 07:38 AM.]

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