You really aren't providing enough technical information to give you a proper answer. Is this for PA? Is this for recording? If it's for PA, I would have separate downward expander's on each of the four microphones. Gates sound completely awful & unnatural and you don't need that much gain reduction to make a substantial difference. Downward expansion of only 10 DB is all that's really necessary. Gating the microphones will sound obnoxious. Compression on those microphones individually is also impractical. That's because, whoever is doing the talking or singing will have plenty of active gain reduction. Meanwhile, the other microphones which are not being performed into will be then turned up " balls to the wall ". This will cause all sorts of awful acoustical aberrations & plenty of awful phasing issues. So each one of these compressors on each microphone, would need to be tied together so that whoever is causing gain reduction will also cause the other microphones not currently being utilized to enact the same amount of gain reduction on those.

The plugging in of above said items would normally be done via the inserts. But since most compressor/limiters while many are stereo, very few offer the opportunity to link their sensors with another stereo pair. Some consoles have this built in along with that ability. Conversely, you could take all vocals, assign them to a bus output or all to a single Auxiliary send. From the auxiliary send, he would go into a single or stereo compressor followed by a downward expander. The output of which would then be double backed into another single input on the console. The 4 vocal microphones, individually would also be eliminated from the 2 track/stereo/mono master output bus while also being combined with your other sound sources & instrumental microphones, playback devices, etc..

If you are multi-tracking this while developing a live PA mix, you wouldn't necessarily want those compressors of the vocal microphones to necessarily be printed to track/tracksBut you could still possibly get away with that also. Just not recommended for recording applications. This kind of processing however rarely works really well on PA systems in especially with folks that don't understand how much more feedback this can cause when not done appropriately and in small quantities. And so, as an audio guy, even with the above recommendations, you're still going to need to ride plenty of levels.

If you are trying to do PA & make simultaneous multitrack recordings, it will require a console with fairly comprehensive inserts/direct outputs/pre-& Post specialized EQ & fairly comprehensive routing. We're not talking of anything of an entry-level beginner attempting this complicated procedure. You really have to have full comprehension of what the console may be capable of. You might find that you are sending things out to output buses only to be sending them to processors and reentering into the console. Meanwhile, the microphones may be going directly to monitors & front of house with equalization & everything while the tracks are being recorded without any of that gobbledygook. So this gets a little dicey times and requires comprehensive consoles. I've actually done similar things on mixers as simple as Mackie 1604's but things can turn into a complicated kludge very quickly with bad level matching/gain staging. It can also mean that PA levels might be correct but recording levels might be low? And you'll just have to live with that and be thankful for what you got. Audio post & mixing can still work wonders especially when combined with computers and their processing capabilities.

Need more help? Let me know.
Mx. Remy Ann David

Wow, thank you!
Let me explain what this is all about. So I became a custodian of our church's PA. Probably because I had some amateur experience in the years past. PA looks like this: three microphones in the front and one in the back, where the organist is. All are plugged into this same mixer. And all are very quiet but working with gain makes them produce a feedback. All microphones are not new, and not the best - as is the rest of the gear... I reckoned that adding signal processor (like Behringer MULTICOM PRO-XL MDX4600) would help with the dynamics and reduce feadbacks. Thats my unprofessional thought...
Would you please tell me what do you think?

I think you'll find that the biggest bang-for-the-buck when it comes to getting enough gain while eliminating feedback, is to make sure you have proper mic/speaker placement and that the microphones are sufficiently directional. Can you describe the shape and size of the room, and tell us about the speaker and mic placement? Better yet, can you draw a diagram or take some photos?

There are two dynamic microphones on pulpits in the front, then one flat - I suspect its a condenser mic - and one dynamic (Shure 58 I believe) in the back. My guess is that the flat one is causing all troubles. I apologize for my lay language, hope you'd understand.

I suggest you take a look at the [[url=http://[/URL]="http://www.shure.co…"]Shure SCM410[/]="http://www.shure.co…"]Shure SCM410[/] automatic channel mixer. This is a device that will automatically pass the audio from only the microphone that is currently being spoken into and suppress the others. It allows higher levels of gain to be set on the PA system overall than would be possible having all mics permanently active.

I have the 8-channel version (SCM810) in my hire stock, and it gets used for conferencing and similar speech events. Although several of the conferences and meetings have been held in churches, I have never tried using it for an actual church service. However, the operating principle of these Shure boxes sounds a good fit for your application.

They come up regularly on Ebay, and I see there are several SCM410s on the US Ebay site at the moment, one with a buy-it-now amount of $299. There are also lots of SCM810s in auction with starting prices lower than the 4-channel versions.

The worst thing you can do in your situation is compress. That will basically have the end result of turning everything up and give you the same result you are getting now when you turn up. The best thing you can do is make sure everyone in front of a mic knows to stay VERY CLOSE to the mic and stay IN FRONT of it. Tell them they should imagine their voice has to go in a straight line all the way down through the handle of the microphone to the wire. It can be very frustrating to get people who have knowledge or interest in audio to use good microphone technique, so you'll have to stay on them. Also, if your mystery mic is an omnidirectional mic, do anything you can to replace it with a cardioid mic. Cardioid mics will only pick up sound from the front. Doesn't matter if it's a dynamic or a condenser, as long as it's cardioid.

Indicating one of the microphones appears to be " flat ", would most generally indicate to me that it is a " boundary " microphone. This type of microphone is generally 100% wrong for any kind of amplification purposes. This is a microphone to record a huge ambient field, such as the entire congregations singing and is certainly not for amplification. There was a similar design to microphone like that also referred to as a " Pressure Zone Microphone " a.k.a. PZM. While certain types of these can be used for amplification purposes they can be quite dicey to use.

Re: feedback problems, ABSOLUTELY NO COMPRESSION SHOULD BE USED. Some limiting may be doable & required. A limiter & a compressor are actually the same devices. The only thing that separates the two is how you adjust the knobs. So a limiter can be a compressor or a compressor can be a limiter. Limiting is the acceptable way to prevent loud blasts of sound into the PA system and keeping people from jumping out of their pews. Limiting basically means it is a very highly set threshold that only compresses the audio or limits the audio when loudness levels become excessive. Setting your threshold so that the limiting device " gain reduction " starts dancing around on the quietest sounds will most definitely ensure you get the best feedback squeal without any amplification other than feedback. And since God doesn't necessarily like anybody's screaming back at God, compression be damned.

There are a number of very effective ways to control feedback. A lot of audio PA guys utilize 1/3 octave spaced, 31 band, graphic equalizers. But you can't adjust those things just willy-nilly. They require an audio spectrum analyzer so you can see the precise frequency/frequencies that are causing feedback. You then reduce those frequencies individually on the 1/3, 31 band EQ. But that's quite a technical process. There have been newer more wonderful devices designed and made abundantly available. They have some internal computer processing that automatically does the above mentioned procedures without you having to do anything much more with them other than turn the system up until you get feedback, Lots of screaming feedback. Of course you do this before the sanctuary is filled with people. This will allow you greater amplification before feedback when the service begins. Of course, like anything else in life, everything has certain limitations. But the jobs these devices do, is quite remarkable.

When first introduced, some of these devices were extremely costly. Some still are. But then there are friends from Germany working with folks in China to bring these devices down in price that the local poor musical groups & places of worship can afford for a couple of hundred dollars and some even less than that. You must read the manual carefully to understand that one must create the feedback first so that the device can evaluate it and eliminate it. I've seen too many people turn their level controls up and win the feedback starts, they pull the level back down immediately. This never lets the device do its job. But when you let the device properly do its job it can provide for a threefold improvement in loudness levels without any feedback and do it better than you could manually. These devices have special built-in equalizers and their costs can frequently relate to how many built-in cutting equalizers at so many different frequencies that it can support. Obviously, the fewer the equalizers, the less expensive the devices. And strangely enough, you won't find any equalization controls on the front panel. Just a volume control at a couple of up down switches that allow for certain different types of environments. Beringer makes the least expensive while Sabine makes some of the more costly types. That is put on the output of the console/mixer before it gets to the loud speaker amplifier.

One should also avoid boosting frequencies on any equalizers, on any mixing devices. Instead, you cut frequencies that are opposite of the frequencies you want to enhance. By cutting frequencies on individual equalizers on a mixer, provides much better audio intelligibility & listenability for all in the congregation.

Dynamic microphones as you have indicated such as the SHURE SM57/58's have less chance of causing feedback than 90% of any condenser microphones with the exception of the ones we call shotgun microphones that are basically condenser rifles whose patterns are very tight. You don't usually find those very often in anything but film/video production work and generally never amplified in those situations. Sometimes, a short shotgun directional cardioid condenser microphone is frequently utilized at the pulpit podium and instead of being 3 feet long or generally under a foot long. Although many pastors prefer to have wireless belt pack lapel/tie tack microphones so they can more easily move about the entire pulpit. Because those microphones are generally omnidirectional, they have to be rather high up, as close as you can get under the pastors chin and not down around their belly as many people place them there on themselves.

Being the audio Guy also means that other microphones not in current use should have their volume levels lower until somebody " steps up to the microphone ". So being the Sunday morning/Friday or Saturday evening audio engineer means that you actually have to engineer audio during the service. This is not something where one generally leaves microphones preset and on all of the time even though many people attempt to do that. The best results will be had when you sit there and ride the levels manually in conjunction with your feedback controller on.

Santa is going to know whether your audio has been naughty or nice. So make sure you don't make an elf of a mess of it.
Mx. Remy Ann David