Inside Automatic Microphone Mixer Systems

An automatic microphone system is a good option whenever multiple microphones (four or more) are being used, particularly if the sound system is intended to run without a live operator
shure mixer

Every time the number of open or active microphones in your church system increases, the system gain (or volume) also increases.

The effect of this is greater potential for feedback as more microphones are added, just as if the master volume control were being turned up.

In addition, unwanted background noise increases with the number of open microphones. Here, the effect is a loss of intelligibility as the background noise level rises closer to the level of the desired sound.

A good solution is to activate microphones only when they are addressed and to keep them attenuated (turned down) when not being addressed.

In addition, when more than one microphone is addressed at a time, the system volume must be reduced appropriately to prevent feedback and insure minimum noise pickup.

An automatic microphone mixing system can be a lot of help in this situation. Essentially, it’s comprised of a special mixer and an associated group of microphones, and it’s function is twofold: to automatically activate microphones as needed and to automatically adjust the system volume in a corresponding manner.

In some automatic microphone systems, ordinary microphones are used and all of the control is provided by the mixer. In others, special microphones are integrated with the mixer to provide enhanced control.

There are several techniques used to accomplish channel activation or (gating) in an automatic microphone system.

A look at a basic automatic microphone mixer setup. (click to enlarge)
In most systems, a microphone is gated on when the sound that it picks up is louder than some “threshold” or reference level.


When the sound level falls below the threshold, the microphone is gated off. This threshold may be fixed, adjustable, or even automatically adjustable.

In any case, the threshold should be set so that the microphone is not activated by background noise but will be activated by normal sound levels.

Traditional threshold systems distinguish between background noise and the desired sound only by level.

However, if background noise becomes sufficiently loud, it may activate microphones unless the threshold is adjusted to a higher level.

Subsequently, if the background noise decreases, normal sounds may fail to gate the microphones on unless the threshold is lowered as well. Threshold adjustment is critical to automatic mic systems of this type.

Some recent automatic mixers incorporate noise adaptive threshold circuitry. These have the ability to distinguish steady signals such as background noise from rapidly changing signals like speech.

They can automatically and continuously adjust individual channel thresholds as ambient noise conditions change.

In addition, some designs can recognize that the same signal is being picked up by more than one microphone.

In that case, only the channel with the strongest signal is activated. This prevents both microphones from being activated when a talker is in between two microphones for example.

Certain other automatic systems, with integrated microphones, can actually sense the location of the sound source relative to the ambient noise and activate microphones only when the sound comes from the desired direction. These “directional gating” systems do not require any threshold adjustments.

There is another circuit within every automatic mixer that continuously senses the number of open microphones (NOM) and adjusts the gain of the mixer accordingly.

With a properly functioning automatic system, if each individual microphone is adjusted to a level below the feedback point, then any combination of microphones will also be below the feedback point.

Many automatic microphone mixers have additional control circuitry, often in the form of logic connections.

These are electrical terminals that can be used for a variety of functions, including: microphone status indicators, mute switches, loudspeaker attenuation, and the selection of “priority” channels.

Some automatic mixers have an adjustable “off attenuation” control: instead of gating the microphone completely off, it can be “attenuated” or turned down by some finite amount, to make the gating effect less noticeable in certain applications.

Another control included on some units is an adjustable “hold time”: when the desired sound stops, the channel is held on for a short time to avoid gating the microphone off between words or short pauses.

In addition, a function which locks on the last microphone activated insures that at least one microphone is on, even if no one is speaking.

Finally, most automatic mixing systems are able to be expanded by adding individual channels and/or by linking multiple mixers together to control large numbers of microphones simultaneously.

An automatic microphone system should be considered whenever multiple microphones (four or more) are being used, particularly if the sound system is intended to run hands-free, that is, without a live operator.

This is often the case not only in the worship facility itself but in fellowship halls, conference rooms, and auditorium systems.

Microphones should be selected and placed according to the normal guidelines (integrated systems require a microphone choice from the selection available for those systems).

It is recommended that the manufacturer or a qualified installed sound professional be consulted on the details of a particular automatic microphone system.

(Copyright Shure Incorporated, used by permission.)

David McLain | The Mixer Guy | CCI SOLUTIONS
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PO Box 481 / 1247 85th Ave SE
Olympia, WA 98507-0481
Voice: 800/426-8664 x255 / Fax: 800/399-8273
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How to Ring Out a PA System

This video demonstrates how to ring out a PA system to eliminate feedback. Releasing May '06, The Ultimate Church Sound Operator's Handbook by Bill Gibson, contains almost 500 pages of full color text, photos, illustrations, and a DVD with audio and video examples teaching everything you'll need to know to be a live sound engineer.

If you have difficulty seeing this training video, click on the title at the top of the article ("How to Ring Out a PA System").

David McLain | The Church Sound Guy | CCI SOLUTIONS
Be seen. Be heard.
PO Box 481 / 1247 85th Ave SE
Olympia, WA 98507-0481
Voice: 800/426-8664 x255 / Fax: 800/399-8273
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Processing on-the-fly!

[Editor's note: Today's article is from a guest author, Jason Mageehan, from England. He's writing about a specific piece of equipment, the dbx DriveRackPA that, as he said in his cover note to me, "quite literally changed my life!" I enjoy the detail of how he uses it; even more, I love the passion with which he writes! Enjoy!]

My name is Jason, based just outside London in the UK. I discovered this website and found some of the articles really interesting and well written, so thought I could contribute something that I have found helpful in my life doing sound in church. Firstly a bit of background to me, I am a semi professional musician, a semi professional sound / recording engineer and a semi professional graphic designer. When I say semi-professional, that means that I get paid, but not often enough to make it a full time career!

So this piece of gear, the DBXDriveRack PA, as a sound guy who has worked in some interesting venues with some even more interesting PA systems, this little beauty has changed my life....oh yes, and when I say 'interesting,' that doesn't always mean 'interesting-good'; often it's 'interesting-bad'!

Let me tell you the story of how I first got hold of the device. Our church quite often will hold a large evangelistic meeting, about a year ago now, we rented the local municipal hall which seats about 1500 people to have Arthur Blessit, a guest speaker preach the word! We had a 2 hour window to set up the sound system, the band, set the stage, basically turn a dry venue into church for the morning. This was the PA system I was given to work with:

  • Meyer Active Tops
  • Logic Systems Twin 15" Passive Subs
  • QSC Amps
  • Allen and Heath GL2400 Mixer
  • Samson Compressors / Gates
  • Selection of In-Ear monitors and passive Foldback Wedges.

Stage monitoring kind of takes second place when you're on a tight limit with time and resources! As a musician myself, I know that if I can hear a basic mix with enough clarity that I can make everything out, I'm happy, and that's easy to do with the right ear, so let's forget about that and focus on the FOH. So, we have a mish-mash system, in a municipal hall with terrible acoustics. Having done sound there before, I knew getting the overall EQ Curve and crossover point would be essential, and to get that done really quick because I had to work with other peoples time constraints as well....I knew I needed some more time.

I rented the DBXDriveRack PA and a measurement mic (RTA Mic) as I had read good things about it, and thought at worst it would be a comprehensive crossover and look nice, and I could go back to the Behringer (yuk!) manual EQ unit the church owns, and at best it would do everything I wanted it to do (crossover and EQ) quickly and effectively.

I have to say, it did do everything I wanted it to do, this room, this PA system, the DBX literally brought it all together and it sounded like a fantastic install, not something threw together quickly for a service. It took about 90 seconds of white noise to be fed out of the speakers at performance levels for it to monitor the sound and make adjustments accordingly that raise and lower peaks in the sound spectrum to give you what is essentially a completely flat speaker response. Having a flat response is clever, but not particularly exciting to the ear, but obviously you can then overlay an EQ curve over the top of your corrected sound, which will then give you exactly the sound you are after.

Put simply, this box does exactly what it says it will do. I have since had my church purchase one. Normally it sits in the install rack running our FOH sound, but when we go out, (and when I have a gig!) I can take it out, use it and re-fit it, set it back to the church preset....easy!

One more story in my life with this unit, a recent fund raiser where I used my own Yamaha 03R digital desk and this unit, with a PA system that is truly a mish-mash of brands....

  • 18" Sound Lab Subs
  • 15" Ross Full Range Speakers
  • LD Systems Tops
  • Lyon Forge Amp
  • Behringer Amp

Now, the system is set up so the Sound Lab Subs, and the Ross Full Range speakers are actually both used as Subs, and the LD systems were the tops. The event was in an Old English church which have lovely acoustics for a choir, but not for a more contemporary set up. Again, I used this unit, and I honestly could not believe it. If I was blindfolded, I honestly would have believed that there was a D&B rig in there, or an EAW, basically I'd put it up against any big brand speaker company even with those speakers.

That may sound flippant, but those of us who've been around long enough, know that however good your system is, if you don't have the ears for it, it will sound bad. What this unit does is take a lot of the hard work out of the equation. I don't use any of the other features on the unit other than the crossover and the EQ facility, but here's a full run-down of the product: [Editor’s note: the DriveRackPA has since been replaced by the DriveRack PA+:].

Some people say it sounds “too digital.” In response to that, I can honestly say, when I use my digital desk and this together, yes, it sounds incredibly pure, and I like that. But what I tell people is this: this unit will very quickly and easily give you an solid foundation to build your soundscape. It has never failed me, and it has saved me on a number of occasions where the situations have been such that I've needed some help from a machine that has an ear that is better than mine.

Would I recommend it? Yes, go and rent one first to see if you like it though [Editor's note: or call CCI Solutions who has one in their "Try It Free" program in the US]. Also, before you make your next speaker purchase, think about this: if this unit costs you £400 [$499 in the US], and one speaker system costs £2000 and another costs £4000, have a serious play around here, because you might find that, like me, you can get a less able system to sound just as good as a top of the range system, and you'll have saved loads of money!

The unit has a tag line; "The cure for the common PA". In my opinion, it is.

Jason Mageehan

How to Find Ground Loops and Prevent AC Hum

Note: This article is from Ebtech. It includes some advertising copy for the Ebtech Hum Eliminator along with the information about ground loops. I usually avoid articles with blatant advertising, but this is an exception because a) the information on ground loops is so good, and b) their product is really quite useful in the right application.

What is a Ground Loop?

When you hear hum in an audio system, it's almost always caused by a loop antenna effect between two or more pieces of gear, across signal lines. A loop antenna is formed by having a loop of wire where the beginning and end of the loop are connected - the loop can be any shape. The loop antenna(e) is basically a form of radio antenna and they tend to pick up the 60Hz AC signal being broadcast by a building's electrical wiring. They also pick up 120Hz, 180Hz, and all the other harmonics of 60Hz and, usually to a lesser degree, electrical noise being broadcast from all over such as radio/TV, hair dryers, etc. These loop antennae are closed circuits usually through the ground wires and hence are commonly called ground loops.

Examples of Ground Loops:

1. Going up the AC power cord ground from the electrical system wiring to a keyboard, going across a signal line ground from the keyboard to a mixer across the signal ground, down the mixer's power cord ground reconnecting to the electrical system wiring.

2. Going across the signal ground from a mixer to a reverb unit, going from the reverb unit across the signal ground back to the mixer and reconnecting inside the mixer.

3. Going up the AC power cord to the mixer, across the signal ground to the amplifier, down the amplifier's power cord ground and reconnecting to the electrical system wiring

4. Going up the AC power cord to a guitar amplifier, going across the input signal ground to an effects device left channel output, from the effects device right channel output to another guitar amplifier, down the second guitar amplifier's power cord ground and reconnecting to the electrical system wiring.

Which connection has the Ground Loop? (AKA Playing Audio Detective)

Identify the ground loop causing the trouble; not all ground loops cause noise or hum. For complex systems you may need to repeat these steps starting with a different piece of equipment in various combinations to locate the problem:

1. Strip the system down to one piece, such as the mixer, by disconnecting all interconnects and AC cords except for the mixer.

2. Add one piece of equipment at a time; hook up AC and interconnects (making sure all grounds are connected and in good condition) then listen for hum or noise.

3. Turn on and off the power each time you switch equipment to avoid pops and shorted outputs.

4. Proceed until you find the offending piece(s) causing the problem.

5. Plug the Hum Eliminator in all lines between the offending equipment and the rest of the system. For example ... insert the line outs of the keyboard into the inputs of the Hum Eliminator, then insert the line outs of the Hum Eliminator into the inputs of the mixer.

It is often helpful to listen through a pair of headphones. Quite often you will only hear hum coming from a particular input channel on a mixer and that is where the ground loop will be. Alternatively, if you hear hum coming out the speakers with all the mixer's channels turned down, it's likely that the problem is between the mixer and amplifier or other equipment that comes after the mixer.

Another common path for ground loops is through a chassis into the rack and then into another chassis. Test this by removing the chassis from the rack. The Hum Eliminator will help but you should also try isolating the chassis from the rack with electrical tape and insulating the rack screws with nylon washers.

Note: Never use the Hum Eliminator between an amplifier and speaker or the equipment may become damaged. Only use the Hum Eliminator on line level signals.

What about clipping or lifting the AC ground or signal ground?

While these methods may or may not remove your hum, they have some real drawbacks!

Removing or disabling the AC ground:

· Can cause electrocution

· Can cause distortion due to floating signal references

· Can cause some pieces of equipment to oscillate or become damaged

· Can cause current and noise meant for the AC ground to be dumped down the interconnect (line level) to another piece of equipment instead

Cutting the shield at one end of the interconnect cable:

· Can hinder the ability of the cable to serve as a signal return

· Can cause distortion and/or clipping of the signal since there is no voltage translation matching (shifting a signal to match ground and power supply).

· Can alter the cable's frequency response.

· Can defeat the shielding effect.

Why using the Hum Eliminator is the safer and better solution:

The Hum Eliminator is completely transparent; its audiophile quality components don't change your sound. With a flat frequency response from 20Hz to 70kHz (way beyond the range of human hearing) the Hum Eliminator is the answer.

The Hum Eliminator breaks the ground loop, keeping all AC grounds intact. It provides isolated signal returns and performs automatic voltage translation matching.

The Hum Eliminator automatically converts from unbalanced to balanced without signal loss. With the Hum Eliminator you can run a signal across a room from a pre-amp, effects unit or keyboard without picking up AC hum from power cords and without the signal loss you get from a DI box. Balanced outputs from the Hum Eliminator benefit from true common mode rejection (CMR), canceling out noise from AC power cords and other sources.

The Hum Eliminator will match any ground potential difference between two pieces of equipment. If the ground of your keyboard is 6 volts higher than the ground of your mixer, the Hum Eliminator will shift the entire signal of the keyboard down by 6 volts to compensate without affecting the keyboard at all

Courtesy Ebtech Audio. Used by permission.

See also this article on ground loops.

David McLain | The Humming Guy | CCI SOLUTIONS
Be seen. Be heard.
PO Box 481 / 1247 85th Ave SE
Olympia, WA 98507-0481
Voice: 800/426-8664 x255 / Fax: 800/399-8273
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An Introduction to Lapel Mics

By Marty McCann

Question from: Ian Stott of Architectural Audio Singapore

"What EQ Settings/curve would you recommend for a lavaliere mic in a difficult environment such as a reverberant church or hall!"

I would recommend a Cardioid pattern lapel mic, even though the pattern is altered when placed on the chest. Some put the mic too far down on their chest. The user should drop their chin to their chest and put the mic directly below that point. The farther down from this point results in even more mid-range chest cavity resonance that colors the sound. This chest cavity resonance and the gain before feedback is a constant battle with lapels (Wireless or hard-wired).

Too often the church customer (who is often rather technically challenged in the first place), tries to resolve the EQ'ing of the lapel mic with the overall main system EQ. This of course is a mistake, because of how it will adversely affect the overall systems performance, i.e., normal vocal microphones and instruments taken direct are negatively affected by the hacking away of the main equalizer in order to chase away frequencies that are problems for the wireless Lav mic only. A dedicated EQ (inserted into the wireless Mic's channel) is the only way to begin to even get any kind of a handle on this application. Even then due to the drastic amount of Mid-range cut necessary to get intelligibility out of the lapel Mic system (this is before feedback suppression), there is often not enough cut remaining in this region for further control of feedback.

Over the years, I have addressed this problem in high visibility, high $'s installations by either using a parametric along with a 1/3 octave to tweak the system, or more recently (since we no longer manufacturer a parametric), I specify ½ of a 2/3 Octave EQ and a 1/3 Octave EQ to process the lapel Mic's channel.

Now here is where the problem is further complicated. In many installations, the lapel Mic is used by more than one individual (sometimes several). Due to the individual nature or timbre of peoples speaking voices, along with the fundamental resonance's of each voice (that is determined both by the vocal chords and the size of the chest cavity), one size does NOT, fit all. The pastor or CEO doesn't understand this at all. At times when it can be determined that certain designated people will be using the lapel system, I have specified 2 (yes 2) CEQ-280a programmable Equalizers, with stored setting for various presenters or speakers.

Now down to the EQ process. Too often the less experienced system integrator or operator will just ring out the mic for feed back. This results in less than desirable tone and intelligibility. EQ for intelligibility first then go for feedback suppression (once again you probably need more than 1 EQ to accomplish both effectively). On the average the required mid-range notch is centered somewhere from 315 to 630 Hz (this is the individual variable) depending on the person speaking. This notch can be two to three octaves wide at the -3dB down points (depending on the lapel mic and user). Because of the small Electret diaphragm and its proximity to the users mouth, there is often more energy above say 8 kHz than is necessary. A variable high cut is a good tool here (that's why I prefer the EQ-31FX over the Q-31FX, the variable low cut is also handy here). Some people's voices exhibit a strong sibilance in the annunciation of Ssss sounds. This of course is mainly at 6.3 kHz on a 1/3 Octave EQ.

When the budget won't allow for two equalizers, one technique is to first start with all of the EQ sliders at the top (this is not a good idea with some cheaper filter designs due to the ripple or poor summing of the filters), then to EQ for tone, followed by appropriate cuts for feedback suppression. With some cheap EQ's this technique would also result in a poor S/N ratio.

While on this subject, we have had tremendous results with the performance of our new PVM-2 wireless headset mic. Because of it's positioning away from the chest and close to the mouth, it needs VERY LITTLE EQ, and can often suffice on the channel strip EQ on a decent mixer. The problem is a lot of people think they look like a Dork with the headset on. In my case, I have overcome the dorky feeling because the end result is soooo much better performance.

Also, some theater productions tape the lapel Mic over the actor's ear or into their hairline (using flesh colored surgical tape). This works well with some of today's smaller Mic elements, such as the PVM-1 Lavaliere microphone.

©2009 Peavey Electronics. Used by permission All rights reserved. Terms/Privacy

David McLain | The Wireless Guy! | CCI SOLUTIONS
Be seen. Be heard.
PO Box 481 / 1247 85th Ave SE
Olympia, WA 98507-0481
Voice: 800/426-8664 x255 / Fax: 800/399-8273

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