by Pat Brown
I received an excellent question from a SynAudCon grad who is in charge of sound at a large Korean church. This seems to be a common problem, so I though my reply would be of general interest. - pb
Dear Mr. Brown,
I have a question about PA system – again. I think I can’t find any fine answer without you.
It’s because of the subwoofer sound. One day, when I visited my sister’s family church that has EAW KF730 (4 each L/R side) and 4 EAW 18″ Subs, I heard pretty bad subwoofer sound. It didn’t have kick’s attack sound and bass guitar’s damping. It was not even tight. So, the Sub sound didn’t help the main speaker much. I couldn’t check the DSP set up at the moment. But, the church staff told me that a good installer set this PA system.
These are my questions.
1. Is the bad sub sound caused by the main speaker? Many installers say EAW KF730 is one of ‘Failed Model’. I know that it’s not a high-end product.
2. Is the bad sub sound from wrong crossover point setup?
3. Is the sub sound problem from wrong subwoofer choice?
Thank you very much.
And, I hope I can see you next year’s seminar.
That’s a tough question! We must first define “bad sound” and make a distinction between “I hear distortion” and “The subs lack clarity, punch or whatever.” It sounds like you are describing the latter, but part of the troubleshooting process is to rule out causes of distortion.
To determine the cause of the bad sound, we must “divide and conquer.” Here are some possible causes of disappointing subwoofer performance, in no particular order.
1. Poor subwoofer design/quality
2. Defective subwoofer
3. Poor room acoustics
4. Incorrect signal processing
5. Poor microphone technique or program sources
6. A bad mix
7. Poor subwoofer array configuration
8. Incorrect amplifier size
9. Poor system gain structure
10. Incorrect placement of sub(s) in the room
11. A mis-polarized subwoofer in an array
or some combination of the above!
Let’s look at each.
Item 1 – EAW makes good products, so I would rule out item 1. While it is true that one model or brand may be preferred over another, no properly designed sub should sound “bad.” Take one out in the parking lot and see how it sounds out there.
Item 2 – Each sub can be tested by a technician to assure that it is functioning properly. Since there are four subs, they can be compared. It is unlikely (but not impossible) that all have failed.
Item 3 – The room’s acoustics has a profound effect on subwoofer performance. This is best described as “coloration.” Again, what is meant by “bad sound.”
Item 4 – The signal processing can be considered as part of the subwoofer – garbage in, garbage out. The crossover and equalizer settings should be checked against what the factory recommends.
Item 5 – How does the sub sound for an electronic source, such as a keyboard, drum machine or iPod? Play some good quality drum tracks over the system. Does it still sound bad? If “no” then the microphone selection and placement should be re-evaluated. How does the kick drum sound acoustically? The sound system can’t take it from “suck” to “tight.”
Item 6 – The mixer operator can certainly ruin the sound of a perfectly good sub by how they mic the low frequency sources, and what they do with EQ and compressors. Unfortunately the solution here may result in someone being unemployed.
Item 7 – When you combine multiple subs, you get a completely different one than the individual units. How they are stacked matters a lot, and depends on the sub placement and the area that needs to be covered. Turn off all of the subs and play a good music track. Turn them on one-at-a-time and listen to how the sub sound changes. Be sure to walk around and listen over the whole audience area. Sometimes “less is better.”
Item 8 – If the system sounds good at low playback levels, but then bad at high levels, they may need larger amplifiers or more amplifiers (or, they may need to TURN IT DOWN). I like to see one amplifier channel for each sub. OverLOADing the amplifiers will result in distortion.
Item 9 – It’s also possible that the signal is clipping the amplifier input. OverDRIVING the amplifiers will also produce distortion.
Item 10 – See item 3. Placement matters, but poor placement choices result in uneven coverage, poor gain-before-feedback, and tonal coloration. Again, what is meant by “bad sound?”
Item 11 – I initially missed this one. Michael Moe pointed it out in his reply below. Subs must have the correct relative polarity to other subs in the system. If one is pushing while another is pulling, you can end up with lower SPL when both are turned on. Polarity testing of all loudspeakers is one of the ABC’s of sound system commissioning.
It is certainly possible to get to the cause of the problem, but it will take some effort. You must “divide and conquer.” I’d start with one of the subs in the parking lot (along with a full-range box). It has to sound good there, before it can sound good inside and in combination with other components. Now, put the same setup on the stage. Still sound good? Keep building it back, ruling out the stuff on the list. Hopefully the cause will be obvious and something simple. The worst case is that the bad sound is caused by a combination of subwoofer selection, placement, room acoustics, signal source, mixer operator, etc.
This is why SynAudCon coffee cups say “It Depends…” on the side. And, don’t forget my favorite Don Davis quote (and they title of one of his books):
“If bad sound were fatal, audio would be the leading cause of death…”
Best,
Pat
December 5, 2011 | Posted in 
Synergetic Audio Concepts, Inc.
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