Thread: Standing Waves or Feedback?

From: Larry Brannin (Crestwood, KY)

Subject: Re: First post, first question

Hi Gang,

After lurking a while, I felt the need to soak up some of this special list

knowledge. I just began my first Sunday running a small Yamaha audio board in

Sunday services. (Only my third Sunday after joining this church) I was told

that about a year ago they moved the board, plus deck etc. upstairs to the

balcony. (The balcony is rectangular in shape with audio board near the right

wall). It was originally placed in the back rear of the Sanctuary but space

dictated the move upstairs.

My problem was that I was told there was a high pitched feedback in the front

of the church for quite some time but I could not hear it way back in the back.

How in the world could I remedy not being able to hear these squeals. I'm not at

all deaf, or inexperienced at audio, but this may be a challenge. Any suggestions

appreciated. I did have a fellow sound guy up front in the second service as sort

of a "spotter", but we are a bit short handed to require two guys per service.

What sayest ye?

Give a man a fish and you have fed him for a day. Teach a man to fish and you

won't have to listen to him whining all day about how hungry he is!

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From: Ray A. Rayburn (Boulder, CO)

Subject: Re: First post, first question

Larry -

Welcome!

>>"My problem was that I was told there was a high pitched feedback in the

front of the church for quite some time but I could not hear it way back in

the back."

If true it sounds like the speaker(s) do not evenly cover the church, or at

least not the balcony. If there is feedback, it should be heard uniformly

throughout the space if the speakers have proper coverage. To have feedback

that is only heard in the front either indicates a drastic problem with the

speaker coverage or that the offending sound is not coming from the sound

system after all.

If the console you are using has a headphone output, try plugging a set of

headphones in and listening on and off through the service. If feedback exists

then you will hear it on the headphones. When forced to mix in a less than

ideal location it helps to have someone you trust to listen for you in a

better location, at least until you have learned to compensate for the

differences between the sound you hear and the congregation hears. Maybe you

could arrange to switch places and alternate Sundays mixing and listening.

This could be educational for both of you. If you must be located in the

balcony try to stay away from the rear wall, and not have any hard surfaces

close behind you.

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From: Blake A. Engel (Chicago, IL)

Subject: Re: First post, first question

Larry,

Isn't it interesting how accepting a church is when it comes to a new person

that knows something about sound?! Anyway, to add a thought to your question,

it sounds as though you're experiencing a standing wave at the front of the

church. If I were to assume the walls on either side (up front) of the sanctuary

are parallel, would I be right?

Here's my thought--if YOU can't hear the squeal, then either

1. it doesn't exist

2. it's actually somebodys hearing aid squealing (seriously!)

3. it's a standing wave

4. you can't hear! J/K

I'm sure you can hear, I'm sure it actually exists...if you eliminate the hearing

aid idea, you're left with a standing wave. That is, a sound wave that's bouncing

back and forth between two parallel surfaces-and as it does, it's being naturally

amplified. As long as you keep adding sound to the room, the squeal will remain.

What is going on during the service when this squeal starts? Is it a constant sound,

increasing in level, decreasing in level, dependent on the loudness of the sound

system/other sounds in the room?

Just a few thought to throw at you--and I'm glad you asked a question--I'm sure

you've seen that everyone here is glad to help and talk things through as much as

needed.

God bless you!

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From: Ray A. Rayburn (Boulder, CO)

Subject: Re: First post, first question

Blake -

<<"you're left with a standing wave. That is, a sound wave that's bouncing back

and forth between two parallel surfaces-and as it does, it's being naturally

amplified. As long as you keep adding sound to the room, the squeal will remain.">>

Standing waves are indeed a natural phenomenon which occur in all rooms (not just

between parallel surfaces - though this will make them more pronounced). They are

caused by the acoustic reflections which occur from all the surfaces in a room.

When a reflected sound meets the original sound or another reflection at some point

in the room it will either add some to the level of the other sound or cancel some

of it out. If the reflection is exactly the same level as the other sound and happens

due to the combination of distances and sound frequency to be exactly the same

polarity, it will result in a boost of 6 dB in level at that point in space for that

frequency. If on the other hand it should happen to be exactly opposite in polarity

it will totally cancel the sound at that point in space at that frequency. If the

levels are not exactly the same, the maximum boost will be less than 6 dB and the

cancellation less than total.

Now when you walk around a room you do not hear all the sound come and go with your

position in the room. This is because the pattern of boosts and cuts is different at

every frequency, and that the spacing is dependent on frequency. At low frequencies

there are large spaces between the peaks and dips in level, while at high frequencies

they are very close together. Add in the fact that we have two ears separated by some

distance, and that we naturally move around while listening, and we can understand

why this standing wave issue is only of any concern at low frequencies. It explains

why you may perceive large changes in bass from place to place in a room.

Some rooms have worse standing wave problems than others. Many years ago Sepmeyer

did a study of room dimension ratios. He showed that for rooms that had all their

room dimensions related by low whole numbers, the standing waves at many frequencies

tended to happen at the same points in the rooms. The worst case was a cube 1:1:1.

Other very bad dimension ratios were 1:1:2, 1:1:3, 1:2:3, 1:2:2, 1:3:3, etc. Rooms

which had dimension ratios that were not simply mathematically related had their

standing waves spread out much more evenly. He gave a list of the best ratios he

found which others later called the "Sepmeyer Ratios".

Some years later another researcher showed how even for room with poor ratios, if

none of the surfaces were parallel, the standing waves spread out more evenly in the

room instead of bunching up. This is one of the reasons acoustical engineers often

recommend non parallel surfaces in a room where sound is important.

This is all background to help you understand that standing waves cannot amplify a

sound, but only make an existing sound uneven across a room. Furthermore this effect

while it does happen at all frequencies, is only audible on normal program material

(as opposed to a pure tone) at low frequencies. For these reasons it is not possible

that standing waves are the reason a squeal (which is by definition high pitched) is

only heard in the front of the room.

Your suggestion of a hearing aid in feedback is a good one, and might well be what

is happening. Unless the loudspeaker's coverage is truly bad it is hard to imagine

how the problem is feedback.

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From: Calvin Wilson (Garland, TX)

Subject: Re: First post, first question

First of all, welcome!

I'd be willing to bet that everyone on this list knows what your going through

with your sound system. Board placement seems to be a pet subject. It amazes me how

truly ignorant the average church leadership is when it comes to sound. (That's not

a put down, it's just fact.) But all is not lost. You've asked the question, now

your going to get plenty of answers. Use what you can. Just remember to deal with

all those from your church in a Christ like manner. Things get better.

Now for my suggestion. You may not find any position in the balcony that is really

suitable for the board. That's because of the way sound moves between adjacent

resonant cavities.(in this case the main sanctuary, and the balcony) As others will

point out, your sound board should ideally be located in the middle of the lower level.

Right where the people are. Most of us never get our boards into that ideal location.

However, you'd be surprised how far you can get with a little education. The trick

is to get those in charge to understand enough to agree that changes are needed,

(without getting too technical) while reassuring them that changes can be made in an

aesthetically appealing way.

I hate to say this, but you really need to try to get the board out of the balcony.

You are simply not listening to the same acoustics as the congregation. You didn't

mention your sound system experience. I'm assuming you're new at this, so... If you

don't have a good book on church sound, I'd like to suggest, "Why Are Church Sound

Systems & Church Acoustics So Confusing?" You can get it at...

http://home.echo-on.net/~jdbsound/bookordr.htm There are others as well. Read the

articles at...http://www.churchsoundcheck.com. Share them with your leadership. I do,

and it really helps.

If the church has the budget, get the sanctuary checked out by an expert. Just ask

the list and you'll get more offers than you can handle. They can design solutions

for most problems.Good luck.

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From: Blake A. Engel (Chicago, IL)

Subject: Re: standing waves

Ray,

<<"...and we can understand why this standing wave issue is only of any concern at

low frequencies. It explains why you may perceive large changes in bass from place

to place in a room.">>

I agree with this, but I've been in churches where they have a SINGLE speaker

(point source) and if you move left/right or up/down, or forward/backward there IS

a difference in the high-freq. response; a BIG difference. I have always assumed

this was because of standing waves.

There are places where you feel very unbalanced; that is, the sound is louder in one

ear than the other--as if you have a deaf ear. (this is with program material, not

pure sine waves). Now, when the sound was loud in my right ear, if someone stood a

few feet on my right, the sound would return to normal. (And most of these churches

had decent acoustics, i.e. low RT60)

If this is not caused by a standing wave, what causes it?

<<"Your suggestion of a hearing aid in feedback is a good one, and might well be

what is happening. Unless the loudspeaker's coverage is truly bad it is hard to

imagine how the problem is feedback.">>

It was kinda' funny when I found out a member in our congregation was hearing the

squeal from another members hearing aid...Its a real possibility! :-)

-Blake

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From: Blair McNair (Elkhart, IN)Subject: Re: Reflections

Blake,

It is entirely possible that the loss of sound on one side, from a particular

vector, is caused by phase cancellation rather than standing waves. Ray mentioned

close reflecting surfaces, I would look for those first but it could be just as

problematic for a reflective path that is only slightly longer than the direct

or other reflected path. When you say that you experience something like hearing

loss in one ear is that of the direct sound or of the reverberant sound field?

If the answer is that the sound (the spatial impression) felt lopsided you

should consider that the issue is likely lateral path reflections which are

not equal in time arrival causing position dependent cancellations.

Usually in sound systems low frequency drop outs are caused by standing waves.

One point is that when you have a standing wave null you will usually experience

it in both ears. Since most sound systems do not have good directivity control

in the lower frequency ranges the stray LF is free to charge the space and develop

modal behavior. In the HF range the system has better directivity control and will

usually cause distinct arrivals other than the direct in certain locations. These

arrivals come in at particular vectors and may easily cause one sided, considering

binaural hearing, cancellations. The shorter wavelengths of the HF also make it

much easier to get cancellation at more specific, smaller, locations.

If the overall sound seemed to get lopsided in a room, with only one speaker device,

it could also be that further back in the room the direct to reverberant ratio was off.

If the speaker directed unwanted or excess energy onto wall surfaces the reverberant

field could exceed the direct energy the further back you move in a room. In that case

the reverberant field would be driving your perception of the sound and it would be

easy to get lopsided imaging.

Well needless to say without detailed information on the room and system one could

only guess at what is going on. I would just like to point out that it is important

to identify what is the direct energy is a system/room and what is the reverberant

energy. The two are quite different and one can destroy the ability to understand

communication. Adequate direct energy in the listening seats and not on the reflecting

surfaces, walls, is a good thing.

 

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