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Old 10th March 2012, 10:48 AM   #1
brig001 is offline brig001  United Kingdom
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Default PA in reverberant auditorium.

I have been asked to provide the PA for a live music night at my daughter's school. I have done this sort of thing before and use my normal stereo speakers (they are built with PA drivers anyway), a small mixing desk, power amps, monitoring etc. The problem is that this room is VERY reverberant and my speakers are not very directional. The main issue being smooth flat side walls, the back has tiered seating so should not be much of an issue. I could hire something more suitable - probably horns, but cost and transport start to become issues.

I would like to add a "reflector" to the outside edge of each speaker to try to stop the sound hitting the side walls - a bit like half a horn. Has anyone any experience of doing this?

Thanks,
Brian.
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Old 11th March 2012, 08:34 PM   #2
brig001 is offline brig001  United Kingdom
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OK, I have made a some progress with this. I have disconnected my crossovers and tweeters and am running the 12" twin cone drivers full range. They beam quite well above about 1kHz or so, so that should reduce unwanted reflections. They top out at about 10kHz, so I might try to bring the tweeter in above that and see how it goes.

I'm still curious about the reflector idea if anyone has any thoughts.
Thanks,
Brian.
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Old 12th March 2012, 05:52 PM   #3
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Running those 12"s above their normal frequency range is not advisable due to their normal frequency roll off, so the highs will be greatly diminished with a dull muddy sound. As well the ability to clearly reproduce the highs without modulated distortion from a 12" will limit the power input. Eg. "Single cone fullrange p.a. drivers are not commonly used". You could try toeing the cabs inwards and down so they are forced to disperse their sound into a more centrally concentrated area.
Slap echo from flat surfaces generally is percievable from 30 milliseconds up. Large rooms are difficult to treat thoroughly from any single ,let alone multiple points of reference in the room, especially on a temporary basis.

Last edited by Top Shelf; 12th March 2012 at 05:55 PM.
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Old 12th March 2012, 06:08 PM   #4
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The issue I would worry about is how many people will be in the room. You need to know the dimensions of the room, what material each of the six or more surfaces are faced with to calculate the in use reverberation time. That will indicate how much pattern control is really required.
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Old 12th March 2012, 10:52 PM   #5
brig001 is offline brig001  United Kingdom
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Thanks both. The room is about 15m long x 20m wide x 10m high with untreated concrete walls, a hard wooden floor and what appears to be a concrete ceiling with skylights - and it was purpose built as an auditorium. I'm no expert in these things, but the either the designers didn't have a clue, or treatments, curtains etc. were all cost reduced to zero after design.

The slap echo was very noticeable when clapping, with multiple reverberations after that, so I guessed at about 100msec. In my simple head, I reckon from the band at the back of the stage (dancers will be in front of them), to a mid seating position, the echo from the side walls will arrive roughly 70msec later than the main sound, so that sounds about right. The tiered seating can hold about 300 people, but only about 100 will be attending.

Top Shelf, I see what you are saying about the speakers, so I will try a couple of cross-overs; maybe at 4kHz and 8kHz, then I can pick the best on the night.

Thanks,
Brian.
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Old 12th March 2012, 10:59 PM   #6
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You need different speakers and graphics equalizer with frequency analyzer. The cheapest option is Behringer 31-band one with LEDs on faders. Increase sensitivity until it starts oscillating, then slowly attenuate the frequency band on which it oscillates untll it stops. Then increase sensitivity and repeat this procedure. After equalizing your mikes and speakers such a way bring down sensitivity a little bit.

Here on the picture, black speakers with 15" woofer and horn tweeter on the stage are wrong. Red arrays are right.

Click the image to open in full size.
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Old 13th March 2012, 06:25 PM   #7
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Quote:
Originally Posted by brig001 View Post
Thanks both. The room is about 15m long x 20m wide x 10m high with untreated concrete walls, a hard wooden floor and what appears to be a concrete ceiling with skylights - and it was purpose built as an auditorium. I'm no expert in these things, but the either the designers didn't have a clue, or treatments, curtains etc. were all cost reduced to zero after design.

The slap echo was very noticeable when clapping, with multiple reverberations after that, so I guessed at about 100msec. In my simple head, I reckon from the band at the back of the stage (dancers will be in front of them), to a mid seating position, the echo from the side walls will arrive roughly 70msec later than the main sound, so that sounds about right. The tiered seating can hold about 300 people, but only about 100 will be attending.
If the concrete block walls are painted then I suspect the ceiling has an acoustical coating. If it were concrete the reverb time would be over six seconds. That is bad enough the room would be useless.

If the walls are unpainted concrete block they would provide enough sound absorption to make this a fairly good room.
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Old 13th March 2012, 09:54 PM   #8
brig001 is offline brig001  United Kingdom
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Hi Wavebourn, I had sort of dismissed (possibly wrongly) any kind of line source speaker because I thought the increased dispersion would make the reverberation worse. I really don't know on this one - any thoughts?

Hi Simon7000, the walls are painted, not sure about the ceiling, and the reverberations from clapping lasted about 2 seconds, so it is quite bad. They have an installed PA which is next to useless. I have heard it used for speeches etc., and it is difficult to hear what is being said from anywhere in the auditorium.

Thanks for the thoughts so far,
Brian.
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Old 13th March 2012, 10:17 PM   #9
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I don't know if it's feasible, but you could try renting acoustical drop cloths, but it would have to be fire rated. you would have to play with them see where you would get your maximum reverberation attenuation. do usually have to be positioned off the wall.
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Old 13th March 2012, 10:29 PM   #10
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Brian;
line sources are good in such environments because they project nearfield farther than ordinary speakers, that means difference in loudness in first and last rows is less, and create less of reflections from floor and ceiling. Also they can distort less, since less excursion of cones is needed for the same loudness.
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