PA in reverberant auditorium.

[brig001]

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.

 

[brig001]

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.

 

[Top Shelf]

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.

 

[simon7000]

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.

 

[brig001]

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.

 

[Wavebourn]

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.

An externally hosted image should be here but it was not working when we last tested it.

 

[simon7000]

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.

 

[brig001]

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.

 

[Top Shelf]

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.

 

[Wavebourn]

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.

 

[Top Shelf]

Wicked idea with the fold up line array there Wavebourn, are those co-axials?

 

[simon7000]

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.

1.5 Seconds is a good number for reverb time. So line arrays should be enough of an improvement to cut it. Of course to drop the reverb time 100 sq M of carpet on the walls would fix the room.

 

[brig001]

Thanks all. It seems like ideally the room would be treated, and to be honest I did think that too, but the chances of getting this done are a bit slim - certainly before this gig anyway. I am warming to the line source idea, and do like Wavebourn's folding speakers - I think I might have a future project there.

I'll see if I can borrow some line source speakers for the gig, failing that I'll play with crossovers and toeing in to get the best I can. I'll report back how we get on.

Thanks all,
Brian.

 

[cyclecamper]

Then again, Wavebourn and I like line sources. Danley may differ, but only because he has point-source speakers that control not only basic pattern but exactly where the power goes (in all dimensions) with great precision. In your case, either line source or point source may be a problem.

The best thing you can do is sell more tickets and fill the place with people.

It may be practical to bring in objects which will break up the echoes into something more random. Get the decorations committee to hang some objects. Good luck. The standing waves are going to suck too if the place is only 1/3 full. Do the skylights open?

Instead of 'reflector' you might think 'absorber'

Aiming directly at the people may save the day if there's enough bodies. But the sound check may be scary before they arrive.

 

[PAULL8485]

In my experiance with PA systems and highly reverberant halls is to have plenty of subs !8" in your hall size no less than 4 crossed over (an electronic crossover) to 1 x 15" (12" if that is all you have) and horn per 2 Subs at stage end, seperate amps for subs and top boxes and same number of 15" or !2" and horn at the back of hall facing stage NO DELAY.
You only need delay if the second lot of speakers are facing same way as stage speakers and about halfway down hall.
the above has worked very well as the back speakers drown out the back slap you may need to adjust volume of back speakers relative to stage speakers, also keep all speakers as close to the ground as possiable and if you can safey pointed down towards the crowd and the best insulation of all is plenty of people on the floor.

 

[mdocod]

You can spend a million dollars on a sounds system for that room, and probably never achieve what could be done with about $10,000 worth of acoustic modifications to the room. I have memories of my middle-school auditorium, all painted concrete walls and hard wood seats. The acoustics were terrible, various gatherings sounded like crap. Speech was completally uninteligable and to be honest, to this day I have no memory of ever understanding much of anything useful that may have ever been said in that room.


I recall performing there (I played the trumpet in jazz band), and it sounded like utter crap most of the time. (When the room was filled with bodies, it sounded betterish). Our "band room" had carpet lined walls and made for a much more acoustically friendly environment. When we visited a "high school" for a regional competition, with a proper auditorium, our performance sounded much better.

-------------

I can't fathom any reason that a line array would improve things in this situation. The wide horizontal dispersion of a typical line build will introduce as many problems as it's vertical dispersion control will solve. I think the best balance here, is to use highly directive speaker design, positioned back in the stage as far as is reasonable, and apply cheap carpet to the rear wall of the auditorium.

Regards,
Eric

 

[pinkmouse]

Hmm, some very strange approaches suggested here. From someone who has worked in lots of rooms like this, the answer is simple, lots of small speakers distributed down the room aimed directly into the audience. That way, the punters act as room treatment and you can keep levels on individual speakers down to minimise exciting room modes. Simple.

 

[brig001]

Thanks for all the suggestions. The school changed their minds on friday, and decided to use their own system. The sound was as awful as I expected. I think there may be some politics going on there.
I am still interested in this, and would like something portable that could be used in this kind of room. Perhaps this is a job for a compact unity horn. Hmm, I need to give this some more thought.

Thanks again for the suggestions,
Brian.