Hire of acoustic baffling

[GreatBigHippy]

Hey all,

I've got a musical coming up in a very reverberant school hall (something like 3-4 seconds), and I've been looking at ways to get some clarity.

The band should be ok in general but if the vocals aren't clear then I really will have problems. As it is only a 200 seater and the band are right in front of the audience I have no chance to turn the levels down on them so the vocals will have to come in over the top as it were (with the necessary eq work done of course)

I have had a few ideas on how to deal with this already

 

1: Instead of 1 set of speakers at the front, go for two staggered sets down the hall, like a delay system. That way I can go for quieter volumes with more direct sound.

2: Hang some very heavy curtains wherever I can (hiring trussing for the back if needed) to deal with the reverb.

or

3: Hire in some proper acoustic baffling which can be installed and chosen for the room.

 

So the question is, are any of these blindingly better than others to anyone? (They all have downsides, budget being one of them )

Also, does anyone know of anywhere in the southwest where baffles can be hired?

 

Ta very muchly

theHippy

[colebrooke]

Hi, I think your curtains idea sounds good - so many school productions don't pay much attention to treating the venue. Spacing your curtains around 6-8 inches from the wall should provide the best results. However if your hall has a long RT60 at low frequencies, curtains don't help much with that... in my opinion you just have to watch that the level of bass and kick drum don't overwhelm everything, as they may tend to bounce around the place, curtains or no!

You mention 'like a delay system'... are you planing to use delays? I would certainly recommend this, in my xp it can really help with poor acoustics. You can really improve the clarity of sound toward the back of the venue.

Sorry I don't know where you can hire baffles!

[paulears]

I've never had much success with delays in rooms with reflective surfaces, unless you have really narrow dispersion patterns. I think the op needs to consider if a speaker at one of the delay positions will be heard by a listener nearer the stage.In a live room, the answer usually is yes, and the delay makes the problem even worse! In such a live room, I'd go for reduced speaker count, not more. A centre cluster, pointing down to the audience seems to work much betterin my experience, with the primary first reflection area bing the audience themselves. Less goes up to the ceiling from the ground reflections.

 

Drapes, if they can be hung always help to some degree - but the room sounds quite large, so that's lot's of drapes and price wise will be a major consideration.

 

I wonder if your reverb time is a bit iffy? 3-4 seconds is concert hall size reverb. A school hall seating 200 shouldn't be that long, but may still sound horrible if the walls, loor and ceiling are all hard. I'm guessing you are quite used the the sound of the room from your use of it before - as most people will be, so maybe the poor sound will be almost expected.

 

The first thing is to try to remove the problems. This of course will be the band. The usual question is that if you are considering huge amounts of expensive drapes, why not move the band into a less prominent position and spend the money on remoting them up? Can they play in another room, with monitors and more controlled acoustics? That does assume your PA is capable of doing them justice, and youhave the necessary skills to balance them up.

 

All this is quite complicated and expensive.

 

Ask a really awkward question. Are the perfomers and musicians good enough to justify this getting complicated. Can the actors act and sing, can the musicians really play well and have good quality instruments? It's quite sad to answer this honestly - and see that whatever you do it will be, well.... not so hot?

 

A bad room, a bad cast and musicians, a basic PA etc etc won't really be improved by a few drapes. 200 people don't generate much box office income, so maybe you should just do the best you can. It isn't the west end, and most audience members are biased anyway - so perhaps this could all be unnecessary. Playing the videos of my college productions with big budgets and clever technical kit still reveal how dreadful 80% of the people in it were!

 

This is often why the teachers don't feel the extra money they get asked for is justifiable - will anybody notice?

[dunk_1984]

Also 200 people create a nice sound absorbtion service, and they pay you to provide it...

 

alternatively just fiddle and see what can be acheived by mabye adding slightly more bass/cut some high to your output... this sometimes helps... delays in a hall that size I doubt will have a huge effect.

[GreatBigHippy]

A school hall seating 200 shouldn't be that long, but may still sound horrible if the walls, floor and ceiling are all hard.

Solid surfaces all the way! Sounds bleedin awful, but as the other post says, 200 bodies do help rather a lot!

 

I think the op needs to consider if a speaker at one of the delay positions will be heard by a listener nearer the stage.In a live room, the answer usually is yes

On further consideration I agree, I think its so bad that even at low volumes the acoustics will make this a bad choice in this circumstance but....

 

A centre cluster, pointing down to the audience seems to.......

Not viable. Only one FOH bar, which is going to be full with lights. And the ceiling is asbestos!

 

Can't remove band, partly due to expense as you mentioned, partly due to the fact that I don't want to explain to parents that their kids had to be put down the hall!

 

Now... drapes I have! I have had a look around and, on stage, in a big heap, are all the stage curtains that were made wrong for us (they installed new curtains... all in blue, not black!) and we have got to keep the blue ones! I'm going to make flags with our school logo on to hang from the ceiling, that way they can stay the whole time, looking pretty but also helping acoustics (should keep things a little calmer in assembly!).

 

I understand about not being able to do a lot about bass frequencies (my other job is studio tech!) but thats not too much of a worry. Only the double bass will be miked, everything else can be damped.

 

Thanks for all yours help. I'll let you know how it goes!

 

theHippy

[MarkPAman]

Solid surfaces all the way! Sounds bleedin awful, .................the ceiling is asbestos!................I'm going make flags...to hang from the ceiling, that way they can stay the whole time........

 

I don't know your room, but I have worked with rooms like it. I think it would be better to hang them (drapes/flags) on the walls if you can. I would imagine that's where the worst reflections will come from.

[colebrooke]

I would still have to say delays could help you. Yes listeners in some positions will hear sound from both sources, but the ear has 20-30ms tolerance for this. Make sure you calculate the delay correctly, make sure the speakers are facing the rear wall, not the center of the room. It can help to add an additional 15ms or so to trick the ear that the sound is still mostly coming from the main FOH. Also if the delays are set 3 to 6dB less than the mains this will help. Doubling the number of speakers adds about 3dB to the diffuse field, which in fairness is not good, however I believe the good outweighs the bad.

It doesn't take long to set up and test - then you can easily switch the delays in and out, to see what sounds better from different positions. In my experience, it normally sounds better with the delays in... in some situations, I have demonstrated this to the client and they have been amazed by the improved clarity of the sound towards the back of the venue.

 

Hope that helps!

[Bobbsy]

I've used delays in specific circumstances and they CAN help. However, in a room as small as a 200 seater I think it could cause more harm than help, particularly if the OP is stuck with speakers that aren't very directional. The problem will be that the required amount of delay will vary by a fairly large percentage between the sides of the audience and those in the centre. Intelligibility could suffer.

 

Especially as they seem to have a lot of curtains to hang, reducing the reflections in the first place seems to be the best option (though I have to say that a 3-4 second RT in a 200 seater is highly unusual/unlikely. The only thing with curtains is that you need to make sure the flame retardent treatment is up to date.

 

Bob

[paulears]

A 200 seater school hall and delays seems totally unnecessary to me. Size wise it is just too small, and multiple sound sources, even delayed ones in small reflective environments just work very poorly - consider the approaches used in churches nowadays.

 

In a big room, then delays work well, but in a school hall, it is going to be really tricky making them work, and setting them up is going to be a real pain, and even with careful setup - then people will be in positions where the delays will be heard at wrong levels, making it worse.

 

You have a lighting bar in about the correct place - surely you could squeeze just one compact cab up there - even if it's just a mono centre feed - it'll help no end, and you could run any music sources through a smallish left/right system. Lose just a single lantern to do it.

 

colebrook - I've had this problem many, many times. Not once have delays sorted it out. Absorb as much as is possible and keep the sound sources high and downwards is the only way I've ever found that helps. However, I'm happy to find you've solved the problem. I visit schools and colleges all over the country - maybe I've been to one of your successes?

[colebrooke]

paulears, yes, you have a point if the hall is smaller / squarer, and to be fair delays do work better in a longer space. Most of my work is touring / hire, not too many installs. I would agree that absorbsion is better, but delays are very easy to set-up in minutes, and their effectiveness could easily be proved or disproved in this case. I'm only too happy to be proved wrong! The reason it works is simple - more people have direct sound from the speakers verses the diffuse field. - you are increasing the critical distance.

 

Oh, I also like center clusters... perhaps could work in this case. They are harder to rig on a small gig (unless you have all the truss there etc) so I don't go that route normally.

 

cheers

 

 

 

(edit for minor error)

[GreatBigHippy]

Thanks for all your input everyone!

I'll try to calculate the actual RT60 for you. Maybe 3-4 is pushing it but I would say close to 3 certainly.

I'm going to talk to the LD and see if I can fit a single speaker on the FOH bar as I agree with Paul that the fact that I'm driving sound into the best absorbers in the house (my audience) would help greatly.

If I have time I really want to try the delays, not because I think they will help massively, but because the discussion has peaked my interest :)

Guess I should change the topic title!

BTW, as acoustics is something I am very much into, could I request an explanation on why hanging drapes on the walls would help more that from the ceiling more centrally?

Its my understanding that a wave has zero energy at the point of contact with a hard surface. As any absorbers will remove a percentage of energy they are more effective when the absorber is hit at either the top or bottom of the wave cycle. As this is where the greatest energy transfer will happen. Any absorber in contact with a surface (assuming zero depth) will therefore have no effect. This is why, in studio applications, bass traps are set away from the wall, with an air gap behind. The longer wavelength of lower frequencies mean that the further from the wall, to a point, the more effective the trap. It is also why bass traps are tuned.

I would have thought that draping the walls would leave the room sounding boxy at best and my best chance of broad spectrum absorption would be to hang them as limp mass traps. They may not be as effective on certain frequencies but will be more effective across the board.

Thats just how I understand it, sorry if the language is a bit off in place. I am really just beginning to get my head around all this stuff and am really keen to learn more, bleedin fascinating stuff acoustics!

Ta very muchly for all your help again!

theHippy

 

Edit: curtains were provided new over summer, for stage, then removed as they were the wrong colour, so yes, flame retardant. Unless we have been screwed :blink:

[Matt Riley]

BTW, as acoustics is something I am very much into, could I request an explanation on why hanging drapes on the walls would help more that from the ceiling more centrally?

 

This is kind of posted for criticism - don't rely on any of it till one of the oldies says it's ok!

 

I'm no acoustics expert - but I am a graduate of the emporium of free sandwiches and helium which is D&B's acoustics course

 

When looking at treatment, you have to pick your battles. In this case guitar amps, PA speakers and the like aren't omnidirectional sources, and normally they are aimed at the back wall, so there's where the first reflections are hitting.

 

If you were at a rock gig, where you had a really well designed flown FOH rig, then the obligatory pairs of 15/2 wedges for each musician, chances are much of the problem would come from the front wall, so that would be where you put your absorbtion.

 

When it comes to spacing the curtains against the wall, if the curtain is over a certain distance away from the wall (dependant on the wavelength - possibly half), then the sound has to go through it twice, thus meaning a curtain with an absorbtion factor of 25% at 1.5 Khz, about 10cm away will mean that, in total 44.75% of the acoustic energy at that frequency is absorbed before reaching your ears again.

 

Which leads me onto suggesting that you might wish, if you've got enough, to double up the curtains. When I've done similar projects before it has only provided a good improvement with proper, heavy serge drape rather than the cheap and nasty stuff, and doubling up some curtains would give you a similar amount of mass to this.

 

Also, remember that not all speakers are equal. A typical cheap 12"/1 speaker won't have anywhere near its published 90 by 40 degree dispersion. Indeed, even the expensive ones are only capable of maintaining relatively even coverage in the range of the horn (about 2-3Khz upwards). A proper, horn loaded speaker will however have a clearly defined coverage pattern often down to much lower - e.g. 300hz. Something like D&B Q7 or Q10 is very loud and compact if budget can stretch.

 

Matt

[DavidLee]

BTW, as acoustics is something I am very much into, could I request an explanation on why hanging drapes on the walls would help more that from the ceiling more centrally?

Its my understanding that a wave has zero energy at the point of contact with a hard surface. As any absorbers will remove a percentage of energy they are more effective when the absorber is hit at either the top or bottom of the wave cycle. As this is where the greatest energy transfer will happen. Any absorber in contact with a surface (assuming zero depth) will therefore have no effect.

You are describing the conditions for resonance - a standing wave where there is an integral number of half wavelengths between two reflecting surfaces, with a node at each interface. For arbitrary reflections of a travelling wave from a hard surface the waveform is simply reflected back such that angle of reflection is equal to angle of incidence. The pressure at the interface - just as anywhere else - will oscillate between extremes of positive and negative maxima. Softening the surfaces will reduce the amplitude of the reflected sound.

 

David

[MarkPAman]

Its my understanding that a wave has zero energy at the point of contact with a hard surface. As any absorbers will remove a percentage of energy they are more effective when the absorber is hit at either the top or bottom of the wave cycle. As this is where the greatest energy transfer will happen. Any absorber in contact with a surface (assuming zero depth) will therefore have no effect.

(My bold)

 

If this were true, then all surfaces would reflect in the same way, which clearly they do not.

 

The reverb you are getting is caused by sound reflecting off hard surfaces, which are walls, ceiling and floor.

You're going to deal with the floor by covering it in people.

 

Now, wherever you hang your drapes they are going to help, but as it seems your room is particularly bad, you want to get the best you can from them. So it seems best to look at what the sound you are producing is bouncing off most, and this is probably the wall at the back of the hall. So you need to get your drapes between the speakers and the wall (but not between the speakers and the audience ). Now the only way of doing this I can think of is to have the drapes between the audience and the back wall, which probably means hanging them on the wall.

 

Drapes hanging from the ceiling, over the heads of the audience will soak up some of the sound that's going up into the ceiling, but hopefully you're not directing too much up there. Certainly some of this sound will bounce back down, but will have done several reflections before it reaches ear level again.

 

At work we had drapes fitted in a particularly reverberant room, a few months ago. The room has a ceiling which slopes from about 2.5m to 7m, and the drapes are on a track at about 2m height, and as close to the wall as we could get them. I have been amazed by how much difference they make, even though there is still a large area of wall not covered above them. Having them on tracks also means we can decide how much we want to dampen the room for different events.

[Bobbsy]

A few thoughts on the acoustic questions posed (and these are random, so this post is going to ramble a bit!).

 

First, regarding whether treatment on the ceiling is the way to go where you have limited resources: the answer is likely "no". There are a couple of reasons why I say that. First, the OP mentions an asbestos ceiling. I'll take this to mean ceiling tiles, in which case there is already some absorbion going on there. Second. the real thing you have to avoid is parallel surfaces...and below the ceiling will be the audience which is probably the most absorbant surface you have in the room.

 

On the other hand, the side walls are very likely parallel to each other giving you a real possibility of creating standing waves. Similarly, the back wall is probably directly opposite the stage/FOH speaker system, giving you a real chance of slap back. Therefore, it's the back wall first you want to deal with, and side walls second.

 

As has been said, for maximum effect, the drapes have to A) be heavy ones and, B), be hung away from the walls. The reasons for this have been discussed. However, the situation with "zero absorbion where something with zero depth is touching" gets rather more complex since absorbtion is only one factor. Diffusion also comes into play and this is where acoustics turns into a black art in my book. Hard surfaces can actually produce a lovely acoustic if they have lots of uneven detail on them...places like the Royal Festival Hall have a good sound despite being mainly hardwood all over the walls. This is because there are lots of uneven, non-parallel shapes incorporated. This is where it gets outside the realms of the amateur...all those apparently random shapes were planned by an expert who could predict the final result.

 

Anyway, that's not really important here where the OP is going to hang some drapes. Good luck with that...I just wish architects would stop designing school halls with hard, box-like parallel walls. Even without a full acoustic planning, just getting out of this mindset could improve the basics!

 

Finally, if you haven't got this already, have a look at the Allen & Heath downloadable RTA HERE. Smaart it's not...but Smaart costs more than £10!

 

Bob