School hall speakers

[sleah]

I'm looking to make a more permanent install of the sound system in one of our school halls. Think of classic 1950's build high school hall, seats about ~350 kiddies.

No problem with spec'ing amps etc.

The speakers I currently have tend to just sit on the stage just inside the pro-arch, they are plastic 12" Skytec 'disco' type speakers.

The sound quality is perfectly fine for what we want so I'm not looking to improve on that and I don't want to bolt these things to the wall as they are bit ugly :** laughs out loud **:

 

My first choice is the JBL Control Pro 4ohm 150w jobbies. I'm thinking of 4 or 6 along the length of the hall. They are well priced, look pretty and mount easily :D

 

On to my questions - do you think the JBL Control Pro's will be up to the job for use during assemblies where the sound is mainly from Powerpoint presentations, videos and music. Maybe occasional radio/lectern mic use.

Music is often played as the kids come in to the hall. It would not be loud like a concert, the 'audience' is silent during presentations.

For concerts, musicals etc I would be using a seperate system so no need to cater for that.

 

Next - can I get away without delays? Distance between speakers would be approx. 7m.

 

Cheers

Simon

[STCM_Tech]

Simon

 

In our school hall, probably the same sort of size, we have four of those JBL Control Pros and it is ample for power points and music.

 

Will

[Bobbsy]

 

<snip>

 

 

Next - can I get away without delays? Distance between speakers would be approx. 7m.

 

Cheers

Simon

 

Assuming the first speakers are level with the stage and you use four per side, the difference between the front speaker and the rear ones would be (back of an envelope rough) 75ms. Considering how reverberant most school halls are anyway, I certainly wouldn't want to use that arrangement without delays--and I probably wouldn't go for that sort of scheme anyway

[timsabre]

Next - can I get away without delays? Distance between speakers would be approx. 7m.

Assuming the first speakers are level with the stage and you use four per side, the difference between the front speaker and the rear ones would be (back of an envelope rough) 75ms. Considering how reverberant most school halls are anyway, I certainly wouldn't want to use that arrangement without delays--and I probably wouldn't go for that sort of scheme anyway

 

I agree, you will seriously hamper intelligibility if you do it this way without delays. For music you might get away with it but it would sound all echoey. You'd be much better off with 2 larger speakers as high as you can get them at the front.

[sleah]

Next - can I get away without delays? Distance between speakers would be approx. 7m.

Assuming the first speakers are level with the stage and you use four per side, the difference between the front speaker and the rear ones would be (back of an envelope rough) 75ms. Considering how reverberant most school halls are anyway, I certainly wouldn't want to use that arrangement without delays--and I probably wouldn't go for that sort of scheme anyway

 

I agree, you will seriously hamper intelligibility if you do it this way without delays. For music you might get away with it but it would sound all echoey. You'd be much better off with 2 larger speakers as high as you can get them at the front.

 

Thanks for the comments. I guessed I probably would need delays. I've no experience of that kind of setup so it was worth asking.

 

I'm trying to avoid the '2 speakers at the front' setup both for asthetics and to avoid the problem of it being too loud at the front and just right at the back.

[paulears]

I'd like to suggest delays would, in this environment work against, not for you. In loads of schools I visit they had, back when the building was new, column speakers, and of course back, no delays. In almost all of these there are hard reflective surfaces, intelligibility was pretty poor. Two plastic boxes on stage sound better. Adding delays will make the thing even worse. Delays, in my own experience are fine when sound reflection in minimal, and acoustics of the building good. In a hall with a rear wall, all that will happen is that the sound will bounce back, and setting up the delays will be difficult, and even impossible, because for any location in the room, the multi-path arrival times cannot be aligned. The usual process of adding delay to the speakers nearest you so the multiple paths arrive at the same time, is wrecked because the rear reflections are so strong. Any direct path will have a single bounce path from the rear wall, and this is made worse by the delays. Unless your room acoustics are really good, my advice would be to NOT even think about delays. The space is too small for it to have any real benefit. The Church experts have got this kind of install as good as it probably can be, and in these reverberant spaces, their usual tack is simply to use directional loudspeakers each with the shortest path lengths possible, to maximise direct sound and minimise arrivals from every other source - so the ratio of good vs bad sound is as good as they can get it. In a school hall with just 8 speakers, 4 a side, every path is visible - an empty floor, rather than all the obstacles in church architecture. All those paths in the school hall will hit the back wall and bounce back, let alone the sides, ceiling and floor! Draw a scale plan, and plot some times and drop off levels. School halls are just a bit better than a swimming pool, and we all know what they sound like!

 

2 speakers, high on the side walls at the front end. If you want less ugly, than two of the commercial types that can be painted - like Tannoys and Celestion and others.

[timsabre]

I'm trying to avoid the '2 speakers at the front' setup both for asthetics and to avoid the problem of it being too loud at the front and just right at the back.

 

That's why you put them high up. If you have them low down, then they'd be too loud at the front, but if you put them high, they aren't.

[Bobbsy]

^^^^

 

This. Two speakers high and hopefully angled down a bit will provide far better intelligibility than your scheme even with delays.

[ojc123]

Ours is a 1950's Hall. 15m square floor area 5m high, 8m x 15m stage. 450 people max. Hard walls and very reverberant. We have two RCF 722As normally high up on the wall and pointing down at the audience. Good for speech, presentations and rock and roll. We hire in a sub for Rock and Roll. They're sitting on the stage at present because of other work going on. And we do have 'too loud at the front' problems.

 

At one stage we had four speakers with a pair half way down the Hall with no delays. It resulted in patchy sound. You could hear an instrument whilst sitting on one chair and not hear it in the chair next to it. It really was that bad. I don't know whether the lack of delays was the problem or if it was the placing of the speakers. I'd go for the high up pair of half decent speakers.

[Shez]

I always find it useful to have both a scale plan and a section to work with on jobs like these. Scissors, paper and a protractor will let you produce triangles for the horizontal and vertical dispersion patterns - lay them out to see what positions will give you the best coverage. (It works for lighting too - there's something much more satisfying playing with bits of paper than trying to do it on a computer. Maybe that's just me...)

[ojc123]

Just to clarify that we have 'too loud at the front' while the speakers are on the stage. That's where they are temporarily placed. It's not a problem when they're up in the air normally. We messed with the angles on paper and in real life to get the best sound over the Hall.

[azlan]

with the speakers up in the air, you will get more even coverage, partly because less sound will be bouncing around in the air over peoples heads, but also because you are making the relative path difference between the closest and furthest listeners smaller (think about a right angled triangle and what happens when you keep one side of the 90* section the same length, but alter the other one, and then consider the inverse square law)

 

e2a: some 4am musings on the whiteboard to try to explain this..

 

as you can see, b gets larger, the difference between a and b decreases (which is the difference between the distance from the speakers to the closest listener and the furthest away), meaning that the sound level actually reaching each persons ears is more uniform.

you also gain an advantage that most the sound will be hitting nice lumps of energy absorbing flesh, instead of reflective hard walls, meaning less natural reverberation, and possibly let the dispersion in the horizontal plane do its job too, enabling everybody to hear what is going on, not just those more or less directly in line with the speakers

 

and here is an example in a fictional school hall 4.5m tall and 12m from stage lip to the back of the seated area

[azlan]

sorry this runs to 2 posts, I left the edit open for too long when I went to get breakfast..

 

So that's great, the path length is more similar with speakers up in the air than with speakers on sticks, but what does this mean in practice? well, what it means is that the 'inverse square law' will have less of an effect than it otherwise would.

 

The inverse square law applies to most things that travel through air in waves (so lights as well as sound), and it basically states that if we assume a wave is radiating in all directions as a sphere (the actual coverage of the speaker is probably not a sphere, but a small section of a sphere) each time the distance doubles, the surface area of the sphere will go up 4 times, and therefore the energy per unit of area is divided by 4, which in audio terms translates to losing 6db every time you double the distance

 

http://imageshack.com/a/img835/6272/91oh.jpg

 

so although by positioning the speakers further away from the closest people, we may be a bit quieter overall, however, we are more even, and as most loudspeakers can go much louder than is needed for speech re-enforcement or normal program music, we just turn them up a bit to boost the initial value.

[sleah]

That's brill! :D

 

Thank you so much for all the helpful information.

I now have another plan - stick with one pair of speakers high up, and just replace the nasty 'disco' speakers with something prettier but larger than the Control 1 Pros.

 

Cheaper too - only one stereo amp instead of 3 (current one may be OK), no LMS unit (already have hard-wall compressor/limiter & EQ) and less cabling.

 

Lovely jubly :D

[Electrolytic]

if you want d&b quality without the price http://www.proaudio-technology.de/2_EN/Produkte/index.html