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Outdoor Acoustic Reflectors

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Outdoor Acoustic Reflectors

Postby mac.churchmouse » Sat Aug 25, 2018 7:28 am

Kia Ora,
Usually requests in the SOS forums are about taming and reducing acoustic reflections but for my next performance project I actually want to do quite the opposite.
In February next year (the Antipodean summer) I am producing a community theatre show in a small, local park. We won't have the opportunity for a PA or similar - the dialogue and the soundtrack will be 'live and acoustic'. So any advice about materials we could use in building the set that would help with projecting the sound?
We are planning to have a reflective cloud above the brick stage, angled towards the audience, plus panels in a semi circle (gaps between) around the performing area. I have wondered about using a recycled material like corrugated iron but have no idea how reflective it might be. We need light, cheap, environmentally neutral, and suitable for painting with graffiti and street art cartoons.
Any ideas gratefully accepted.
Nga mihi nui - much thanks, Glenn
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Re: Outdoor Acoustic Reflectors

Postby blinddrew » Sat Aug 25, 2018 10:54 am

What's the rest of the environment like? Are there any natural features you can use to your advantage? Bare rock walls? Natural amphitheatres? Or are you stuck in the middle of a field?
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Re: Outdoor Acoustic Reflectors

Postby Wonks » Sat Aug 25, 2018 11:55 am

I;d suggest hard flat (or slightly concave) surfaces rather than corrugated. Corrugated will act more like a diffuser and spread the sound out in all directions. If the audience is pretty much sitting directly in front of the stage, then Ideally you'd build the set behind you in a near-parabolic curve rather than a semi-circle, with the performers in the middle as much as possible. This will project the reflected sound directly forwards as much as possible,. A semicircle will be better if the audience is more spread out and also sitting well to the sides of the stage.

There will be a slight smearing of the sound as a result of the time delay between reflected and direct sound, but not enough to reduce intelligibility. The only possible drawback with trying to reflect as much of the rearward travelling sound as possible back again, is that higher frequencies are more directional, so most of the sound going backwards from someone speaking to the audience will be in the low mids and bass frequencies, which would increase the amount of low frequencies in the sound and may end up making the speech less intelligible than before. Or it may make the sound more natural to people sitting further away, I'm not at all sure here!

Presumably there's a no-generator rule in the park?
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Re: Outdoor Acoustic Reflectors

Postby Bob Bickerton » Sat Aug 25, 2018 12:38 pm

Kia ora Glenn

I guess what you’re talking about is a sound shell which were commonly used pre-amplification and there are many fine examples in Aotearoa. A quick search on google will bring up lots of pictures, but of course most of those are permanent structures.

We successfully used MDF for a soundshell, actually in a theatre with poor acoustics, but that will only last a few minutes if it rains! Probably plywood would be better and potentially could be reused for other purposes after the event.

Of key importance is to direct the play and actors with this in mind. Actors will need excellent projection, the audience will need to be positioned very close and you’ll need to be well away from any ambient noise such as roads. Even a very well designed soundshell made of solid materials will only have minimal effect.

Depending on where you are, I’d have a dry run in an existing soundshell structure so you can hear the (small) difference it’ll make.

Ngā mihi

Bob
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Re: Outdoor Acoustic Reflectors

Postby Wonks » Sat Aug 25, 2018 1:44 pm

The Hollywood Bowl in LA is such a sound shell design, with the shell being built in 1929 in pre-decent PA times to help project the sound (though it was a far from perfect design).
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Re: Outdoor Acoustic Reflectors

Postby mac.churchmouse » Sun Aug 26, 2018 3:30 am

Bob Bickerton wrote:I guess what you’re talking about is a sound shell
A sound shell encompasses the concept perfectly. And a nosy at successful versions of those will give me some basic ideas for form (The Wellington Botanical Gardens has a lovely little one, and of course there's a superb Deco number on the Napier foreshore).

The structure will only be semi-permanent so ply would do fine. Any one have thoughts about other materials? I have seen photos of people using the plastic equivalent of corrugated cardboard - as used in Real Estate signs - to create outdoor sets and art works. But I have no idea if that will have any acoustic reflective value. Its certainly light, cheap, and waterproof.

Perhaps Hugh or another of the SOS gurus might have a technical explaination of what makes material acoustically reflective - is it mass or perhaps hardness or density? Bearing in mind that I am guessing that what I want to do is 'amplify', or focus forward, frequencies for clarity and distinction of diction - so in the 1 to 5 khz area?
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Re: Outdoor Acoustic Reflectors

Postby Bob Bickerton » Sun Aug 26, 2018 3:38 am

Here lies the dark arts of acoustic reflection co-efficients and acoustic absorption co-efficients. If you can understand this paper you'll go along way towards figuring out what's going on: https://cds.cern.ch/record/1251519/files/978-3-540-48830-9_BookBackMatter.pdf

Good luck!

Bob
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Re: Outdoor Acoustic Reflectors

Postby ef37a » Sun Aug 26, 2018 9:10 am

The material properties that make it acoustically reflective are probably high stiffness (Young's Mod'? ) and low internal damping. Mass would be important I suppose since if it "moved" that would absorb energy. Ply would seem to be the most convenient material although I think a facing of hardboard would help being denser and harder?

Plasterboard would be even better I think but less easy to handle but then you might get hold of some scrap, broken sheets for nowt and attach them to a timber or scaff' frame?
Paint it as a "cyc'"?

Shape of structure is a poser? I don't think you want a parabola? That would need all the actors at one spot. A parabolic mic "collector" takes in sound waves and directs them to a common point.

Dave.
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Re: Outdoor Acoustic Reflectors

Postby mac.churchmouse » Sun Aug 26, 2018 12:01 pm

Bob Bickerton wrote:Here lies the dark arts of acoustic reflection co-efficients and acoustic absorption co-efficients. If you can understand this paper ...
Aye, there lies the rub!
Will find a quiet corner and perhaps a pint of something pale and see if I can figure out what it actually means.
Thanks (for the excuse for a beer at least)
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Re: Outdoor Acoustic Reflectors

Postby Wonks » Sun Aug 26, 2018 1:55 pm

Absorbent coefficient is basically the inverse of the amount of reflection at the given frequencies. So the less absorbent a material is, the more reflective it is.
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Re: Outdoor Acoustic Reflectors

Postby mac.churchmouse » Tue Aug 28, 2018 6:30 am

Thanks for all the help and advice. Starting to get a picture of what's involved so can now talk with vague intelligence to my designer.
If anyone's in Sydney late February 2019 keep an eye out for the Sydney Summer Star Trek! We're recreating "The Trouble with Tribbles" (1967) so it's going to be mad fun.
Nga mih nui, Much thanks Glenn
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