Large Scale Parabolic Acoustic Mirrors

[Vax]

(image not to scale)

 

I'm interested in creating a pair of 15 meter diameter parabolic dishes facing each-other at an outdoor event taking place on a dry lake bed with a set of stairs leading up to a platform that would allow a group of people to talk or sing in the focal point, allowing long-range transmission of audio between the two points over a relatively large range, preferably more than 200 meters.

 

First off, is this even feasible at this distance? Will there be too much noise pollution or loss of voice quality or amplitude?

 

The temporary structure will need to be assembled on-site. I believe sectional panels could be cheaply built using cloth, chicken wire, and adhesive laid on a form. Will these panels suffice acoustically? Is density of the material a big factor here?

 

Thanks for any help

-Vax

[swansont]

I suspect this will not work for a group, since these work for sounds at the focal point. It might be that only one person gets the benefit of the dishes at each end.

[Enthalpy]

Seen and used some with 3m diameter at La Villette. It works very well, fun and convincing. Do it.

 

Only one person can speak and one listen at a time because the focus is narrow. At La Villette they had sorts of crosshair rings at both parabolas, copy that. But maybe D=15m isn't mandatory for 200m range?

 

Sound quality is perfect, no special noise, BUT I expect wind may bring the sound wave of-axis and then the receiver won't focus any more. Try to put figures on wind shear near the ground. Also, the temperature gradient near the warm daily ground lets sound go up and lost. Maybe it'll work only on calm night, as you can experiment without parabolas: you hear a source from a longer range at night.

 

One more attenuation is when wind blows against the propagation direction. This has nothing to do with a longer path. It's the wind shear near the ground that lets sound go up and lost.

 

Instead of making the reflectors yourself, I suggest to use (borrow!) existing radio dish antennas without their primary source. Usually metallic, stiff and smooth, heavy enough for sound, of precise shape... Perfect. You may assemble D=15m from seven smaller D=5m. You can make the first experiments at D=60cm with satellite TV dishes.

 

One manageable method to produce your own dishes (not my preferred choice) is to make a horizontal shape in sand using a rotated profile, and put fibreglass-epoxy on it.

 

To check material density (kg/m2), compare it with a half-wave of air. It must be significantly heavier. Mind the precision of the shape!

[Vax]

Instead of making the reflectors yourself, I suggest to use (borrow!) existing radio dish antennas without their primary source. Usually metallic, stiff and smooth, heavy enough for sound, of precise shape... Perfect. You may assemble D=15m from seven smaller D=5m. You can make the first experiments at D=60cm with satellite TV dishes.

Unfortunately, I will not be able to borrow any of the materials as they will be destroyed in a fiery blaze at the end of the (art) event.

 

 

To check material density (kg/m2), compare it with a half-wave of air. It must be significantly heavier. Mind the precision of the shape!

I have access to high-precision surveying equipment, so precision will not be a problem. I was thinking along the lines of spraying foam and carving it into 2 different forms for setting/curing the panels, to be used 12 times each per parabolic dish.

 

The material to build the panels with is still a big mystery to me. Ideally it will be sturdy and dense enough. I am looking for an adhesive solution that burns relatively cleanly.

I have budgeted $5,000 US for materials. Fortunately all labor required will be covered voluntarily.

Edited November 6, 2012 by Vax

[Enthalpy]

You might assemble smaller flat panels to make the paraboloids. They could consist of triangles of plywood screwed on a wooden truss. It even burns.

 

3400Hz suffices to transmit voice (like telephones, corresponding to 100mm wave length. +-15mm accuracy is enough.

 

Have some means of measurement and adjust the screws at the truss to 5mm, then the curvature must stay within +-10mm.

 

Take 15m focal length (define precisely where the users must stand!) or 30m curvature radius, then the triangles can have 0.55m radius or 0.95m base or 0.39m², so your 177m2 reflector needs 454 triangles and half as many adjusted holding points. Well, not so nice after all.

 

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You parabola can be a sphere. At 15m focal length and 15m diameter, the discrepancy is only +-1.5mm; a shorter focal length is possible.

 

This enables to build a tripod and check the reflector's shape with a string, or perhaps shape a sand reflector with a vacuum cleaner at the string, then use the sand form to produce the dishes. not really easy.

 

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If you use centrifugal force to shape the parabola, rotate at 1 turn in 10.8s to get 15m focal length. Cast concrete, rotate, wait until it hardens. How to rotate hundreds of tons? You find it.

 

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The dish's material could be cotton impregnated with wax or paraffin. At least, it burns cleanly.

 

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If you have 45 sectors (at least near the rim) instead of 12 then the panels can be flat between them. You could force plywood with screws to follow the spokes' curvature.

 

Did you consider the effect of wind? Your reflector is a huge sail.