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Old January 19th, 2015, 11:28 AM   #1
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Acoutic fresnel reflector ??


Please forgive the mis-spell in the topic title which I cannot correct. My keyboard is old and of a dead-end mechanical design of cheap plastic on cheap plastic sliding surfaces which get gritty and bind. One day I'll buy a new one.

This is in effect a double post which evolves out of a parallel discussion on parabolic mikes but impinges upon a different acoustic mechanism and possible theory. If the webmasters delete this one I shall not be all fraught and upset.

Those of us who are older may be familiar with the effect of a large vertical area of corrugated wall or fence upon a sharp explosive sound. There is an artifacted echo return which is time-stretched but also reproduced as a sharp high pitch. Corrugated iron is a sinewave shape.

My assumption is that the high pitch is created by the incremental arrival of each short intense echo in rapid succession over time from each of the angular corrugated surfaces which collectively direct their echo towards a common point. That point is not apparently critical along the wall's length so long as there remains sufficient wall area to provide the return.

More critical it seems is the distance from the wall of the listener in attenuating that return. Too close and each peak in the corrugation will mask the echo from its neighbour, thus eventually attenuating by reducing the effective reflective area to the listener.

Too far and the return is more akin to a normal echo, far less time-stretched as all reflecting elements of each corrugation are more equidistant from the listener. There seems to be a sweet spot distance from such walls.

So for the scientists among you, my question is this, could much lower audio frequencies be amplified to a listening point by superimposition of each individual increasingly delayed echo from the sine-shaped upright corrugations. I guess there would be all manner of frequency bands of amplifying and nulling.

If unwanted higher "artifact" peaks were graphed out of the audio recording, could an amplified lower frequency recording be constructed from that echo return additive to the direct sound from a distant object like an aircraft?

I definitely heard an enhanced return from a corrugated airport hangar wall, of sound of turbulent airflow over an aircraft's stalled wing. My subjective estimate is that there might have been as much as 12db worth.

The aircraft was in a favorable position for sound to be reflected off a gently sloping but almost planar paved apron angled towards the aircraft, reflected up at a shallow angle to the corrugated iron wall then concentrated back to my head-height where I was camera-operating.

I have been possibly chasing my tail playing with a parabolic reflector mike which amplifies higher audio frequencies.

A mike, or even a row of them along a "sweet spot" line parallel to the corrugated hangar wall and flight of the performing aircraft in a "sweet zone" might do the trick with the lower frequencies of air-buffeting and airflow detachment from a wing of an aircraft flying aerobatic figures.

Any advice will be appreciated.

Last edited by Bob Hart; January 19th, 2015 at 07:58 PM. Reason: error
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Old January 19th, 2015, 12:27 PM   #2
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Re: Acoutic fresnel reflector ??

If you do a Google search for: acoustic lens and click the [Images] option, you will see a wide variety of techniques, theories, and gadgets.
I remember back when JBL used acoustic lenses resembling Venetian blinds over the front of their HF horns to more widely disperse the HF content. But they have invented very clever variations on horn technology (such as the "Image Control Waveguide" in their LSR305 monitors which are getting rave reviews) to achieve even better results than those "acoustic lens" techniques from decades ago.

There are also people working with arrays of small microphone elements (like Panasonic WM-61) and using them like those flat "phased array" antennas which are now rather common in radar units.

However, both of those are only dealing with the DIRECTION issue, and neither has any actual acoustic GAIN as you would get from a parabolic reflector. Of course, if what you are recording is loud aircraft (or steam engines, etc.) then maybe acoustic gain isn't much of an issue.
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Old January 19th, 2015, 08:07 PM   #3
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Re: Acoustic fresnel reflector ??


Thank you for your response. Interesting stuff. Like the parabolic reflection system, it seems the higher frequencies are favoured with industrial application in the ultrasonic frequencies being the most advanced in development.

A parallel stripline of mikes in a suitable position along a corrugated iron wall might be a goer but incapable of portability. Unless the mikes were on some sort of demountable clothesline pulled up tight, it would be an impractical and dangerous item to have set up on an airport apron where there is no local air-traffic control.

Focused stereo might be a possibility which is extremely difficult with parabolic systems.
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