> This is the first time I have heard the term "Airy disc" used in the
> context of acoustics.
John,
The theory is identical with any wave focussing. Knowing the size of
the "fuzzy blob" focus area describes most of the characteristics of a
dish microphone and lets you work out what is happening.
> ... in practice it will be distorted over a measurable volume of
> space. From my perspective this practical "focus" will likely be a
> non-uniform, asymmetrical blob rather than a sphere or disc.
It is more like an apple core shape with the narrowest bit, or best
image, at the focus point. This is what you see when you are focussing
a camera lens. Bypassing the geeky explanation, the diameter of a
sound Airy disc is more or less the wavelength of the frequency being
focussed, in other words 300mm or 1 foot at 1 KHz.
Stick a small mic in this position and you will only pick up a
fraction of the reflected sound, therefore the dish gain is low, but
at 15KHz and 20mm diameter it is about the same area as the mic
capsule so the gain will be high and the focus will be narrow.
My argument with using a dish for stereo is that a parabola loses its
focus pattern off center, so why not use a spherical dish and at least
maintain the same focus at any angle? The huge wartime concrete
reflectors used on the English Channel coast to listen for German
planes were spherical.
Point a dish at the sun - with the mic(s) removed for safety -and you
can see the focus patterns with a piece of paper, or at least the
patterns for high frequencies. A spherical dish has a ring focus which
can be used for positioning the mics for stereo.
I'm writing a full length piece on sound focussing which will give
chapter and verse. :-(
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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