Hi David,
Thanks for elaborating, I will look forward to your sound focusing piece.
To give a rough estimate of the capsule spacing in the PCM-M10, I have measured
the microphone grills at about 57mm on center. I don't use a dish very often so
this is a bit of a diversion for me, however if it would be useful for anyone
here, I could mount my M10 in my Telinga dish and see what happens.
John
--- In
<naturerecordists%40yahoogroups.com>,
"Avocet" <> wrote:
>
> > 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
>
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
------------------------------------------------------------------------
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/naturerecordists/
<*> Your email settings:
Digest Email | Traditional
<*> To change settings online go to:
http://groups.yahoo.com/group/naturerecordists/join
(Yahoo! ID required)
<*> To change settings via email:
<*> To unsubscribe from this group, send an email to:
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
------------------------------------------------------------------------
|