> I'm curious to know if you have something to suggest for generating such
> kind of wideband noise without a speaker=C2=85.
Gianni,
For high frequencies and ultrasound (testing bat detectors) I use compresse=
d
air from a nozzle that doesn't "whistle". I've used a 12V car tyre inflator=
which pulses, but this helps to smooth the average spectrum. One effect you=
get is from close reflections like the ground or buildings, so you need to=
use it outdoors and recognise those reflections.
For mid to high frequencies, a garden hose gives a fairly smooth but uneven=
spectrum, but you can still compare mics.
For low frequencies if you don't have access to a large waterfall, you will=
need to use loudspeakers, and allow for the colouration they produce. For
mic testing, we used to use a padded box alongside a mic with a known
characteristic in comparison.
The ideal is an ancheoic chamber, but not everyone has one of those. :-)
Mic manufacturers will compare their mics to a known instrumentation mic
like a B & K, also using warble tones to even out resonances. As a
consequence, all published mic response curves have been "smoothed" out.
That why different "flat" mics don't all sound the same. Not many people
realise that. :-(
David Brinicombe
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