> The main problem is that I have no anechoic chamber,
Ivano,
I go under a pile of bedclothes for mic HF noise tests.
> however I have a frequency generator up to many MHz and a 300 MHz oscilloscope
The weak link I've found is a sound emitter. One solution was a ceramic
tweeter with the horn removed. I've also used electrostatic tweeters but not
sure if you can still get them. Also try headphone earpieces, but nothing
gives a flat response over a wide ultrasound range. While experimenting, I
had the benefit of a bat detector with a wide frequency range. I was using
Knowles capsules, as was my bat detector.
> I have a vacuum pump but nothing in which to put the array to test the vacuum!
You need a bell jar or equivalent on a rigid base. I got hold of a bit of
half inch thick aluminium plate but it probably doesn't have to be that
thick. The vacuum pipe and sealed (solid) wires connect through this.
I used a large gallon glass jar, but these are usually plastic nowadays. You
have to be careful that they will stand a vacuum. Alternatives are a Perspex
tube closed with a quarter inch Perspex plate glued onto one end.
Alternatives are a hemispherical glass mixing bowl or similar ovenware. Bear
in mind that the force over a 25cm diameter is about half a tonne.
You need to form an airtight seal by either grinding the base of the "bell
jar" against the base plate or making a flexible seal. Note that the harder
surface grinds away faster than the softer face using grinding paste.
Possibly a toughened glass bowl will not like this and an alternative is to
make a silicone rubber seal, using smooth clingfilm to get one non-sticky
face. The vacuum joint is finally sealed with silicone grease.
All the above is of course assuming that your mic capsules can withstand a
vacuum.
David Brinicombe
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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