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In article <1990Dec6.002534.18546@vicorp.com>, ron@vicorp.com (Ron Peterson) writes:
> >I am thinking about building an electronic instrument.
> >My problem is the mouthpiece.  What should I use?  What I need is some
> >kind of device that outputs a digital or analog signal proportional to
> >the pressure of an airstream applied to it.  The sensor should be cheap
> >and immune to moisture and should respond to changes quickly.
> >have any suggestions?
>
> A breath controller option is available for DX-11 (and probably other)
> synthesizers.  It is a headset like device with a small tube that extends
> out to a small whistle-like mouthpiece.  The tube has a wire running through
> it so that it can be bent to accomadate different head sizes.  The air
> you blow into the mouthpiece presses against a resistive strain gauge
> which can be read electronically.

	I'll offer a few off-the-wall suggestions for the design of an
electronic mouthpiece.  Readers should bear in mind that I don't claim
any particular expertise in the design of electronic musical instruments,
with my last project being a theramin about 25 years ago.  Also, my
personal musical ability is limited to piano, organ and Polymoog.  I
could not play a wind instrument to save my life. :-)

	It would seem to me that one wants to sense more than a simple
gage pressure change.  I suspect that air *velocity* with a directional
vector is more desireable.  Velocity can be sensed using a differential
pressure transducer connected across both sides of an orifice plate,
thereby providing a signal which varies in both magnitude and polarity.

	Low-cost differential pressure transducers employing silicon
strain gage technology providing say, 100 in-H2O FS range are readily
available.  Frequency response to several hundred Hz is possible.  Such
transducers are often intended for the HVAC market, and are available
from such vendors as Honeywell, SenSym, etc.  It is possible to purchase
suitable transducers in single quantity for $ 75.00 or less.

	One should be careful in selecting a transducer which will withstand
moisture without damage.  Some differential pressure transducers have only
"dry" ports, while others have one "wet" port and one "dry" port.  In this
application, *both* ports should be rated as "wet".

	In addition, the pressure transducer should be connected to the
pressure ports across the orifice plate using tubing that is as short
and large in diameter as possible to assure maximum frequency response.
	
>    If I remember correctly, playing a sax involves using the tongue or
> teeth to damp the reed also.  Maybe a waterproof microswitch or another
> strain gauge could be used for that.

	A possibililty here is to mold an outer mouthpiece covering using
silicone rubber or polyurethane, such that a *hollow* chamber is formed
between the covering and the body of the mouthpiece.  This hollow chamber
would be filled with silicone fluid and connected to a gage pressure
transducer such that increased mouth or teeth pressure against the
mouthpiece results in an increased amplitude from this pressure transducer.

	For initial experimentation, a low-cost disposable pressure
transducer intended for arterial and venous blood pressure measurements 
would be quite suitable.  H-P, Spacelabs, Mennen Medical and Graphic
Controls all offer such products which typically sell to hospitals for
between $ 25.00 and $ 35.00.  While it might be difficult to purchase
just one device, a "friend" working in a hospital might be able to procure
one.  While such a medical transducer only has a single pressure port and
is not truly differential, most of such products will sense a negative
pressure.  Electrically, this type of device will look like a four-arm
bridge.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231   {boulder, rutgers, watmath}!ub!kitty!larry
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