This probably won't work but I'm trying to understand:
Why can't we use a horn coupling to the transducer?
My understanding is that is what a horn does, it is a air
transformer which couples compacted air in the throat to the
ordinary air at the mouth of the horn.  Are the frequencies too high
(~40K?) and the wavelengths too short to be useful?

>----------
>From:  Robert Nansel[SMTP:bnansel@NAUTICOM.NET]
>Sent:  Thursday, November 20, 1997 2:52 PM
>To:    PICLIST@MITVMA.MIT.EDU
>Subject:       Re: Serial Sonar Unit (SSU)
>
>Thur 11/20/97 Wayne Foletta wrote:
>
>
>>From graduate research work I and my colleagues did at Stanford on
>>medical ultrasound (both CW and pulsed Doppler) in the early 70's we
>>found:
>>1. Impedance match transducer to circuit and transmission medium (be it
>>air or water) for highest sensitivity.
>
>This is the big trouble with using piezo transducers in air. Piezos are
>"stiff" transducers, that is, they generate large forces but small
>displacements; they couple very nicely into like-stiffness media, such as
>water or solids, but poorly into a low-density medium like air. Another way
>of saying this is that there is a large impedance mismatch between air and
>piezo. I've heard some folks have done stuff with impedance matching
>layers. One group I read about used a one-quarter wavelength silcone-rubber
>matching layer to achieve better coupling into air. They were working at
>1-2 MHz, though the principle should be applicable to lower frequencies
>with suitable adjustments in dimension and material.
>
><Snip>
>