The fabrication and modeling of novel, capacitive, ultrasonic air tran
sducers is reported. Transmission experiments in air at 11.4, 9.2, and
3.1 MHz are shown to correspond with theory. The transducers are made
using surface micromachining techniques, which enable the realization
of center frequencies ranging from 1.8 to 11.6 MHz. The bandwidth of
the transducers ranges from 5% to 20%, depending on processing paramet
ers. Custom circuitry is able to detect 10 MHz capacitance fluctuation
s as small as 10(-18) F which correspond to displacements on the order
of 10(-3) Angstrom, in a bandwidth of 2 MHz with a signal to noise ra
tio of 20 dB. Such detection sensitivity is shown to yield air transdu
cer systems capable of withstanding over 100 dB of signal attenuation
a figure of merit that has significant implications for ultrasonic ima
ging, nondestructive evaluation, gas flow and composition measurements
, and range sensing. (C) 1996 American Institute of Physics.