  We used an optomechanical sensor to study the ultrasound generated by
manually operated piezoelectric spark igniters. These low-energy sparks produce
short-duration acoustic shock-wave pulses, with sub-microsecond rise times and
frequency content extending well beyond 2 MHz in air. The same source-receiver
combination was then used to demonstrate broadband characterization of solid
(polymer and glass) plates in a simple setup, where single spark events yielded
high-SNR data without the need for critical alignment. This setup also enabled
us to estimate pressure excursions approaching 105 Pa at millimeter-scale
distances from the spark. The results are in large part made possible by the
small size, wide bandwidth, and high sensitivity of the optomechanical sensor,
and might be of interest for air-coupled ultrasound applications in
non-destructive testing.
