This paper describes preliminary observations of ultrasonic wave propagatio
n in air-saturated defatted cancellous bone from the human vertebra. Using
a broadband pulse transmission system, attenuation and phase velocity were
measured over a wide frequency range (100 kHz-1 MHz). The observed behaviou
r was consistent with that expected for the decoupled slow wave predicted b
y Blot's theory. Velocity was lower than that of free air, and there was ma
rked frequency-dependent attenuation and velocity dispersion. The tortuosit
y (alpha) of the trabecular microstructure was estimated from the high freq
uency limit of the dispersion curve, with a mean value of alpha=1.040+/-0.0
04 obtained in five specimens. Ultrasonic measurements in air represent a v
aluable new approach, capable of yielding parameters that directly characte
rise bone structure. Furthermore, they may give useful insights into wave p
ropagation in bone in vivo, where the trabecular framework is saturated wit
h marrow fat rather than air. (C) 1999 Elsevier Science B.V. All rights res
erved.