The Doppler effect is a familiar phenomenon in linear acoustics. The nonlin
ear Doppler effect is also possible when a moving target exhibits nonlinear
responses. In: these cases Doppler frequency shifts arise at harmonics or
combination frequencies in the scattered waves. For collinear geometry of p
rimary beams, the nonlinear Doppler frequency shifts at the second harmonic
and the combination frequencies are found to be the same as those for the
linear scattering of those frequency components by the target. However, for
highly, noncollinear primary beams, the difference frequency Doppler frequ
ency shift can greatly exceed the linear one. This phenomenon is mainly stu
died in the present work. Experimental verification done with flowing bubbl
es in a tube shows good agreement with the theory. The suggested difference
frequency Doppler technique can find applications in medical diagnostics o
f blood flow as well as in industries and in oceanology. (C) 1999 Acoustica
l Society of America. [S0001-4966(99)03709-1].