[填空题]
A dramatic demonstration, ca ,utt)og 5aqjg- q8p:mlled “singing rogra4hc+ ;sd n:ds,” involves a long, slender aluminum rod 4rads;c+n:hg held in the hand near the rod’s midpoint. The rod is stroked with the other hand. With a little practice, the rod can be made to “sing,” or emit a clear, loud, ringing sound. For a 90-cm-long rod,
(a) what is the fundamental frequency of the sound? $ \times10^{ 3}$ Hz
(b) What is its wavelength in the rod, m
(c) what is the traveling wavelength in air at 20 $^{\circ} $ C? m
