Calculation is made of the transmission of harmonic compressional waves through a finite solid filter structure consisting of a metal rod loaded at equal intervals with heavy metal collars. The theory is then tested experimentally for the special case where the finite structure (consisting of 6 sections) is terminated by a finite rod with a free end. The structure acts as a low pass filter with a cut-off at about 2000 cycles and no further transmission up to 12,650 cycles. The transmission in the transmission band shows a series of peaks whose positions, with one exception, are accurately verified experimentally. 42 R. B. LINDSAY AND A. B. FOCKE
The absorption of supersonic waves in both water and in an aqueous suspension of lycopodium spores was measured at seven different frequencies ranging from 990 kilocycles to 2500 kilocycles in approximately equal steps. A piezoelectric quartz plate was used in each case as a sound source and was driven by an electron-coupled oscillator and two-stage amplifier capable of yielding 100 watts. The radiation pressure of the sound waves against a cone suspended in the field was measured directly with a chainomatic balance. The differences in the absorption coefficients found for lycopodium suspension and for water were compared with the classical theoretical expectation as derived by Sewell. These differences approach more nearly the theoretical value as the frequency increases. This may be due to the relative nonrigidity of the spores at lower frequency-a situation which would violate the theoretical boundary conditions.
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