146 Lecture 9 
metal housing 
nickel wire 0Smmé single layer coit 
50 windings 
SSS 
Uh 
SS 
CZLTT TL TL T 
ORITTTAL AT LR 
SP NIP CLL OL. 
VILL LLL 
rubber support 
ferrite core, 1,6mm@ 
perspex tube, 3mmé 
Fig. 9.8. Nickel wire hydrophone. 
wire in the presence of a permanent magnetic field transduces the extensional 
wave in the wire into an ac voltage. The frequency-resolving power of this type 
of nickel wire hydrophone depends upon the "gap-width" of the coil relative to 
the wavelength in the wire. An arrangement similar to the piezoelectric micro- 
phone described above was, therefore, used by Kuttruff [22] and is illustrated in 
Fig. 9.8. The magnetization wave propagating in the wireis detected by a ferrite 
body at the end of the wire. A corresponding signal voltage is induced in a coil 
wound around the ferrite body. The sensitive part of this microphone, the nickel 
wire, is easily deformed. The microphone has a rather large frequency band- 
width. However, mechanical and magnetic losses in the nickel wire which in- 
crease with frequency play a more decisive part in the characteristics of this 
arrangement than do the corresponding effects in quartz. 
9.3. PROPERTIES OF LIQUIDS EXPOSED TO HIGH SOUND INTENSITIES 
9.3.1. Increase of Sound Absorption 
The first effect to be discussed is the increase of sound absorption in liquids 
which are exposed to high sound intensities. The physical reason is obvious. 
Because of the nonlinearity of certain properties of the liquid, higher harmonics 
are generated due to acoustic overload under a sinusoidal strain. The absorption 
is naturally higher for the higher harmonic frequencies than for the fundamental 
frequency which leads to an increase of the observed total loss. Figure 9.9 is 
taken from a paper by R.T. Beyer and V. Narasimhan [12]. In this figure, 
a/y? [a= absorption coefficient, y= frequency](atm/Mcps) as proposed by R.B. 
Lindsay. At low sound levels, the normal absorption coefficients are indicated; 
but at higher sound levels, a significant increase in the absorption is observed. 
The absorption rises approximately in proportion to the quotient of pressure 
divided by frequency. 
9.3.2. Generation of Higher Harmonics 
With sinusoidal excitation of sound, higher harmonics are generated by the 
nonlinear behavior of the liquid medium. Direct measurement of these overtones 
can provide information on the nonlinear characteristics of the liquid. Such ex- 
periments have been carried out in many laboratories in recent years. Es- 
