ABSTRACT
Sound consists of mechanical waves. Unlike electromagnetic waves, sound needs a medium through which to propagate. Sound propagates by alternately compressing and expanding the particles of the medium. Sound waves can be displayed graphically as local variations in acoustic pressure against time (Figure 1.1). The time from the peak of one cycle to the peak of the next is the wavelength. Wavelength is related to frequency:
Wavelength propagation velocity/frequency
The speed at which sound passes through a medium, i.e. its propagation velocity, is related to the density of the medium and is greater through dense materials. Sound intensity (power per cross-sectional area) is attenuated as it travels through a medium. Most of the energy loss is through heat resulting from the compression of the medium. Attenuation increases as frequency increases. Energy loss is measured in decibels per centimeter per megahertz and ranges from 0.5 dB/cm to 1.1 dB/cm/MHz in soft tissue. The acoustic impedance of a material is the product of its density and propagation velocity and is measured in rayls (one rayl is equal to one kilogram per square meter per second).
