MEASUREMENT USING SOUND

Principles of Sound

Sound waves are small fluctuations in pressure, density, and velocity that can propagate through matter of any form: solid, liquid, or gas. Unlike electromagnetic waves such as light (see the section on light), sound cannot propagate through a vacuum. Sound is called a longitudinal wave because the motion of the particles occurs in the same direction as wave propagation. In contrast, waves on the surface of the ocean are transverse waves because the motion of the particles is mainly perpendicular to the direction of wave propagation ( Fig. 30-23 ).

In 1842, Christian Johann Doppler first described the apparent change in pitch of a sound that occurred when either the source of the sound or the listener was moving.^[14] This Doppler effect now has several applications in patient monitoring, including precordial and esophageal Doppler ultrasound monitoring of local blood velocities or cardiac output.

Figure 30-23 Transverse (A) and longitudinal (B) waves. In transverse waves (e.g., ocean waves), the particles move perpendicular to the motion of the wave. In longitudinal waves (e.g., sound waves), the particles move back and forth in the direction of the wave. The actual matter in either wave does not move much. The energy transfers without mass transfer.

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When a sound source is moving toward the listener, the apparent pitch increases, and vice versa. The amount of frequency shift depends on whether the listener or the sound source is moving ( Fig. 30-24 ; see Appendix 7 ). Because changes in the frequency of sine waves can be measured precisely, the Doppler principle provides a very accurate method of measuring the velocity of moving sound reflectors. At the high frequencies often used (≥5 MHZ), objects as small as red corpuscles can scatter enough sound for detection.

Sound has been used for many years in medical diagnosis and monitoring. It can be used as a diagnostic method in two ways: passive and active. In a passive examination, the sounds generated by the patient are studied. The basic examination of this type uses the stethoscope. In an "active" examination, acoustic energy is transmitted into the patient, and the resulting interaction of this energy with the patient is analyzed for information. Both types of examination use the same physics principles.