Fundamentals of Acoustics

Description

Typical examples of such a force include the tensile force produced when a spring is stretched, the increase in pressure produced when a fluid is compressed into a lesser volume, and the trans- verse restoring force produced when a point on a stretched wire is displaced in a direction normal to its length. It is the action of this elastic restoring force, coupled with the inertia of the system, that enables matter to participate in oscillatory vibrations and thereby generate and transmit acoustic waves. Many different types of vibration occur in the generation and propagation of acoustic waves. In a narrow sense, their frequency is limited to the range from about 20 cycles/sec to 15,000 cycles/sec, which produce the auditory sensation of sound for the average person. However, in a broader sense they also include both the ultrasonic frequencies above 15,000 cycles/sec, which although inaudible have important practical applications in numerous fields, and the inaudible infrasonic frequencies below 201 cycles/sec. The modes of vibration range from the simple sinusoidal. vibrations produced in the adjacent air by a mounted tuning fork vibrating at its fundamental frequency, through the complex pattern of vibrations generated by a bowed violin string, to the nonperiodic vibrations associated with a noise or an explosion. In studying such vibrations it is advisable to begin with the simplest type, i.e., a sinusoidal vibration that has only a single frequency component.