ABSTRACT
The control of low-frequency noise and vibration has traditionally been difficult and expensive and, in many cases, not feasible because of the long acoustic wavelengths involved. If only passive control techniques are considered, these long wavelengths make it necessary to use large mufflers and very heavy (or very stiff and light) enclosures for noise control, and very soft isolation systems and/or extensive structural damping treatment (including the application of vibration absorbers) for vibration control. As long ago as the 1930s, the idea of using active sound cancellation as an alternative to passive control for lowfrequency sound was first proposed by Coanda in patents published in 1931 and 1934, but with incorrect explanations of the physics. In 1936 and 1937, patents were published by Paul Lueg that contained correct explanations and used the idea of cancellation of sound propagating in a duct as an illustration of the principles. This work is often cited as the beginning of active noise control. The idea was to use a transducer (control source) to introduce a secondary (control) disturbance into the system to cancel the existing (primary) disturbance, thus resulting in an attenuation of the original sound. The cancelling disturbance was to be derived electronically based upon a measurement of the primary disturbance. It is this alternative means of noise and vibration control which emerged from such modest beginnings to which this book is dedicated.
