Sound Intensity

Abstract

Sound intensity is a vector that describes the flow of acoustic energy in a sound field. The idea of measuring this quantity directly, instead of deducing it from the sound pressure on the assumption of some idealized conditions, goes back to the early 1930s, but it took about 50 years before sound intensity probes and analyzers came on the market. The introduction of such instruments has had a significant influence on noise control engineering. This chapter presents the energy corollary, which is the basis for sound power determination using sound intensity. The concept of reactive intensity is introduced, and relations between fundamental sound field characteristics and active and reactive intensity are presented and discussed. Measurement of sound intensity involves the determination of the sound pressure and the particle velocity at the same position simultaneously. The established method of measuring sound intensity employs two closely spaced pressure microphones. An alternative method is based on the combination of a pressure microphone and a particle velocity transducer. Both methods are described, and their limitations are analyzed. Methods of calibrating and testing the two different measurement systems are also described. Finally the state of the art in the various areas of practical application of sound intensity measurement is summarized. These applications include the determination of the sound power of sources, identification and rank ordering of sources, and the measurement of the transmission of sound energy through partitions.