THEORY OF MODULATED AIR STREAM SOUND SOURCES
调制气流声源的原理

Modulation of air stream is an efficient method of sound production. Man's speech organs, sirens, electro-pneumatic loudspeakers as well as common whistles are all based on this principle. The features of air stream sound sources are their high efficiency and possibility of high power outputs, which afford them practical value. The sound production by modulating air streams were discussed by previous authors and theories-given, but some were oversimplified and others too involved to allow practical computations. In this paper, air flow vs pressure characteristics of air stream sources are derived from the basic theory of gasdynamics and graphical method is presented for the estimation of sound radiation under given conditions of plenum pressure and nozzle-to-horn throat area ratio. It is established that although the air flow characteristics as well as the radiation relations are nonlinear, the radiated sound power is determined mainly by the radiation of the fundamental, and approximations give rise to simple expressions for the radiated sound power, the air stream yield and efficiency, with an error less than 1 dB from exact results. Charts are given to facilitate design and question of optimum condition is discussed.