ABSTRACT
Determining the source strength of airborne sound is the most difficult part of any noise reduction program of a machine. However, it is possible to model certain sources so that their characteristic features will help identify the source of sound in a machine so we can take appropriate measures to reduce sound. Among these sources, monopole, dipole, and quadrupole are very important. This chapter discusses a baffled piston, which serves as an important acoustic source in many applications. For modeling the source of sound, the concept of velocity potential can be advantageously used. Just like dipoles were obtained combining two monopoles vibrating out of phase, it is possible to obtain quadrupoles by combining two dipoles vibrating out of phase. Quadrupole sound sources are generally found in gas flow due to viscous stresses within gas. The sound power of quadrupoles can be obtained by considering the action of two dipoles vibrating out of phase with each other.
