Acoustic Mode Shapes Research Articles

Estimation of possible excitation frequencies for shallow rectangular cavities.

The physical processes that sustain discrete-frequency oscillations of cavities adjacent to compressible flow are modeled analytically, yielding a formula which predicts excitation frequencies as a function of Mach number and cavity geometry. These physical processes are similar to those used by Powell in describing the mechanism underlying the production of edge tones. The empirically determined constants appearing in Rossiter's formula for excitation frequencies are computed from the model. It appears that instability of the shear layer as well as interaction between the shear layer and the cavities' trailing edge is required to sustain discrete-frequency oscillation. It is suggested that the simultaneous excitation of two or more discrete frequencies (which are not harmonic), as have been observed in practice, correspond to the simultaneous participation of two or more vortex sheet displacement modes. The model analyzed yields qualitatively correct acoustic mode shapes in the cavity. Theoretical results include the calculation of possible excitation frequencies over the range 0.8 ^ M ^ 3 for rectangular cavities and show their dependence on cavity depth. Analytic results are in general agreement with experimental data.

Point-to-point correlation of sound pressures in reverberation chambers

The reverberation chamber is such a fundamental tool in general acoustics and, in particular, architectural acoustics, that most of its properties have been under intensive study for some years. However, the point-to-point correlation of the sound pressures has received little attention except for an elegant paper by Cook et al. published in 1955.The behavior of the point-to-point correlation with position and direction is a measure of the diffuseness of reverberant field. It is also an indication of the minimum permissible spacing of microphones used to obtain independent estimates of sound pressure level. It is undoubtedly the limiting factor in the realism of high-intensity noise tests as applied to major sections of missiles or space vehicles. A highly correlated field tends to be selective in the excitation of structural modes. The favored modes depend on the nature of the correlation. The reverberant field tends to be non-selective. One would like its correlation to conform as closely as possible to some standard formula, so as to simplify the estimation of correction factors relating the test severity to the severity of excitation in flight.By means of a calculation in terms of the actual acoustic mode shapes of a rectangular chamber, the present paper shows that under certain conditions the point-to-point correlation can can asymptotically approach Cook's formula. However, significant fluctuations away from the formula may be expected at the lower frequencies. Furthermore, a sound source position away from the comer, and in particular a position of high geometric symmetry, can result in selectivity in the excitation of acoustic modes, and large deviations from the formula. Similarly, an array of highly correlated sound sources can result in deviations.