Abstract:

The number of modes below the limit frequency, as well as the modal density as a function of frequency, has been provided for acoustic enclosures. The exact number of modes can be obtained using the Blevins model for rectangular acoustic enclosures. The Blevins model is particularly useful for the estimation of number of modes in the mid and lower frequency range. The exact results are compared with the approximate averaged results of the Maa model. The average cumulative number of modes and average modal densities obtained by Blevins model are in agreement with the Maa results in the region of ten times the frequency of the first oblique mode, provided that the shape of the enclosure does not deviate significantly from the cube form, as would happen in the case of a long corridor. For the purpose of applications in Statistical Energy Analysis the approximate formulae of Maa and Courant are adequate. A computer program that estimates the number of modes and the modal density of rectangular enclosures in one-third octave bands is available.