Wave Effects in Isolation Mounts

Abstract

Both theoretical and experimental studies of wave effects in isolation mounts have been made. From the standpoint of vibration isolation, wave effects are important in the sense that the vibration isolating properties of a mount are changed by their presence. The well-known “lumped parameter” theory of vibration mounts predicts that the vibration isolation of a mount increases at 12 db per octave for frequencies well above the resonant frequency of the spring-mass system. This theory holds true only when the wavelength of the elastic wave in the mount is large compared to the dimensions of the mount. Standing waves occur, as would be expected, which in certain frequency ranges decrease the vibration isolation properties of the mount by as much as 20 db. For practical mounts, wave effects are most detrimental in the most audible frequency range (500 to 1000 cps). The theoretical and experimental treatments are in good agreement, and indicate various methods for improving the vibration isolation properties of the mount. Experimental data concerning isolation mounts fabricated of various materials are presented.