Amplitude-sensitive attenuating earplugs

Abstract

The most simple amplitude-sensitive attenuating earplug is made of a plate with one little hole that is inserted in a perforated plug (like Racal Gunfender). That hole presents an acoustic impedance which has essentially a viscous resistance and a nonlinear dependence on the particle velocity in its center [L. J. Sivian, J. Acoust. Soc. Am. 7, 94–101 (1935); U. Ingard and H. Ising, ibid. 42, 6–17 (1967)]. The dimensions of that hole have been optimized and a new configuration, i.e., an empty cavity with two perforated plates, has been designed. This ‘‘filter’’ of small dimensions can easily be fitted into different commercial perforated earplugs. To better understand the experimental results and optimize the acoustic nonlinear ‘‘filter’’ dimensions, a theoretical model has been developed. Moreover, an experimental setup is described which allows visualizing the particle velocity inside the cavity with a particle imagery velocity camera which helps to understand the actual behavior of such a nonlinear cavity.