Effects of cup, cushion, headband force, and foam lining on the attenuation of an earmuff

Abstract

The use of hearing protectors is considered the only short-term solution available to severe noise problems. In this paper the noise leak pathways through the protector to the ear channel were investigated. Noise attenuation measurements were carried out for a circumaural earmuff protector to quantify the effect of various parameters and modifications on the earmuffs, such as: internal foam lining, vibration of earmuff cover, type and force of contact between the earmuff and the head face. The assays were undertaken using an artificial head according to ANSI S12.42-1995 and a diffuse sound field according to ISO 4869-1. The coherence function between cup vibration and sound pressure inside the cup has shown that a great amount of sound energy is transmitted to the inside of the cup at low frequencies: 100–320 Hz. The foam lining of the cup clearly attenuated the sound in the frequency range between 2000 and 8000 Hz. The attenuation measured with a complete earmuff and the same without the cushion has shown that sound leakage occurs at frequencies ranging from 125 to 2000 Hz. A Vaseline coating between the head surface and the cushion improved the attenuation by less than 5 dB (100–200 Hz). As the headband force increases to a maximal deformation of the cushion, attenuation reaches 20 dB at 200 Hz. The increase in the headband force is however a limiting factor to the use of the earmuff, as it can get too uncomfortable. In this work the bone conduction is negligible.