The ageing effect on the acoustic performance of earmuffs: an investigation involving the forestry compartment

A study was undertaken on 26 earmuffs of the same model used by forestry workers, in order to investigate the change of their acoustic performance with age. Some of the hearing protectors were tested when they were new, other ones after two different periods of use. The measurements were performed in the laboratory by means of a Neumann KU 100 dummy head equipped with ear simulators including microphones. Two loudspeakers, driven by a signal generator, produced a random incidence field. The test signal consisted of a 90 dB pink noise and measurements were carried out in one third octave bands within the range 125-8000 Hz, according to the frequency response of the system chosen for the signal reproduction. The right and left cups of each earmuff were simultaneously characterised by determining the insertion loss of the test signal, according to a procedure similar to that indicated in ISO/TR 4869-3 standard. Results showed that the main factor controlling the acoustic performance of the used earmuffs was the presence of damages, causing a deterioration which became more and more serious with increased usage. When the hearing protectors were in good condition, on the contrary, the insertion loss reductions due to their ageing were quite small and limited at the mid-high frequency range. In addition, no progressive significant deterioration appeared with increased usage.