Impulse Noise

In their review Clifford and Rogers (1) come to the conclusion that workers in industries with impact noise, as well as soldiers exposed to supersonic blasts from armament and explosive devices, appear to be more at risk for hearing loss than are their counterparts exposed to continuous noise. Alternative considerations for hearing protection are dictated because of a disproportionately increased biophysical response in comparison to continuous noise. Impulse noise is a significant and distinct problem that requires a new strategy for hearing protection.

A review of current clinical and occupational literature suggests that impulse noise may be more damaging than continuous sound. Statistical measurements such as kurtosis hold promise for the quantitative prediction of hearing loss. As sound energy to the cell increases, the mechanism of cochlear damage shifts from biochemical injury to mechanical injury. Outer hair cells appear to be more sensitive than inner hair cells to impulse noise because of their energy requirements, which lead to increased production of reactive oxygen and nitrogen species and self-destruction by apoptosis. Hearing protective devices currently in use for impulse noise include hunters’ hearing devices, active noise-reduction headsets, and various in-ear plugs, including nonlinear reacting inserts.

Existing equipment is hampered by the materials used and by present-day electronic technology. Antioxidants administered before sound exposure show promise in mitigating hearing loss in industrial and combat situations. New materials with improved damping, reflective, and absorption characteristics are required. Hearing protective devices that allow passage of ambient sound while blocking harmful noise might improve the compliance and safety of those exposed. Sensing devices that instantaneously and selectively hyperpolarize outer hair cells are discussed as alternate protection.

In the study “Effects of impulse noise and continuous steady state noise on hearing”(2) the effects on hearing induced by occupational exposure to impulse noise were compared with those induced by exposure to continuous steady state noise.

Three groups exposed to impulse noise, one group exposed to continuous steady state noise, and an unexposed control group were studied. The hearing thresholds of the groups were measured by a puretone audiometer three times in two workdays. None of the groups showed significant differences between the hearing thresholds measured in the morning, at midday, and in the afternoon.

Group 1 with the shortest duration of exposure and group 2 with the intermediate duration of exposure to impulse noise had the highest thresholds at 6000 Hz in both ears.

Group 3 with the longest duration of exposure to impulse noise had the highest thresholds asymmetrically, at 4000 Hz in the left ear and at 6000 Hz in the right ear. The group exposed to continuous steady state noise also had the highest thresholds asymmetrically, in the left ear at 6000 Hz.

It was concluded that the longer the duration of exposure to impulse noise the wider the region of the frequencies that showed raised threshold shifts in both ears. Impulse noise seemed to produce permanent threshold shifts at 4000 and 6000 Hz after a shorter duration of exposure than continuous steady state noise.

Source:

1. Impulse noise: theoretical solutions to the quandary of cochlear protection

R.E Clifford and R.A. Rogers
Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts, USA. (2009)

2. Effects of impulse noise and continuous steady state noise on hearing

S Mäntysalo and J Vuori
The Institute of Occupational Health, Department of Psychology, 00290 Helsinki, Finland (1984)