The Influence of the Acoustic Properties of Motorcycle Helmets on Temporary Hearing Loss in Motorcyclists

Noise is an unavoidable component of motorcycling. The noise sources are varied, and include the helmet itself which also filters the noise passing through it. Here helmet noise transmission characteristics have been analyzed using insertion loss measurements and loudness matching in a behavioural study. Results demonstrate the action of the helmet as a spectral filter and confirm previously published data showing attenuation in the frequency range above 500 Hz. Highlighted here for the first time is an amplification of noise below 500 Hz. The loudness matching task data allowed the generation of equiloudness functions which show the effect of the helmet on riders' perceptions of loudness. The generated curves are here compared to the relevant international standard (ISO226) and show that loudness was strongly influenced by the helmet. The noise experienced by a motorcycle rider on a 30 minute journey can result in a temporary hearing threshold shift of over 15 dB. Due to the filtering characteristics of the helmet this threshold shift is highly frequency dependent. To quantify the frequency dependence of the resulting temporary hearing threshold shift pure tone audiometry was conducted before and after exposure to white noise, with and without a helmet in a laboratory setting and after on-road noise exposure. Of particular note is the finding of increased hearing sensitivity at high frequencies following certain type of motorcycle noise exposure. The difference is discussed in the framework of the filtering characteristics of the helmet.