Assessment and Standardization of Personal Hearing Protection including Active Noise Reduction

The performance of passive hearing protection is normally quantified by the sound attenuation or insertion loss (IL). The IL allows prediction of the noise level at the eardrum for a given ambient noise spectrum. A subjective test (with test-signal levels at the threshold of hearing) is normally used to measure the sound attenuation. Active noise reduction requires a different assessment method. Due to self-noise and level dependency, assessment methods operating at threshold level cannot be used and have to be replaced by objective methods. The effect of high noise levels and impulsive noises may introduce a non-linear behaviour of active systems. Therefore, the use of artificial heads is applied to avoid the risk of introduction of temporary or permanent hearing loss. Comparison of results from subjective and objective test methods will be discussed. Prediction of the noise dose, representative for a certain noise condition, can be obtained by consideration of the environmental noise spectrum, the insertion loss of the hearing protector and an estimation of the variance of the insertion loss among individual users. Examples of such a prediction (by using a spreadsheet) will be given at the lecture. Speech communication quality is an important issue for use at operational conditions. The noise level at the ear is one of the major variables that define the speech communication quality. Subjective and objective assessment methods for speech communication systems will be presented and discussed. Prediction of the speech intelligibility of a communication system (in a similar condition as presented for the noise dose) will be demonstrated by using an objective intelligibility measure. Some performance measures for hearing protection, speech communication and criteria for speech quality are standardised by international bodies. International standards are provided by ISO, CEN, and IEC.