EAR MUFF PERFORMANCE IN IMPULSIVE NOISE AS A FUNCTION OF ANGLE OF INCIDENCE

The European directives regarding noise exposure limits require knowle dge of the attenuation of impulse noise obtained from hearing protecto rs. To clarify this matter a European joint project was performed. Our part of the project was to investigate the influence of sound inciden t angle on the attenuation of ear muffs, especially for impulse noises . The impulse response in the ear canal of 16 subjects was measured in an anechoic chamber at the incidence angles of 0 degrees to +90 degre es in steps of 15 degrees and from 180 degrees. This was made without ear muff and with three different types of ear muffs, Bilsom Viking 24 21, Peltor H7A and Willson 358A, respectively. These impulse responses were then convolved with three types of impulses. Mean and standard d eviation of attenuation values for the peak of the impulses were calcu lated (linear assumptions!). In most cases the lowest attenuation of t he peak was found at 180 degrees. Attempts were made to predict these peak attenuation values from SNR (Single Number Rating), PNR (Predicte d Noise level Reduction) and octave-band attenuation values proposed i n ISO DIS 4869-2. The latter gave mostly good (error < 2 dB) or in som e cases fairly good (error < 4 dB) predictions, the PNR-values gave fa irly good predictions except for the impulse with the main energy in t he 4 kHz octave. The SNR-values gave bad predictions except for the im pulse with the main energy around 1 kHz. This investigation supports t he idea to predict the attenuation for various types of impulse noises from attenuation data proposed in the ISO DIS 4869-2 standard, if the frequency region containing most energy is known and considered.