Direction-Dependent Insertion Loss of a Full-Face Motorcycle Helmet

The acoustic performance of a motorcycle helmet is normally evaluated as a single insertion-loss spectrum obtained for one direction of sound incidence. Such a description says nothing about how the attenuation varies with the direction from which sound arrives, nor about what the helmet does to the level difference between the two ears on which horizontal localisation depends. This work sets out a measurement and analysis approach for the missing description and applies it to one full-face specimen, which serves as the demonstration case and as a reference data set rather than as a sample representing helmets as a class. In this work, the insertion loss of a full-face helmet is, therefore, obtained binaurally by employing a head-and-torso simulator in a hemi-anechoic room, at closely spaced azimuth positions across the frontal horizontal plane and for the unhelmeted, closed-visor and open-visor conditions. The complete matrix is acquired in two independent sessions, each with a separate fitting of the helmet, and every derived quantity is reported together with the range it takes across those two fittings. Insertion loss is strongly frequency-dependent and becomes negative at low frequencies, where the enclosed cavity amplifies the sound field, and its magnitude varies with the direction of incidence. Closing the visor adds attenuation at mid and high frequencies but deepens the low-frequency amplification, which is consistent with the cavity-resonance interpretation. Referring to the ear under consideration, the broadband insertion loss increases towards the side on which the source lies. Fitting the helmet compresses the interaural-level differences available across the frontal hemisphere and flattens their variation with azimuth angle near the median plane. Both fittings reproduce the compression. The span of the band-averaged interaural-level difference falls to 32% of the unhelmeted value in one of them and 33% in the other, while the mean of the individual per-band spans between 500 Hz and 8 kHz falls to a fraction lying between 43% and 60% depending on the fitting, so that the direction of the effect is a property of the specimen, and the per-band magnitude is, in part, a property of the fitting. The results are reported as measured insertion loss and interaural-level-difference changes for this specimen; no listening experiment was performed, and no inference is drawn about localisation performance or about full-face helmets as a class. A directional, two-eared characterisation nevertheless yields information that single-direction reporting cannot, and the protocol and the data given here are intended as the reference against which further specimens can be compared.