Nigel Holt

Aeroacoustic sources of motorcycle helmet noise

The prevalence of noise in the riding of motorcycles has been a source of concern to both riders and researchers in recent times. Detailed flow field information will allow insight into the flow mechanisms responsible for the production of sound within motorcycle helmets. Flow field surveys of this nature are not found in the available literature which has tended to focus on sound pressure levels at ear as these are of interest for noise exposure legislation. A detailed flow survey of a commercial motorcycle helmet has been carried out in combination with surface pressure measurements and at ear acoustics. Three potential noise source regions are investigated, namely, the helmet wake, the surface boundary layer and the cavity under the helmet at the chin bar. Extensive information is provided on the structure of the helmet wake including its frequency content. While the wake and boundary layer flows showed negligible contributions to at-ear sound the cavity region around the chin bar was identified as a key noise source. The contribution of the cavity region was investigated as a function of flow speed and helmet angle both of which are shown to be key factors governing the sound produced by this region.

Noise mechanisms in motorcycle helmet noise

A unique set of results on the acoustics of motorcycle helmets has been gathered during road tests on a rider wearing a representative modern helmet. The data were collected during a study of the noise which can cause hearing damage and, possibly, distraction in riders. They consisted of simultaneous measurements of noise at the rider’s ear and unsteady pressure on the helmet surface, combined with GPS measurements of rider position and speed. These signals have been analyzed to reduce the coherent structures in the turbulent flow responsible for noise generation. The identified structures appear to be produced by a vortex street shed by the motorcycle windscreen. The internal and external pressures proved to be poorly correlated over most of the frequency range, which has been identified as a result of the insertion loss of the helmet. The implications of these findings are that the majority of the variation in helmet noise is a function of such extrinsic factors as motorcycle configuration and rider build and posture. Efforts to reduce the harmful effects of noise in motorcycling should, then, move to studying the whole system of rider, helmet, motorcycle, and external environment.

The effects of windscreen flow on noise in motorcycle helmets

Vortex shedding from a motorcycle windscreen results in three flow regions in which the riders helmet may be immersed. First, the helmet may be completely in the free stream. Second, it may be in the path of vortex shedding from the windscreen. Third it may be underneath the shed vortices and shielded from the free stream by the windscreen. On-track noise tests were conducted and showed a difference in sound pressure level of more than 10dB and a change in spectra content, due to changes in riding position and helmet angle. Similar tests carried out in a wind tunnel, using simultaneous microphone and flow visualization measurements, allowed the identification of the flow regions. The potential contribution of vortex shedding to the noise was assessed using wavelet analysis to identify intermittent flow structures.

The experimental measurement of motorcycle noise

The noise source mechanisms involved in motorcycling include various aerodynamic sources and engine noise. The problem of noise source identification requires extensive data acquisition of a type and level that have not previously been applied. Data acquisition on track and on road are problematic due to rider safety constraints and the portability of appropriate instrumentation. One way to address this problem is the use of data from wind tunnel tests. The validity of these measurements for noise source identification must first be demonstrated. In order to achieve this extensive wind tunnel tests have been conducted and compared with the results from on-track measurements. Sound pressure levels as a function of speed were compared between on track and wind tunnel tests and were found to be comparable. Spectral conditioning techniques were applied to separate engine and wind tunnel noise from aerodynamic noise and showed that the aerodynamic components were equivalent in both cases. The spectral conditioning of on-track data showed that the contribution of engine noise to the overall noise is a function of speed and is more significant than had previously been thought. These procedures form a basis for accurate experimental measurements of motorcycle noise.

The sources and effects of noise exposure in motorcyling

We report on the Bath Motorcycle Collaboration, an interdisciplinary collaborative research effort involving the Departments of Mechanical Engineering and Psychology at the University of Bath and Bath Spa University in the United Kingdom. The group has taken a broad approach to the problem of noise in motorcycling, examining its sources, transmission, and effects. Noise‐induced hearing loss is a problem which can affect professional riders and racers as well as leisure riders and commuters. To study the problem, extensive wind tunnel tests have been conducted to provide detailed aerodynamic measurements and flow visualization around the helmet. These results have then been compared with and validated using on‐track data covering realistic riding conditions. Insertion loss measurements combined with loudness matching tasks on groups of volunteers have been used to investigate the process of noise transmission through the head/helmet system. Hearing threshold shift measurements have been conducted to quanti...

Motorcycle helmets and the frequency dependence of temporary hearing threshold shift

Temporary hearing threshold shifts (THTSs) as a result of exposure to noise vary as a function of the noises spectral content. However, to date THTS has been measured and predicted in a way that does not take account of frequency variation-most notably in standards such as British Standard 5330. We therefore carried out pure tone audiometry on participants before and after exposure to white noise in order to quantify the frequency dependence of the THTS. Moreover, as this research group has previously shown that motorcycle helmets act as spectral filters, attenuating noise in the region above 500Hz and amplifying noise in the regions below 500 Hz, this was done both with and without a motorcycle helmet. As our previous findings would suggest, the pattern of threshold shift is a function of the filter characteristics of the helmet, including an increased sensitivity at higher frequencies. There was also greater than expected reduction in sensitivity at frequencies where the helmet amplifies incident noise. The results indicate an acoustic effect of helmets which has not previously been reported.