Despite their suave appeal, convertible cars could have to carry a health warning owing to the threat they pose to the occupant's hearing.
Writing in the Journal of Laryngology and Otology, St Louis University researcher Anthony Mikulec and his colleagues measured the noise levels experienced by passengers travelling in five different makes of cabriolet, including a Porsche 911, at a range of speeds and with the roofs both on and off.
Guidelines relating to levels of noise exposure suggest that sustained exposure to sounds louder than 85 decibels (dB) is likely to permanently damage hearing, probably by metabolically stressing the hair cells in the inner ear where soundwaves are converted into brainwaves.
In the study, with the roof off and moving at about 55 miles per hour (88 km/h), three of the five cars were already exposing passengers to sound intensities close to this threshold. When the speed was increased to over 70 miles per hour (120 km/h), sound levels at or above 85 dB were recorded in all of the cars, and in one model sustained levels of nearly 100 dB were measured. This effect was negated when the roof was in place, with passengers experiencing sound levels of about 75 dB, well below the limit considered safe.
These results suggest that long car journeys in open-topped vehicles are likely to expose passengers to sustained noise of 100 dB - the equivalent of holding a running chainsaw - particularly when the traffic is heavy as this increases incidental noise, or if the car radio or music is played since this will be turned up even louder to compensate for the ambient sounds.
According to Dr Mikulec, "In light of the results of this study, we are recommending that drivers be advised to drive with the top closed when travelling for extended periods of time at speeds exceeing 85.3 km/h".
So, topless round town then, but not elsewhere...
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