The study, conducted by the Saint Louis University School of Medicine and The Ear Institute of Texas, tested five different convertibles from the past decade: 2001 Porsche 911 Carrera 4, 2004 Nissan 350Z, 2005 Ford Mustang, 2005 Saab Aero and a 2009 Saturn Sky turbo.
The vehicles were tested by a passenger who used a sound level meter to take between eight and 10 measurements near the driver’s ear.
In all cases, the radio and air conditioning were turned off, there was no rain or other bad weather, the horn was not used and there was no conversation between the vehicles’ occupants.
Interestingly, the Porsche – the oldest of the group – was the only convertible not to have a maximum sound recording in excess of 85 decibels. It ranked first in the study, although the Saab and the Mustang also returned average noise levels less than 85 decibels.
According to the National Institute of Occupational Health and Safety in the US, exposure to noise greater than 85 decibels is not recommended for prolonged periods.
With the roof up, no excessive noise levels were recorded in any of the vehicles.
Saint Louis University School of Medicine’s Dr A.A. Mikulec summarised the findings:
“When the convertible automobiles were driven with the top open, high levels of noise were consistently recorded. Although driving for short distances under such levels of noise exposure is unlikely to cause a significant degree of noise-induced hearing loss, our study demonstrates that long duration driving at high speeds with the convertible top open will increase the driver’s risk of hearing damage.“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 exceeding 85.3km/h.”