That's according to a study conducted by the Saint Louis University School of Medicine and The Ear Institute of Texas, and recently published in the Journal of Laryngology and Otology. The study was conducted using five different vehicles: A 2009 Saturn Sky 2.0 Turbo, an '04 Nissan 350Z, an '01 Porsche 911 C4, an '05 Saab Aero Convertible, and an '05 Ford Mustang GT Convertible. The sound level measurements in 80 percent of the cars, when driven at 55 mph with the top down, were all greater than 85 decibels (db). As bad luck would have it, that is the maximum noise level recommended in such industrial workplaces as assembly plants, stamping plants, and steel mills. Exposure to noise above 85 db for prolonged periods is not recommended, according to the National Institute of Occupational Safety and Health, which suggests that the higher the noise level, the less time you should spend being exposed to it.
Take the speed up to 75 mph with the top down, and the mean noise exposure inflicted on the driver in the study was 89.9 db. Factor in the frequent exposure to higher noise "spikes" while driving on the highway, like, for example, when cruising next to a motorcycle or one of those big honkin' trucks, and it gets even worse. Motoring at even higher speeds with the top down meant drivers were exposed to 90 db levels, roughly the same volume as a police siren or a pneumatic drill going off just one yard away. Even at 55 mph, the top-down noise level was comparable to that of a construction site.
The study was conducted using a sound level meter, operated by a passenger. The meter was used to record a series of eight to 10 sound level measurements at various points along the car's journey, at various speeds. The meter was placed near the driver's left ear. During all of the tests, the radio was turned off, and there was no conversation between the driver and the passenger. Also, the air conditioning was turned off, the car horn was not used and there was no rain or other inclement weather to contribute more noise.
The Saturn Sky 2.0 Turbo had the loudest mean noise values of any car tested at any given speed, producing 98.7 dB when driven at 75 mph with the top down. The Nissan 350Z produced the overall maximum reading: 104 dB when driven at 75 mph with the top down. The Ford Mustang GT Convertible fared better, with a loudest mean noise exposure of 84.7 dB and a maximum value of 88.6 dB, both measured at 75 mph. The Saab Aero Convertible was the quietest car, with a mean noise level of 71.2 dB when driven both at 55 and 65 mph.
The cars were also tested with their convertible tops up. The Porsche 911 C4 was the noisiest car with the top up, producing 77.8 dB when driven at 75 mph. When the cars were tested with the top up, no damaging or excessive noise levels were recorded. Obviously, convertible drivers will also be exposed to additional noise when listening to music. With the top down, the music volume levels required while driving at the above speeds will add significantly to the noise exposure level.
"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," summarized Dr. A A Mikulec, from the Saint Louis University School of Medicine, who oversaw the study. "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 85.3 kmph (53 mph)."