Scientists at the St. Louis University School of Medicine and the Ear Institute of Texas have partnered to research the matter, in a paper called "Noise Exposure In Convertible Automombiles," and find that if you routinely drive at more than 55 mph in your convertible you might be subjecting your ears to unsafe noise levels.
The researchers aimed to quantify the noise that drivers are exposed to in a convertible with the top down, compared to with the top up, at 55, 65, and 75 mph. Measuring five different vehicles—a 2009 Saturn Sky 2.0 Turbo, 2004 Nissan 350Z, 2001 Porsche 911 Carrera 4, 2005 Saab Aero Convertible, and 2005 Ford Mustang GT Convertible—the researchers found that each of these (in some cases) produced excessive (85 dB) noise levels, as set by the U.S. National Institute of Occupational Safety and Health Administration (OSHA).
Saturn Sky, Nissan Z Crank It Up To 11
Of those vehicles tested, the '05 Saab Aero Convertible was the quietest, with a modest noise level of just 71.2 dB at either 55 mph or 65 mph. The 2001 Porsche 911 was the loudest, measuring 77.8 dB at 75 mph with the top up. With tops down, the results are quite different, with the Saturn Sky registering an ear-splitting 98.7 dB and the Nissan 350Z at 95.3 dB, both at 75 mph. Even at just 55 mph, the Saturn Sky topped 91 dB. For reference, OSHA's daily permissible workplace levels include three hours at 97 dB or two hours at 100 dB.
"Drivers of convertible automobiles with the top open may experience noise levels of 104 dB or more at highway speeds," the authors sum. To compare, a loud rock concert is 110-120 db, and the threshold of pain about 125 dB.
An intensity change of 10 dB is perceived as roughly twice as loud, while 20 dB higher is about four times as loud. So, at highway speeds with the top down, the Saturn Sky would seem more than twice as loud than the Mustang GT or Saab Aero.
Earplugs not a smart solution
"We recommend that such drivers be advised to drive with the top closed when exceeding 85.3 km/h, or to use noise reduction devices such as earplugs or noise-cancelling headphones (taking into consideration the safety and legal concerns associated with these devices, if the driver is less aware of such roadway noise as car horns and emergency vehicle sirens)," conclude the researchers, who add that while occasional use under such levels isn't likely to cause hearing loss, more frequent driving is risky.
And playing the radio loudly with the top down could increase levels, they note. The researchers reported that for each measurement, the climate control system and radio were turned off, and the readings were taken in dry weather.
Surely, it's news that Thelma and Louise would have thrown to the wind. But if you're traveling longer distances in a convertible and want to preserve your hearing, close the top.
[Cambridge University Press, Cambridge Journal of Laryngology & Otology]