Leave it to the Brits to address such a common question with scientific rigor and analysis! Philip Michael, Niel Opie and Michael Smith, from Worcestershire Royal Hospital in the UK, published a short but information-packed article in the August edition of the journal Otolaryngology-Head and Neck Surgery (2010;143:219-22). They looked at seven different types of convertible cars (Audi A4, Porsche 911, Aston Martin V8 Vantage, Morgan Roadster, Bentley Continental GT, Toyota MR2, and a Mazda MX5) driving at 50, 60 and 70 mph with windows raised and down. They placed a noise sensor on the traffic side of the car (on the right in the UK- they drive on the wrong side of the road) and took 3 separate measurements in each condition (scientific method at work). All tests were done on similar roadways and in non-rush hour traffic to minimize data contamination.
What did they find? There was no difference in noise levels between the different types of car or at the speeds measured. All of the averaged (mean) testing results with the windows down exceeded the OSHA criterion limit of 85 dB (89.1 dB +/- 0.7 dB). Just rolling up the windows of the cars,still with the top down, dropped all noise levels below accepted levels to avoid hearing loss (84.2 +/- 1.0 dB). These results did not take into account the impact of rear wind deflectors since not all of the cars had them available. Noise levels were increased when the test cars were passed by loud vehicles (i.e. big trucks).
What do I do if my only chance to get a tan is when I am driving my car with the lid off? Well, consider keeping your windows up and you may want to wear hearing protection while you are driving. Hearing protection is typically not a good idea when you drive since it can limit your ability to hear traffic and other environmental sounds around you. In short, ear plugs can be dangerous and I do not recommend that option. You can tolerate 85 dB for up to 8 hours so take breaks if you need to do the entire long drive with the top down. If you turn up your radio to hear the music with the lid off of your car, be aware that you may be increasing your noise exposure as well. You can download one of may portable noise meters (I chose dBMeter Pro from the ITunes app store because it had 33 ratings). and can actually get a quick and dirty test when you are driving to assess your risk as well.
This is the first paper to document a risk for hearing risk while driving a convertible with the top down. The paper did not talk about the risk of tinnitus (ear ringing). I recommend you take in the data as one more factor when you are planning a fun day in your car. Always remember, you are either a Porsche 911 owner or someone who wishes they were.