Compared with larger headphones that cover the entire ear, some insertable headphones, like the white ones sold with iPods, increased sound levels by up to nine decibels. That may not seem like much, but because decibels are measured in logarithmic units, it can mean the difference between the noise output of an alarm clock (about 80 decibels) and that of a lawnmower (about 90 decibels).

The other problem, a second study found, is that insertable headphones are not as efficient at blocking background noise as some larger ones that cover the ear, so there is more incentive to turn up the volume.

To be sure, no one is certain what levels of noise the average MP3 listener is experiencing. But a large study of iPod users between 18 and 54 in Australia last month might provide some insight. The study, by the National Acoustic Laboratory in Sydney, found that about a quarter of the people surveyed kept their iPods at volumes that could cause long-term hearing damage.