The earbud-style headphones packaged with iPod and MP3 players can cause serious hearing loss, a Northwestern University professor and audiologist said last week.

"Insert earphones can boost the signal by as much as six to nine decibels. That's about the difference between the sound of a vacuum cleaner and a motorcycle," said Dean Garstecki, a professor at Northwestern in a statement. "It's a significant difference."

Earbud headphones, the type that rest inside the ear, can lead to permanent damage after just an hour of high-volume music in the 110-120 decibel range, equivalent to the noise level of a concert, Garstecki said.

The large amount of storage space on iPods also contributes to problems, said Garstecki. "Not only do you have a signal that is likely to be more intense, you're likely to be using this device longer than people have been using CD players and Walkmans in the past. This increases the likelihood of hearing loss down the road."

Not surprisingly, rock and rap are the most dangerous, since they're typically played at a higher volume than, say, classical or jazz.

To minimize hearing loss, Garstecki recommended listening no more than an hour a day, switching to over-the-ear headphones, and dialing down the volume on iPods to "6" or lower.