In a study published last year in the journal Ear and Hearing, Dr. Brian Fligor of Harvard Medical School looked at a variety of headphones and found that, on average, the smaller they were the higher their output levels at any given volume control setting.

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.

A large study of iPod users between 18 and 54 in Australia last month 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.

It can be rectified simply by using a costlier ear-covering headphone. The question is how much it will affect the iPod "cool factor".

And "cool factor" is very important for Apple bottomline.

Sales of portable digital-music players are set to grow 57 per cent this year after more than doubling (144%) in 2004 according to the results of a global survey.

Shipments of MP3 players - of which Apple's iPod is the best-known - rose by 116 per cent in 2004.

The market research group iSuppli said electronics producers stand to benefit from consumers' willingness to pay more for "cool" products, which Apple has exploited and which Sony is targeting with its new lineup of Walkmans.

Source: NY Times & Reuters via Silicon.com