They are already changing the way we listen to music but a Sussex academic says the iPod could soon change the world.

Dr Michael Bull, regarded as the world's leading authority on personal stereos, has launched a study into the impact of the 21st Century device on society.

Apple's iPod can hold up to 10,000 songs so all but the most obsessive music fans could fit their CD collection in their pocket.

The tiny device with its white bud-style earphones is the year's must-have gadget.

Dr Bull, 51, who lectures in media and culture at the University of Sussex, believes people living in their own sound bubbles tune out the world.

He began his research in the Seventies when Sony Walkmans were introduced.

He has now turned his attention to their sleek successor, which sells for between £250 and £400.

He said: "The iPod is the first cultural icon of the 21st Century. There is nothing else like it in terms of style, functionality and consumer desire.

"The potential for continual play means you never have to tune in to your environment. You're perpetually tuned out.

"More people will exist inside their mobile bubbles and the streets could become scarier and more alienating.

"People can go from A to B, get on a bus and buy a ticket or go shopping without having to interact with anyone else.

"People also become so immersed in the act of listening it jars them to come out of that space. It can take them a while to reorganise themselves back into the real world."

Dr Bull said this change in the way we interact would be cemented in a few years when personal stereo and mobile phone technology combine.

With everyone tuned into iPods or similar devices, Dr Bull thinks we may not be able to help each other in emergencies.

He said: "With these things in your ears you won't even hear the cries for help."

But Dr Bull doesn't paint an entirely pessimistic outlook.

He said: "On the positive side, it allows people to reclaim their time for themselves. Also, everyone who wears one says it is a happy experience.

"Listening to music you like is the most effective way to make yourself feel better.

"A lot of female Walkman users I spoke to felt safer with their stereos because they did not have to look anyone in the eye who might have been staring at them. They can be the aural equivalent of sunglasses."

Dr Bull lives with his wife Rosalinde and one-year-old son Theo in Montpelier Crescent, Brighton.

His interest in the sociology of personal stereos began while he was on holiday in Greece in 1992. He said: "I was on the beach and saw someone wearing a Walkman.

"It struck me as strange because the surroundings were so beautiful and quiet and I couldn't understand why anyone would want to listen to anything else. It got me thinking about why people feel the need to change the soundscape they are in."

The iPod's technology is creating its own community.

Dr Bull said: "It holds a much broader appeal than the youth-orientated advertising implies, with users being professional executives and highly educated.

"They particularly appreciate the function allowing files to be sent to other iPods, creating a social community."

But he said not everyone would be tempted by the iPod - himself included.

He said: "Some people don't like the earphones and I'm one of them.

"Other people just don't have the time to use an iPod, like mothers trying to look after their children, and some are worried about health hazards.

"Then there are some people who just don't like music."

Dr Bull is keen to hear from as many iPod users as possible. He has compiled a questionnaire, available from m.bull@sussex.ac.uk, and the findings will be the foundation of his book Mobilising The Social: Sound Technology In Urban Experience, due to be published in spring 2005.