Talking train window adverts tested by Sky Deutschland

A German firm is proposing to transmit adverts via train windows so that the sound appears to "come from inside the user's head" when passengers lean against them.

The idea would use bone conduction technology, a technique that transmits sound to the inner ear by passing vibrations through the skull.

The concept has been developed by ad agency BBDO Germany on behalf of broadcaster Sky Deutschland.

It is already proving controversial.

Comments posted under a video showing off the concept include "This is a violation to a person's right to rest" and "I think I'd take a sledgehammer to the window."

The Talking Window campaign idea was shown off at the International Festival of Creativity in Cannes last month.

The video shows passengers on a German train being surprised to hear ads urging them to download the Sky Go app on to their smartphones to watch streamed video.

The audio is created by a special Sky-branded transmitter made by Audiva attached to the windows.

"Tired commuters often rest their heads against windows," says the ad.

"Suddenly a voice inside their head is talking to them. No-one else can hear this message."

Details posted online note that bone conduction technology has previously been used in hearing aids, headphones for swimmers and runners, and devices used by magicians to make someone think they have had a message planted in their head.

Google also plans to use the tech in its forthcoming Glass headset.

BBDO Germany said it had had a positive response to tests using prototype transmitters placed in public transport in Munich and Aachen.

"If our customer Sky Deutschland agrees, we will start with the new medium as quickly as possible," spokesman Ulf Brychcy told the BBC.

"At present, this is limited to the German market. If we look into the future: everything is possible.

"Some people don´t like advertising in general. But this is really a new technology. [It might] not only be used for advertising, but also for music, entertainment, mass transport information, weather reports and so on."

A spokeswoman for Sky Deutschland said it had yet to make a decision on whether to run the campaign.

Although the firm shares the same logo as the UK's BSkyB's satellite TV service, the two are separate companies, albeit both part-owned by News Corp.

BSkyB said it had not been aware of the campaign before the BBC brought it to its attention, and was not planning to launch anything similar.