Billboards. Radio ads. Pop-up banners. You see and hear advertising everywhere these days, and a new concept literally bring ads into your skull.

Sleepy commuters in Germany, who rest their heads on public train windows, have recently gotten a surprise when they heard an ad that no one else can, as explained in the video above. They're not dreaming: a transmitter on trains in Munich and North Rhine-Westphalia sends high-frequency vibrations to the windowpane, which a person can hear through bone conduction.

This ad campaign by agency BBDO in Dusseldorf, Germany, was launched with prototypes on behalf of broadcasting company Sky Deutschland, back in January. The ad that plays to a person's inner ear is about Sky’s new mobile app.

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"Passengers got surprised and enjoyed this new form of advertising," BBDO Germany told Mashable. "Highly encouraging first reactions by commuters and our client." The agency plans to roll out this type of advertising nationwide in Germany.

BBDO also said this technology could be used beyond advertising: e.g. music, entertainment, transport information, weather, etc. The same sound transmission technology has been used in some headphones and hearing aids.

While marketers might like this pervasive advertising medium, we wonder whether commuters would get annoyed when they're just trying to rest. After all, sleep is one of our few escapes from the constant bombardment of screens, notifications and ad messages. There could also be privacy concerns.

What do you think about this approach to promotion? Are the window ads innovative or too pervasive? Share your thoughts in the comments.

Thumbnail image courtesy of YouTube, noris100.