A US company claims it is ready to build a microwave ray gun able to beam sounds directly into people's heads.

The device – dubbed MEDUSA (Mob Excess Deterrent Using Silent Audio) – exploits the microwave audio effect, in which short microwave pulses rapidly heat tissue, causing a shockwave inside the skull that can be detected by the ears. A series of pulses can be transmitted to produce recognisable sounds.

The device is aimed for military or crowd-control applications, but may have other uses.

Lev Sadovnik of the Sierra Nevada Corporation in the US is working on the system, having started work on a US navy research contract. The navy's report states that the effect was shown to be effective.

Scarecrow beam?

MEDUSA involves a microwave auditory effect "loud" enough to cause discomfort or even incapacitation. Sadovnik says that normal audio safety limits do not apply since the sound does not enter through the eardrums.

"The repel effect is a combination of loudness and the irritation factor," he says. "You can’t block it out."

Sadovnik says the device will work thanks to a new reconfigurable antenna developed by colleague Vladimir Manasson. It steers the beam electronically, making it possible to flip from a broad to a narrow beam, or aim at multiple targets simultaneously.

Sadovnik says the technology could have non-military applications. Birds seem to be highly sensitive to microwave audio, he says, so it might be used to scare away unwanted flocks.

Sadovnik has also experimented with transmitting microwave audio to people with outer ear problems that impair their normal hearing.

Brain damage risk

James Lin of the Electrical and Computer Engineering Department at the University of Illinois in Chicago says that MEDUSA is feasible in principle.

He has carried out his own work on the technique, and was even approached by the music industry about using microwave audio to enhance sound systems, he told New Scientist.

"But is it going to be possible at the power levels necessary?" he asks. Previous microwave audio tests involved very "quiet" sounds that were hard to hear, a high-power system would mean much more powerful – and potentially hazardous – shockwaves.

"I would worry about what other health effects it is having," says Lin. "You might see neural damage."

Sierra Nevada says that a demonstration version could be built in a year, with a transportable system following within 18 months. They are currently seeking funding for the work from the US Department of Defence.