Physicists report [ArXiv] they’ve made a metamaterial that reverses the Doppler effect for sound – so that you’d hear sirens rising in pitch as they race away from you.

Metamaterials are structures whose unusual properties – such as controlling light or sound waves – depend on how their parts are arranged, rather than on the atoms they are made of. So-called ‘invisibility cloaks’ which hide objects from light by bending electromagnetic waves are the most famous examples. Chul Koo Kim, of Yonsei University, Korea, and colleagues have now created a practical acoustic metamaterial, a thin tube which manipulates sound. So far, though, it affects only sound waves traveling in effectively one dimension – that is, inside the tube.

The tube is segmented by tensioned elastic membranes, and punctured with side holes. The researchers placed a sound detector inside this tube and linked it to a loudspeaker. When they moved a sound source along the outside of the tube – approaching, passing, and receding from the detector – the acoustic vibrations set up inside the tube propagate with negative phase velocity, creating a tone-shift from low to high pitch.

Kim tells Physics World that the invention is a stepping stone to an acoustic cloak which could hide objects from sound waves. Acoustic superlenses – which could achieve subwavelength resolution in ultrasonic imaging – are also an option.

Physicists have generated an inverse Doppler sound effect before, but by using a nifty electronic setup combining electromagnetic pulses and radiofrequency shock waves.

The key to this more practical acoustic magic flute was making an air column which displays both negative modulus (elasticity) and negative density as sound waves propagate through it. (This is the equivalent, in acoustic terms, of the negative refractive index required for a material to bend back electromagnetic waves).

Each concept has been demonstrated separately before but Kim’s team put them together: air moving in and out through the side holes affects the density of the tube’s air column, while the tension of the internal membranes affects its elasticity.

The researchers have still to move into 2D and 3D, and their current device relies heavily on absorbers at each end of the tube to prevent soundwave reflections, Kim told Physics World. Which means we’ll be waiting for the naw-nee siren for a while yet.

Image: The acoustic metamaterial from Chul Koo Kim/Physics World