New Acoustic Prism Splits Sound Like Optical Prisms Split Light

Scientists at EPFL have invented an acoustic prism which separates different frequency components of sound similar to an optical prism.

While a water droplet can naturally split the light into its constituent frequencies, an acoustic prism is entirely man-made. The Swiss Federal Institute of Technology in Lausanne AKA the EPFL (École Polytechnique Fédérale de Lausanne) in Switzerland has developed such a prism.
An acoustic prism is an engineered metamaterial which consists of a repeated arrangement of small units. The device can be used to find the direction of sound using only a single microphone.

Some Previous Developments in Acoustics

During the last few years, many interesting concepts and applications have emerged in acoustics. Some of these are obtained by extending the ideas utilized in electromagnetics to acoustics, such as acoustic superlenses which can focus ultrasound waves similar to an optic lens. Such devices hold the potential application in high-resolution clinical imaging.
Another example of the application of electromagnetics and acoustic is the acoustic cloak. The cloak can hide an object from sound waves. To this end, the device changes the propagation and reflection of sound waves and makes it seem as though the cloak and any object beneath it are not present. Future applications of an acoustic cloak include sonar avoidance and design of auditoriums or concert halls.
World’s first 3D acoustic cloak was designed two years ago at Duke University. The pyramid-like cloak consists of a number of plastic plates with many holes poked through them.

World’s first 3-D cloaking device designed at Duke University. Image courtesy of Duke University.

This is only one of the amazing developments in acoustics which once seemed to be infeasible. There is a long way to go before these technologies could be used in practical cases, but the current small-size solutions are yet a big stride.

Acoustic Prism of EPFL

Researchers at EPFL have developed an acoustic prism which can separate a sound wave into its constituent frequencies without resorting to the usual analog or digital signal processing techniques.
The acoustic prism is fabricated by connecting 10 aluminum unit cells. There is a hole, called a stub, on each of the unit cells. A membrane separates the two adjacent unit cells of the acoustic prism. The membranes introduce a frequency-dependent delay when sound waves travel through the structure from one unit cell to the next one.

The acoustic prism, the unit cells, and the membranes. Image courtesy of Acoustical Society of America.

The acoustic prism, which is called an acoustic leaky-wave antenna, can be used in both receive and transmit modes.

Transmit Mode of the Acoustic Prism

In the case of the transmit mode, we can apply a sound source to one end of the acoustic prism and the device will radiate different frequency components of the source from different stubs of the prism. The study shows that the high-frequency components of the sound radiate from the holes which are closer to the source, whereas the low-frequency components escape from holes that are further away.
This frequency-dependent radiation of the sound wave has an overall functionality similar to that of an optical prism—explaining the name “acoustic prism”.
Interestingly, similar to an optical prism, the angle of radiation from the stubs depends on the frequency of each sound component.

A simple schematic of the leaky-wave antenna. Image courtesy of Acoustical Society of America.

Receive Mode of the Acoustic Prism

The acoustic prism exhibits a fascinating phenomenon when used in the receive mode. In this mode, a microphone is placed at one end of the prism and a source at a distance of, say, 4 meters away is used to apply the sound wave. The spectrum of the signal picked up by the microphone exhibits a band-pass response as shown in the following figure.

The acoustic prism acts as a band-pass filter in the receive mode. Image courtesy of Acoustical Society of America.

This figure shows both the theoretical and the simulation results.
As the frequency of the source varies, the power received by the microphone exhibits a peak at the center frequency of the band-pass filter. However, there is a big difference between the response of an acoustic prism and that of a usual analog or digital band-pass filter.

As shown in the above figure, the center frequency of the achieved band-pass filter depends on the angle of the incident sound wave. Therefore, by analyzing the power received by the microphone, we can determine the location of the sound source.

The acoustic prism holds the potential to challenge current techniques of sound source localization which rely on multiple-sensor beamforming.

For more information, please read this recently-published paper from The Journal of the Acoustical Society of America on metamaterials in single-microphone direction finding which presents the concepts of the acoustic prism in great detail.
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