Laser Prizes

Listening with light – the membrane-free laser microphone

Dr. Balthasar Fischer, founder and CEO of Xarion Laser Acoustics GmbH in Vienna, receives the first prize of the 2016 Berthold Leibinger Innovationspreis.

They are familiar from spy movies: directional listening devices that use laser beams to listen in on conversations from a distance. The laser beam measures the vibrations of objects, thus enabling the playback of sound emitted in closed rooms. However, what does not work nearly as well in real life as in the movies, has absolutely nothing to do with the invention of Dr. Balthasar Fischer. Not only does his laser microphone work perfectly, but it is also based on an entirely different principle. It measures sound waves in the air or in liquids directly on site. This mode of operation is also different from that of any conventional microphone. Its applications can thus be found less in voice recordings than in acoustic metrology, including non-destructive testing with ultrasound, process control for machine tools or as a sensor for photoacoustic imaging in medical technology.

In the past, all sound recordings were based on the principle of measuring the effect of sound on a membrane or other acoustic sensor. Sound is made up of vibrations in the air. These vibrations spread out at the speed of sound, making everything they encounter vibrate as well. This is why lightweight membranes make good acoustic sensors and sound waves can be converted into electrical vibrations, or electrical signals, using various electrical effects. Although this works rather well, it also has technical limitations. The weight and measurements of the membrane restrict its use to only a relatively small range of frequencies. Conventional microphones work well in this range and it is more than sufficient for the range of frequencies audible to the human ear.

Then why create a laser microphone, what can it do better? These are questions that Balthasar Fischer knows the answer to. The Swiss scientist first got his degree in physics and then changed both his location and academic field. He studied sound engineering in Vienna, because, in addition to his interest in physics, he is also a great fan of music. As he straddled both worlds, those of sound and physics, he asked himself a fundamental question: can sound be measured with light and, if so, how? This gave rise to the basic idea of a membrane-free microphone, one that could record the vibrations in the air without a mechanical agent. Since light does not have an inert mass, it should be the ideal acoustic pick-up. But how? The vibration of a sound wave is made up of alternating denser and thinner air – or liquid. The density of a substance has an impact on the propagation speed of light; this is known from refraction effects such as the seemingly bent handle of a spoon in a glass of water. Fischer now had the idea of using this effect by measuring the periodic changes in the speed of a laser beam between two mirrors that should occur when sound waves pass through them.

Balthasar Fischer took his patented idea to Vienna University of Technology and developed it there as part of his doctoral work. After first completing extensive theoretical work, he was finally able to demonstrate that it would be technically possible. In 2012, two years after receiving his doctorate, he had developed the technology to such an extent and its potential applications were so tangible that he turned his attention to its commercialization by founding Xarion Laser Acoustics. The advantages of his microphone are its very extensive range of frequencies, which is 60 times greater than the range audible to humans, a very high sensitivity for “low tones” as well as its robustness in the face of transmissions of structure-borne sound or interference through electromagnetic waves. All of which make it easy to understand why this comparatively more sophisticated laser microphone is not vying with the classic microphone to make sound recordings.

Balthasar Fischer is first planning to market the advantages of the membrane-free microphone in the industrial sector. For example, the broad spectral sensitivity for ultrasound waves enables early recognition of wear in machine construction. And its extensive bandwidth coupled with its greater sensitivity significantly increases the resolving power of ultrasound tests, for example in finding small defects in carbon fiber components that cannot be detected with conventional ultrasound sensors. Medical imaging is another sector in which the membrane-free sensor could find application. Quite promising images of tissues have already been taken. In terms of sensor size, its measurement sensitivity is greater than that which is even only theoretically possible from the piezoelectric sensors commonly used in medical technology today. However, Fischer is thinking even further ahead with his company. As a next step, a second chip-based generation of his membrane-free microphone will make further applications possible, this time perhaps for consumers as well. In addition to an application for active noise cancelling, its use as a precise directional listening device for the car may also come into play.


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