Making sound visible requires optical methods, which is why an optical laboratory is increasingly becoming standard equipment in every acoustic research group. This applies in particular to the acoustics of musical instruments, which seeks answers to questions such as how certain instruments produce sounds of the highest quality, how musicians produce such sounds and how instrument makers can influence them.
Electronic Speckle Pattern Interferometry
Electronic Speckle Pattern Interferometry is also called ESPI, which stands for Electronic Speckle Pattern Interferometry. It is a method of analyzing oscillations by detecting deformations.
An ESPI is based on the interaction between two speckle patterns, both generated by the interference between a reference beam and the image of an object irradiated by a laser. Usually the images show an object before and after deformation. The interferometer presented here uses an image subtraction to relate two images to each other. The method of measuring the interrelation between the two was originally achieved by a film. The system set up at the Institute of Musical Acoustics works with a high-resolution black-and-white camera, which is connected to the computer via a LAN connection. The control of laser, vibration excitation, image acquisition and processing takes place directly on the computer. This makes it possible to analyze vibration patterns in real time.
But what are speckles?
If a surface is illuminated with laser light, speckle patterns are created. These are based on the principle of interference phenomena of coherent light. If a surface has irregularities in the order of the wavelength, i.e. in the nm range, it is called optically rough. If the reflected laser light is projected onto a screen in the far field, a granular structure becomes visible, which is called a speckle pattern. A single spot of light within this pattern is called speckle. The unevenness of the surface represents scattering centres from which spherical waves of different phases emanate, which interfere in the far field. This creates a spatial structure with randomly distributed intensity minima and maxima.