Acoustic shadows

This project was planned as a follow-up of my ArtWeek installation. The idea arose in discussion with Neil Jones. I met Neil in the NHM by chance during the buildup phase of ArtWeek. We have been in touch since and talking about my project and related topics.
Neil is a geologist and works in the field of seaquakes, so he has a background in seismic phenomena, wave propagation in water, etc. From this context, we came to the idea of translating my work with shadows to the realm of sound waves.
Briefly, the idea was to have a box filled with a medium (may it be water, air, or something else) and a solid object in the middle (which could be shell-like composed of materials of different densities and properties). At least one side of the box would be some kind of screen, able to sense and visualize upcoming wave fronts. On the opposite side of the box, a (preferably movable) sound wave source would be installed, sending out waves into the box which will pass the medium, be distorted by the object and finally reach the screen and trigger visualization. Onlookers could move the source and thereby produce different patterns at the screen. Having a distortion object with a dense core and a more traversable outer layer would add the possibility to expose the invisible inner structure in terms of the outcome pattern.


Sarah Duit



Acoustic shadow


An acoustic shadow is an area of decreased acoustic pressure behind an obstacle which blocks the direct path between a sound source and a receiver (listener/microphone/?). In daily routine, this effect is less obvious and distinct than shadows of light because of the greater wavelength of acoustic waves. So, everyday objects are large in comparison to the wavelength of light but not to that of sound. All parts of sound which have a greater wavelength than the size of the obstacle are diffracted around the object. Due to this diffraction, there is no clear-cut border of the acoustic shadow but a wider area of more or less pronounced sound changes and effects.
An area of almost complete lack of waves will only be established if the measure of the obstacle d is at least five times greater than the wavelength ?.


An object size of double the wavelength for example will still diffract almost all of the sound around the obstacle.
Based on the presumption of an object with a size of approximately 10 cm which should be casting a completely wave free acoustic shadow, the calculated wavelength would therefore be 0.02 m at the most. This wavelength corresponds to a minimal frequency of approximately 76 kHz in water or 17 kHz in air. This would be in the area of ultrasound which covers frequencies above the human hearing range, starting from 20 kHz. In air, ultrasound propagates with an attenuation strongly increasing with frequency while water gives only little attenuation. For a wavelength double the object size, which results in almost complete diffraction, frequencies would be 30 kHz (water) or 7 kHz (air). A wave of these properties would not produce a complete shadow but distortion patterns.


Fact Box

Acoustic shadows
March 01, 2012