Acoustic shadow
An acoustic shadow ( Engl. acoustic shadow) or a scarf Labs overshadowing occurs when there are obstacles on the direct sound path from the sound source to the listener or microphone, for example, columns or people.
Acoustic shadow, the shadow boundary
The resulting shading is an area of reduced sound pressure or sound pressure levels on the sound source side facing away from an obstacle and is not sharp shadows, but, as in the diffraction (Physics ) of light, a wide area with more or less pronounced coloration. The acoustic shadow will be "illuminated " by the effect of sound diffraction, ie the sound barrier - or shadow border - is more or less washed out. All sound components, the wavelength of which is greater than the dimension of the obstacles are flexed around the obstacle.
Calculating as a function of wavelength
Is the wavelength λ is small compared with the extension of the obstacle D, the underlying result is a shadowing, wherein the transitions are slidably. So, as a effect of an obstacle in the sound field a " Verdumpfung " the sound image and indeed the greater, the greater the obstacle is. Only when the extension of the obstacle is greater than five times D to its wavelength λ, generated, an audible verdumpfender shadowing for this frequency f, and higher frequencies. The space behind the spread obstacle is the shadow space.
The shadow boundary is: .
Here, the sound velocity C = 343 m / s at 20 ° C. Is the diameter of the obstacle only twice as large as the wavelength, the noise is still bent almost completely around the obstacle.
Only a very large obstacle poses behind a wave shadow, ie a nearly wave- free region.
Effectiveness of an obstacle for noise shielding
The effectiveness of a sound obstruction is strongly influenced by diffraction effects. 1968 presented the Japanese Maekawa a simple model for calculating the sound shield on flat, long walls. As a base served a series of measurements whose results were expressed as a function of known from optics Fresnel number. Result is an empirical law by which you can see the sound level difference, which is caused by the obstruction charge. Generally it can be said that all of the points lie below the line of sight of the source (that is, a bd = 0), a level difference of at least 5 dB have. The model also allows negative Fresnel numbers to -0.1 for points above the line of sight.
And for points above the line of sight
Since this law was only determined empirically, it has a limited application range for distances between noise source and receiver of less than 100 m and noise barriers of at least 1 m. Distance plays a vital role, as can be expected for such short distances of approximately linear noise propagation - weather phenomena and temperature gradients have no effect.
Shading in room acoustics
In practice, many partitions are used as sound barriers for acoustic separation of the sound waves in order to exploit the acoustic shadow. The intrusion of noise in the shadow area behind an obstacle is of practical importance because it is responsible for the limited effects of the so-called sound screens.
Shading of the ear cups by the head as an obstacle
The mutual shading of the ears through the head as an obstacle to the ear distance d = 21 cm gives only an effective acoustic shadow of a fairly high frequency of f. These are f = 5 × 343 / 0.21 = 8200 Hz
Nevertheless, this result but shifts the frequency response that allows the ear, not just - over the term - right from left, but forward from behind to distinguish up from down ( directional hearing ).
This effect is produced in about surround sound simulations on appropriate high-pass filter on certain channels.
Medicine
In the sonography refers to the canceling behind a highly reflective ( " hyperechoic " ) structure as acoustic shadowing. A common clinical practice cause of acoustic shadows are gallstones.