$images of diffraction of sound$

$images of diffraction of sound$

This resembles any object of similar extension perpendicular to the wave propagation.Īccording to above consideration it is now obvious that such an obstacles influences sound propagation according to it’s size relative to the wavelength. We lift that wall from the floor and double it’s height „h“, to achieve similar bending above and below the wall. Similar to this bending effect, sound diffracts behind a slit in a wall into shadow areas. Hence, similar to the upward bending in front of the wall, there is again a downward bending behind the wall. We assume the wall to transfer no sound itself. If „h“ = 1/2 wavelength, the delayed red path signal cancels with incoming negative pressure on the blue path, and less sound energy is bent around the wall. Since the travel of sound is always at (nearly) constant velocity, regardless of direction, red arrives delayed compared to blue on it’s way towards the edge of the wall. Similar to that, the red path energy is to some part reflected back and causes an even higher pressure buildup at the wall. Therefore, some pressure will also expand vertically and „bend“ the wave’s blue path upwards. The local pressure at the wall is higher than above, where the green passes the wall unimpeded. When the blue path hits the wall, it causes a pressure buildup directly at the wall and partially reflects back. Now we place a small vertikal wall, like a noise barrier from floor to a certain height „h“, in the propagation path.Īnd watch three different sound paths red, blue and green. The distance between locations with positive and negative pressure deviation maxima is a half wavelength along the dispersion path. As we know, the signal is carried by sinusoidal varying deviations from atmospheric pressure, traveling forward with sound velocity.

$images of diffraction of sound$

Let us take a closer look at our assumed plane wavefront. Obstacles with a size similar to the wavelength show a much more complicated impact on wave propagation. Obstacles that are very small compared to the wavelength have no influence on wave propagation. The angle of incidence is equal to the exit angle. A ray-tracing model covers this perfectly. Such large obstacles compared to the wavelength indeed act like mirrors to the sound. Think of an ocean wave that hits a rocky beach. At normal incidence, the SPL directly at the wall is twice that of the incoming wavefront.

$images of diffraction of sound$

Ray tracingĪ wall that is much larger than the wavelength reflects the wave backwards. To simplify our discussions as before (Link wavelength), we assume the signal carried by the wave to contain only a single frequency. Our consideration starts with a plane wavefront that hits a stiff, heavy and flat wall at normal incidence.

$images of diffraction of sound$

Therefore, objects in our daily environment influence sound propagation differently: they both reflect and diffract sound, depending on shape and size relative to the sound signal’s wavelength. The easiest way to describe sound propagation is by comparison with light rays.īut the wavelength of light is far below a millimeter while that of sound ranges from millimeters to meters. Interestingly, sound waves bend around objects. A complex mixture of reflection and diffraction happens to sound that hits an object of similar dimension as the wavelengths of the sound signal.

$images of diffraction of sound$

Images of diffraction of sound