The more rigid (or less compressible) the medium, the faster the speed of sound.
The speed of sound in a medium is determined by a combination of the medium’s rigidity (or compressibility in gases) and its density. If you like to investigate further, there is a nice wikipedia article about diffraction.\) makes it apparent that the speed of sound varies greatly in different media.
I will cover this in a separate blog.Īnd of course reality is much more complex than our simple model. Very high frequencies with short wavelengths produce diffuse reflexions on a rough or curved surface.īy the above we neglected another important phenomen: absorption. Higher frequencies with smaller wavelengths will be influenced, reflected and diffracted. Frequencies with wavelengths much larger than the object will pass unimpeded. 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. At what minimum angle relative to the centerline perpendicular to the doorway will someone outside the room hear. Sound with frequency 1220 Hz leaves a room through a doorway with a width of 1.18 m. When the blue path hits the wall, it causes a pressure buildup directly at the wall and partially reflects back. Use 344 m/s for the speed of sound in air and assume that the source and listener are both far enough from the doorway for Fraunhofer diffraction. 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. 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. Find the number and angular directions of the diffraction minima at listening positions along a line parallel to the wall. The angle of incidence is equal to the exit angle. Sound with a frequency 650 HZ from a distant source passes through a doorway 1.10 m wide in a sound absorbing wall. 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. 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. 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.