Propagation of sound

Propagation of sound

When a sound wave propagates, there are several effects that have an impact. Since these effects are all relevant for room acoustics, they are discussed here as a foundation. The wavelength "λ" is essential in the propagation of a sound wave. The wavelength is inversely proportional to the frequency "f" and also depends on the speed of sound "c" in the medium in question, which in the case of air is normally about 343 m/s. The formula for calculation is λ = c/f.

Diffraction

An important effect is sound diffraction. Sound waves can be diffracted "around the corner" under certain circumstances. The following always applies: If the wavelength of a sound wave is large in relation to an obstacle, the sound wave can bend around it. Inversely, the obstacle must be larger than the wavelength of the sound wave to have any impact on its propagation. This effect usually results in the lower cutoff frequency at which room acoustics modules are still effective. For example, if a sound wave is significantly larger than a diffuser and its structure, it will not be effective at the corresponding frequency.

Reflections

Reflection of a sound wave occurs whenever the sound wave encounters an area of high or low density or, for us, higher or lower acoustic impedance. This does not occur during normal sound propagation in air, but only when the wave encounters an obstacle such as a wall.
When the sound wave encounters a sound-reflecting wall made of materials such as concrete, wood or glass, the acoustic impedance changes abruptly. This means that virtually the entire sound wave is reflected back by the wall. As in the field of optics, the angle of incidence equals the angle of reflection. One might think of a laser hitting a mirror. Such strong, directional reflections often cause problems in room acoustics.
In order for a reflection to take place, another condition is important: The dimensions of the obstacle must be at least as large as the wavelength of the sound wave. (See: diffraction) This means that a low-frequency oscillation that hits a small obstacle is only reflected to a very small extent and is mostly diffracted around the obstacle. This fact is enormously important if you want to slant walls or sound reflectors to guide sound waves away from the listening or recording position. The wall or the reflector must be large enough to guarantee a full reflection.

Interferences

Interference refers to superposition patterns of sound waves. If, for example, two sound waves arrive in phase at a certain point in space, theiy add up to twice the sound pressure (constructive interference). The signal becomes up to +6 dB louder at this point. If, however, the sound waves arrive in exactly opposite phase to each other, their sound pressure curves cancel each other out and the sound pressure can be reduced by -∞ dB to zero (destructive interference). In room acoustics, these patterns occur primarily at low frequencies whose wavelength coincides with one of the room dimensions. This is referred to as room modes or resonances. The wave is reflected back and forth between the walls, forming local peaks or cancellations. To solve this problem, low-frequency absorption can be introduced into the room, or parallel walls can be avoided from the outset.
A relevant example in the high-frequency range is a loudspeaker aimed at a listening position and the presence of a sound-reflecting side wall, ceiling or floor. In addition to the direct sound, which travels directly from the loudspeaker to the listening position, the early reflection from the corresponding wall also arrives with a time offset. Depending on the wavelength, the resulting phaseshift results in summation or cancellation of the soundwaves. When plotted over frequency, the resulting filter resembles the shape of a comb, which is why it is often referred to as a comb filter. In the context of neutral and linear sound reproduction, this effect should be avoided by all means, especially in professional listening situations. This is achieved by damping or equipping the critical surfaces just mentioned with diffusers.