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. Such strong, directional reflections often cause problems in room acoustics. One might think of a laser hitting a mirror. As in the field of optics, the angle of incidence equals the angle of reflection. This means that virtually the entire sound wave is reflected back by the 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 does not occur during normal sound propagation in air, but only when the wave encounters an obstacle such as a wall. 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. For example, if a sound wave is significantly larger than a diffuser and its structure, it will not be effective at the corresponding frequency. This effect usually results in the lower cutoff frequency at which room acoustics modules are still effective. Inversely, the obstacle must be larger than the wavelength of the sound wave to have any impact on its propagation.
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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. Sound waves can be diffracted "around the corner" under certain circumstances. Diffraction An important effect is sound diffraction. 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 wavelength "λ" is essential in the propagation of a sound wave. Since these effects are all relevant for room acoustics, they are discussed here as a foundation.
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When a sound wave propagates, there are several effects that have an impact.