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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. (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. 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.
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For example, if a sound wave is significantly larger than a diffuser and its structure, it will not be effective at the corresponding frequency.
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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. 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. Reverb and standing waves can be controlled by adding absorption materials to a room.When a sound wave propagates, there are several effects that have an impact. Figure 3 - Example of a sound wave diffracting around a gap in a surface AbsorptionĪbsorption is the loss of sound through an absorbent material. High-frequency waves have high directivity and can easily be blocked, whereas low frequencies have low directivity and spread far and wide. For spreading to happen, the wave must be larger than the object. Figure 2 - Example of an incoming sound wave refracting as it hits the water Diffractionĭiffraction is the bending of waves around small objects and the spreading out of a wave through small openings.Īll waves tend to spread out at the edge when they pass through a gap or past an object. Since temperature decreases with height, the speed of sound also decreases with height. With sound waves, it is more common for the sound to refract when it encounters a change in air temperature. Refraction is the process where a waveform changes direction as it passes from one medium to another - the speed of the wave changes as this happens. Figure 1 - Example of an incoming sound wave, reflecting back off a large surface Refraction Reflection is responsible for producing echo, reverb , and standing waves. The reflected sound will have a different frequency characteristic than the direct sound if all frequencies are not reflected equally. Higher frequency sound can be reflected by both small and large objects. Low-frequency sound has a long wavelength and so can only be reflected by large objects. For sound to be reflected, the object must be physically as large, or larger than the wave. Reflection is the process whereby part or the entire wave is returned when it encounters a boundary. Sound waves react in different ways when they interact with an obstacle reflection, refraction, absorption, and diffusion.