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Acoustical size is measured in terms of the wavelength of the sound. The effectiveness of an obstacle in diffracting sound is determined by the acoustical size of the obstacle. Obstacles capable of diffracting (bending) sound must be large compared to the wavelength of the sound involved. The process by which this change of direction takes place is called diffraction. Obstacles can cause sound to be changed in its direction from its original path. This is considered a desirable effect due to the fact that the air conditioning noise is reduced.ġ0 Diffraction of Sound Wavefronts and rays of sound travel in straight lines, except when something gets in the way.
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Example: when a sound wave (noise) traveling in an air conditioning duct suddenly encounters the large open space of the room, the impedance mismatch reflects a significant portion of the sound back toward the source. Mismatches in impedance give rise to reflections, which cause numerous undesirable effects, but can sometimes be desirable effects instead. A standing wave is a resonance condition in an enclosed space in which sound waves traveling in one direction interact with those traveling in the opposite direction, resulting in a stable condition.ĩ Reflection of Sound from Impedance Irregularities This concept is used in microphone design to create highly directional microphones known as parabolic mics.Ĩ Standing Waves The concept of standing waves is directly dependent on the reflection of sound. The precision with which sound is focused is determined by the shape of the concave surface.Ī parabola has the characteristic of focusing sound precisely to a point. Plane wavefronts of sound striking a concave surface tend to be focused to a point. This amounts to a diffusion of the impinging sound. Reflection of plane wavefronts of sound from a solid convex surface tends to scatter the sound energy in many directions. The pressure at the face of a perfectly reflecting surface is twice that of a perfectly absorbing surface. The mid/high audible frequencies have been called the specular frequencies because sound in this range acts like light rays on a mirror.Ĥ Angle of Reflection Sound follows the same rule as light: the angle of incidence is equal to the angle of reflection. Sound above 300 – 400 Hz is best considered as traveling in rays. Below 300 – 400 Hz, sound is best considered as waves. The image source is located the same distance behind the wall as the real source is in front of the wall. Like a mirror, the reflected wavefronts act as though they originated from a sound image. The diagram shows the reflection of waves from a sound source from a rigid, plane wall surface. As the sound waves encounter obstacles or surfaces, such as walls, their direction of travel is changed, i.e., they are reflected. 1 Reflection, Diffraction, Refraction, DiffusionĪcoustics Reflection, Diffraction, Refraction, DiffusionĢ Reflection of Sound If a sound is activated in a room, sound travels radially in all directions.