$reflection refraction and diffraction of sound waves$

$reflection refraction and diffraction of sound waves$

Some diffusion can be obtained by carefully placing book cases and other furniture in a room. The anechoic chamber at University of Salford where every wall is sound absorbing DiffusersĪcoustic Diffusers are used to disperse reflections spatially – to spread out reflected sound energy over a wide range of angles – as shown in the diagram above with the black path. The sound quality would be like listening outdoors, where only the direct sound from a source is heard (assuming soft ground and an absence of nearby buildings). It can be a tricky balance for an acoustic designer – too much absorption, and the room will sound dead. This absorption can be a specialist product such as those made of mineral wool, open cell foam, or recycled fibrous material like paper-waste, but absorption can also be provided by more commonplace object such as curtains, sofas or carpets. One solution to reflections is to apply absorption to the wall, which turns acoustic energy into heat – this is a kind of damping. There are two basic forms of acoustic treatment to deal with reflections: absorption and diffusion. These later reflections all blend in together, and cause reverberation. The higher-order the reflection, the further the wave has travelled and the later they arrive. Second order reflections are like shots played off two cushions, third order off three etc etc. You can imagine the waves bouncing around like balls on a pool table.

$reflection refraction and diffraction of sound waves$

In small rooms, first order reflections tend to be loud and arrive very soon after the direct sound. The diagram below shows a plan view of a small rectangular room, highlighting a first order (involving only one reflecting surface) reflection from the top wall in red.

$reflection refraction and diffraction of sound waves$

(Timbre is a French word which describes sound quality). These reflections can cause the sound to be coloured, or to put it another way, the timbre of the sound will change. The direct sound arriving at your ears directly from your hands is quickly followed by reflections from the walls, ceiling and floor. Try this experiment – go into a small bare room like a bathroom and make noise – a handclap will do. Hear the difference between a room with strong reflections and one with no reflections (anechoic chamber)

$reflection refraction and diffraction of sound waves$

Reflection refraction and diffraction of sound waves