Abbingdon Music Research LS-77 owner manual G.5 Standing Waves, G.4.2.2 Porous absorbers

G.4.2.2 Porous absorbers

These absorbers include mineral wool, foam, carpets, curtains, and so on. They can be very effective, but the thickness of the material has to be taken into account. Thin layers will only absorb the highest frequencies. To absorb a given frequency (and all frequencies above) the thickness of the absorber must be the quarter of the wavelength of that frequency. Alternatively the front of the material must be placed at a distance of one quarter of the wavelength from the nearest surface.

Sofas and armchairs form very large and effective absorbers, especially if additionally being filled up in their hollow cavities, placing acoustic foam or felt behind wall hanging rugs can dramatically improve their absorbtion without adding to the visual impact. Placing heavy underlay felt underneath carpets can improve their absorption. Special absorbing foam such as the melamin based BASF BasoTect® foam that is also used inside your LS-77 can be surfaced with thin cloth or even silk paintings to give good looking yet highly effective absorbers.

G.5 Standing Waves

Standing waves exist in all kind of rooms. The shape of the room, the dimensions of the room, and the relationship between the dimensions of the room, allare important parameters that will determine the frequencies around which the phenomenon exists as well as the distribution of these standing waves. But how do they occur?

Imagine a sound source.

When the sound is emitted the sound wave will propagate in all directions if no obstacles are in sight. This will of course happen with the speed of sound. Now, if the sound source is placed inside a room, the sound wave will hit the boundaries of the room. If the boundaries consist of acoustically hard (reflective) surfaces, the sound is reflected. If the angle of incidence is 90º the sound will be reflected right back where it came from.

Under certain circumstances the sound wave will meet itself again. For instance if the sound is reflected between two parallel walls. This becomes a problem, when the sound wave not only meets itself, but when it meets itself in phase. And this will happen when the distance between the walls is half a wavelength of the radiated sound

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