Porous absorbers are a treatment for acoustic problems, as well as to reduce noise and resonance. To understand how they work, we must first look into the physics of it all.
The Physics of Porosity
Due to viscous boundary layer effects, when a sound spreads in a small space, it loses energy. It can lose energy, too, when it moves through irregular pores. Also, this occurs due to thermal conduction from the air to the absorber material. This effect is more noticeable at lower frequencies. Air is a viscous fluid, so the friction with the pore walls spreads the sound energy.
For the absorption of porous absorbers to be effective, you must have interconnected air paths through the material. In other words, for this you require an open structure. The vibration of the material can result in minimal absorption loss.
Types of Porous Absorbers
The most common porous absorbers are carpets, acoustic foams, acoustic tiles cushions, curtains, and fiberglass, as well as cotton and mineral wool. They are popular as a treatment for acoustic problems, as well as to reduce noise as well as resonance.
As mentioned before, carpets are one of the most common examples. The problem with them is that they can’t achieve good acoustics. Also, they can only absorb high frequencies that go from about 1 kHz onwards. Additionally, textiles might seem like a good option because you can find them easily, in forms such as curtains and clothes. Curtains are excellent absorbers when they are thick enough. In contrast, very thin curtains have a very low resistance and therefore they hardly absorb any sound at all.
Typically, many manufacturers use special acoustic fleece. Their thin fabric provides the ideal flow resistance and it’s fire proof. If the perforated portion is too high, the panel only serves as a mechanical carrier for the acoustic fleece. In opposition, if the perforated portion is small, the panel and the air volume behind it act as a Helmholtz resonator.
Another example is mineral wool. People employ it in its pressed form or in rigid boards in acoustic ceilings. These boards come in a variety of colors and finishes. You insert them into rail systems. Mineral wool can be an additional layer on perforated panels. It, too, improves sound insulation in lightweight interior walls. When used in this way, cotton wool can replace the mineral kind.