Absorption

Sound Absorption is the conversion of sonic energy into thermal energy.

When air becomes energized by sound waves passing through it and we could watch the air through a slow-motion microscope we would see air moving back and forth along the direction of the sound wave. As the air moves we would also notice small pressure changes; the air moving forward is slightly compressed (higher pressure) and the air moving backwards is slightly expanded (lower pressure).

Other absorber technologies exist, such as an Impedance-Matched Capacitor-Resistor Circuit, which allows lower frequencies to be absorbed than thickness would normally permit. Additionally, resonant absorbers can dissipate sound energy by means of a sympathetic membrane which also converts bass energy to heat. Common sound absorbers include: flat wall panels, corner loaded bass traps, curtains/draperies, foam, soft furniture, and a clothed human body.

As the wave passes by our ear, it is these pressure changes that push and pull on our ear drum and become what we call sound, hence “sound wave”. Sound absorption, on the other hand, takes place where the silent movements of the air rub against a surface.

We are familiar with the most common form of a sound absorber; a panel of open cell foam or compressed fiberglass that is open enough to allow sound waves to penetrate into the volume of the absorber where the motion part of the sound wave can rub over the multitude of microscopic surfaces in the sound panel and cause enough friction to dissipate energy from the sound wave.

This type of sound absorber directly involves the sound wave itself and how much it absorbs depends on the fraction of the wavelength it is in contact with. A 2” deep sound panel absorbs high frequency sound down to about middle C of the piano, 260 Hz. The wavelength of 260 Hz is 4.33 feet and the ¼ wavelength is about 13”. The depth of the sound absorbing panel is about 1/6th of the ¼ wavelength dimension. This is a wavelength type sound absorber in that it reacts to the distributed acoustic nature of sound, the wavelength.

There is another kind of sound absorber, particularly useful in low-frequency absorbers, bass traps. Instead of interacting with the wavelength of sound, they interact with the frequency of the sound. The frequency type of sound absorber is designed similar to an electronic circuit, except they use acoustic resisters, acoustic capacitors and acoustic coils to create an acoustic circuit device. This is referred to “lumped parameter” acoustic design which is very different from the “distributed parameter” version of wavelength acoustics.

Initially ASC was a factory that was dedicated to building the lumped parameter type bass trap, the TubeTrap. Over time we realized our clients also wanted their sound panels to both look and sound like the TubeTrap style, where each product is voiced to have a blend of lower frequency absorption overlaid with a % of higher frequency diffusion. And so we build a full bandwidth product line: Lumped parameter bass traps + distributed parameter treble sound panels.

Not all absorption is good. Too much absorption over deadens the room. Our built-in diffusers help to compensate for the absorption being added into the room by backscattering 25 to 50% of the incident upper frequency energy back into the room. We use both the reflective and diffractive diffusion techniques to smoothly distribute room ambience as quickly as possible.

We use a second technique for developing acoustic control in the room. The amount of sound power absorbed depends on the surface area and efficiency of the absorber, but also how loud the sound is in front of the absorber. We place our absorption in locations where the sound is the loudest so we remove acoustic energy from the room before it has been reflected multiple times within the room. By this we lower the level of the reverberation in the room while at the same time speed up the decay rate of the reverberation in the room.

Room Modes: Reverberation is sonic energy stored in a chaotic form in a room. Room modes is sonic energy stored in a very organized form in a room, where sound is reflected over and over again between the two same parallel walls. This is the first order type of modes, second and 3rd order modes also exist in rooms but they die out relatively quickly.

When a sound wave impacts a wall at an angle (2nd and 3rd order modes) some of the wave has motion parallel to the surface, where friction occurs and sound is absorbed. When the wave impacts the wall squarely (1st order mode) there is no air motion along the surface of the wall and therefore no friction, hence no absorption. Adding wavelength type sound panels do not work because there is no air movement on the reflecting walls. The only type of sound absorber that works on 1st order modes is the frequency type bass trap, like TubeTraps.

Other absorber technologies exist, such as an Impedance-Matched Capacitor-Resistor Circuit, which allow lower frequencies to be absorbed than thickness would normally permit. Additionally, resonant absorbers can dissipate sound energy by means of a sympathetic membrane which also converts bass energy to heat. Common sound absorbers include: flat wall panels, corner loaded bass traps, curtains/draperies, foam, soft furniture, and a clothed human body.

The vast majority of ASC products incorporate a Sound Absorption component. Below is a partial listing:

Products