A brief introduction to acoustic treatment
Acoustic treatment and soundproofing are two totally separate concepts. To start thinking about acoustic treatment, imagine a room constructed from thick concrete - walls, floor and ceiling. It's soundproof, but it sounds terrible because of the strong reflections from the hard, parallel surfaces. So let us consider how we would apply basic acoustic treatment to this room...
Acoustic treatment should achieve one of two things. Either it should make the room sound pleasant, or it should make it as dead as possible. Sounding pleasant is best, but it is difficult to achieve in small rooms. In a small room, such as the smallest bedroom of your apartment or house that you want to convert to a studio, it is often more practical just to deaden it as much as possible. The reason for this has to do with the frequencies of the standing waves that are created in small rooms, but that's a more advanced topic for another occasion.
Suppose though that you have a fairly large room, around twenty square meters or more. It's still made of concrete remember. What you want to do is get rid of the ugly reflections and replace them with a pleasant acoustic ambience in which to make music.
What the bare room has is a small number of very strong reflections. Small number - very strong. This is important. So firstly you would need to consider absorbing some of the reflected sound. This can be achieved in two principle ways...
It's easier to give examples of porous absorbers than to embark on a long-winded scientific description. Carpets, curtains and cushions are all great porous absorbers. The "Three C's" of acoustic treatment perhaps. But there's a cheaper material that makes an excellent porous absorber - mineral wool. You will find this at a builders' merchant in the form of loft insulation. It's intended for thermal insulation, but it works extremely well for absorbing sound too. Fiber glass loft insulation can work too, but mineral wool is more commonly used professionally. The only downside of mineral wool is that it sheds fibers and must be contained behind a fabric barrier. But you would want to do this to make it look good anyway.
Porous absorption is great, but it has one limitation - it only works well at medium and high frequencies. To work well at low frequencies it has to be VERY thick, or spaced away from the wall. A room that is treated only with porous absorption will be dead at high frequencies but still reverberant at low frequencies. Subjectively the room can sound rather oppressive, to the point of not being a pleasant place in which to work. Having said that, a room that has only porous absorption will be better than a room that has no absorption at all. Those bare concrete walls, remember?
Panel and membrane absorbers
Panel and membrane absorbers are basically the same thing, but the way in which the terms are commonly used changes according to the nature of the material used.
What we would normally call a panel absorber is a wooden panel mounted on a frame that spaces it away from a wall. The space behind the panel is completely enclosed. Sound energy is absorbed in the meshing of the wood fibers as the panel vibrates in response to sound. The panel absorber can be tuned to a specific range of frequencies, according to a formula that includes the mass of the panel and the spacing distance. It is perfectly practical to tune panel absorbers to work well at low frequencies, where porous absorbers struggle.
What we would normally call a membrane absorber uses a flexible surface, such as a material with a rubbery consistency. Where a hard panel will reflect some sound energy and mid and high frequencies, the flexible membrane will provide absorption in itself, regardless of the air-space behind. It too can be tuned, but the panel absorber is more precisely tunable to specific problem frequencies.
Absorption isn't the whole of the story in acoustic treatment. Going back to the bare concrete room with just a few strong reflections. These will occur at certain frequencies dependent on the dimensions of the room. Having just a few strong reflections, creating resonances at certain frequencies, is subjectively unpleasant to the ear. It is much better to have many weak reflections, which will create a diffused sound field within the room.
This can be done with hard, irregular surfaces. You can buy diffusers, or make them yourself. A bookshelf filled with books of different sizes actually works quite well and provides a combination of diffusion and absorption.
In summary, good acoustic treatment consists of a combination of porous absorption, panel/membrane absorption, and diffusion. Although the very best results will be achieved through scientific methods and calculations, surprisingly good results can be achieved through educated guesswork and experimentation.
One more point...
Although professional recording studios have separate recording rooms and control rooms, at Audio Masterclass we recognize that most home recording studios have just a single room. Ideally the requirements in terms of acoustic treatment are different for recording and for monitoring, but there are methods and techniques that can be used to get the best results according to individual circumstances.