Sound Absorption V Sound Proofing
Soundproofing is the process that stops or significantly reduces sound movement from going from one area to another. Whether this be the stopping of sound created within a room leaving that room, or the sound from the outside of a room entering into a room.
The control of acoustics within a room referred to as “sound treatment” is the process of controlling the residual sound within a room so that the sound interference within the room is reduced eliminated or attenuated to so that the sounds impact in the room is almost negligible. Acoustic foam will help with its use the emphasis is on dealing with the sound that is in inside the room and nothing to do with the sound that has left the room. Acoustic foam sound treatment helps to control sound interference so that its effect on the rooms acoustics is minimal or non existent improving the finished recorded music for a producer or mixing engineer.
The simplest way to understand the distinction between these two is by considering a sound source in a room usually the speakers producing the music or beat a producer is working on. This sound comprises of a whole spectrum of frequencies from sub 100Hz to 10Khz and over.
Figure 1 attempts to illustrate this. As the sound created impacts the walls of the studio (Arrow A) three things happen.
i)- Some of the sound energy is absorbed by the wall. This will depend on the frequency of the sound as well as the absorption properties of the wall. Sound energy that is absorbed is converted in to heat (D) by the vibrating molecules of the wall as they “soak” up the sound energy. This portion of sound will have no impact on the sound in the studio as such is as good as non-existent.
ii)- A portion of the sound will pass through the wall and emerge as sound wave C after having undergone diffraction downward as it passes through the wall (Arrow B). Generally sound frequencies whose wavelength is greater than 4 times the thickness of the wall will penetrate the wall. This again depends on the thickness of the wall and structure e.g. if it has a filling or composite makeup. As the sound wave C has left the room it is of no use to us. Incidentally this portion of sound is what will create noise complaints from your neighbors.
iii)- The final portion of the sound will get reflected back into the room (A1) . The amount of the sound reflected will depend on the frequency of the sound, the angle of incidence as well as the reflective properties of the surface onto which the sound is incident. This is the portion of sound that acoustic control and sound treatment attempts to deal with. This is because it is still in the room and will have an effect on our resultant mix.
Scenarios i & ii are what soundproofing deals with i.e. using construction to absorb as much sound as is incident on the walls of a room so that the energy of sound wave C is reduced significantly not to disturb those it is not intended for. Soundproofing will reduce sound leakage or entry into any room.
If you are reading this article and are looking for soundproofing materials and panels, the best advice I could give is to stop right here and head over to a sound proofing expert or building merchant to purchase soundproofing materials. If however you are interested in improving the quality of the sound in your room please read on.
Scenario iii is what acoustic control or sound treatment deals with – Improving the quality of the sound left in any room. Figure 2 shows what happens to the same sound wave impacting a wall fitted with an acoustic panel. A portion of Sound wave A will still get absorbed and converted into heat (D) and will still emerge on the other side of the wall (C) pretty much unscathed. The big difference is what happens to the sound that would have otherwise been reflected back into the room (A1).
The profile and composition of the acoustic panel will determine what happens to the reflected sound wave A1. Using a convoluted profile or wedge profile will mean that rather than having one strong reflected sound wave A1 you will end up with several weaker waves A1 A2 A3 scattered in several directions. The process of absorption, diffraction and reflection is true for every layer of material placed on the wall facing the incident waves. Absorbent materials such as foam will absorb more reflected sound than rigid hard panels that will reflect more of the incident sound waves
Considering that it is the lower frequencies that will penetrate walls due to their much longer wave lengths it is easy to see why a panel of foam 2 inches or so thick will have little impact on sound that is already “determined” to pass through your walls. If sound energy is powerful enough to pass through a 7 inch brick wall what difference will 2 inches of foam make? Answer “None”! Be very wary of dealers who sell any type of foam as sound proofing foam!
Sound treatment products
Acoustic treatment products focus on the sound that is reflected internally by the boundaries that create the room, i.e walls, ceilings and floors. The idea being that whilst you may not completely eliminate the sound bouncing around your room you should be able to tame it so that it doesn’t have a huge effect on the overall sound of your mix.
Applying acoustic foam or other absorbent material helps in doing just that, waves that would otherwise have been reflected by the surface of the wall hit the acoustic foam or panel and either get scattered in various directions (depending on the profile) or absorb some of the energy. Waves bouncing off the surface face the same fate again getting absorbed by the foam panel and getting diffracted in different directions as the different portions of the wave front emerge at different times due to the non planar profile of the panels.
Acoustic foam with a uniform open cell structure is the main type of foam used for this purpose. The open cell structure presents the wave front with several air filled pockets which contains air molecules which vibrate as sound penetrates them, this vibration helps reduce the power of the waves reducing the energy with which they are reflected at.
The more open cell the foam is the better the airflow resistance and the better it is at reducing sound energy incident on it. The three types of foam commonly found in acoustically treated spaces these are Melamine foams, Polyurethane Polyester foams and Polyurethane Polyether foam.
Pro-Acoustic acoustic solutions
If you are looking to give your sound an added edge or improve the accuracy of the sound you are mixing check out our range of acoustic treatment products. Our range of pro acoustic tiles and bass traps are made from acoustic foam to give you that added absorption you require and greater aesthetics.
If you are interested in the detailed physics behind absorption and diffusion I recommend reading Acoustic Absorbers and Diffusers by Trevor J. Cox , Peter D’Antoni F. Alton Everest A Master Handbook of Acoustics