I always like to read what other authors think are important areas of acoustical control needed in small room scenarios. In the book entitled, “Home Recording Studio Basics, What You Need To Know To Build A Home Studio”, the author talks about many things when it comes to electronics and studio recording. They also venture into the acoustical region and complied a list of “10 Guidelines For Room Acoustic Treatment”. Lets examine each one and look at how applicable they are.
Absorption, Diffusion, and Reflectivity
The first guideline is about acoustical basics. They talk about how only three things happen to sound energy in our rooms. Sound is either reflected, diffused, or absorbed. This remains true today. Reflections off of our wall surfaces are common in small room acoustic situations. Sound energy bounces around in the room from our wall, ceiling, equipment, and even our floors. Sound absorption occurs when sound energy is converted to heat energy and then is lost forever. Diffusion is used in our light fixtures with the lens that spread the light out in different directions. It is ironic that we use diffusion and absorption to treat reflections. We use two to deal with one.
Guideline # 2 talks about reflectivity in our rooms especially our control or monitoring rooms. It states that reflectivity is in abundance in our rooms and must be controlled. No argument on this point. All room boundary surfaces produce reflections. Our walls,ceilings, and even our floors assist in reflection accumulation within our rooms.
Guideline # 3 deals with the materials that are used in our rooms. They state that some rooms are “harder” than others. This means that some materials such as concrete has a much higher reflectivity than wood and frame construction techniques. Concrete is used to isolate sound energy and keep it from entering into other areas. It will have a higher degree of reflectivity than a gypsum wall board because of its design purpose and composition. Sound absorption technology can be added to the concrete room to minimize any sound absorption issues.
The fourth guideline discusses bass trapping. The authors particularly focus on the area below 200 Hz. which is a good start point because most rooms will have low frequency issues in this area and real issues below 100 Hz. They mention that one should place the sound absorption material in the area behind the speakers. This is a good area to start because it is a area of high low frequency pressure. Additional areas must also be addressed.
Diffusion vs. Absorption
Guideline five talks about the mechanical difference between diffusion and absorption. Diffusion takes energy and spreads it out without any loss of signal. Absorption takes electromechanical energy and converts it to heat which means that the energy converted to heat through the absorption process is changed to a new energy form and lost forever.
The six point made is all about acoustic foam. It states that the thicker the foam the more absorption at lower frequencies. Well, it says lower frequencies but does not define what they are. It talks about using foam of 6-12″ to use for bass absorption. This is a good thickness for lower frequencies but not the real low frequencies. Lower frequencies below 100 Hz. need special absorbers to deal with it. Foam is good for middle and high frequencies but will not work for frequencies below 80 Hz. unless it is 10′ thick.
Room Treatment Placement
Point 7 discusses the placement of sound absorbing materials throughout the room. Their position is that it is best to distribute sound absorbing material throughout the room and not just in one place. Overall a good idea except when it comes to low frequency absorption. Low frequency absorption must be placed in areas of high sound pressure created by resonances which are mostly influenced by room dimensions. Areas of highest pressure are behind the speakers and in the corners and wall boundary intersections of our rooms. All room modes end up in the corners of our rooms.
Common Building Materials
Guideline #8 discusses using common building and domestic types of materials for room treatment using mainly absorption type materials. The authors suggest using upholstered furniture, vertical blinds, draperies, wall hangings and carpet. The authors state that these materials have both absorptive and aesthetic benefits. All of these materials do have absorptive qualities and can be pleasant to look at. However, non of these materials will have an impact on low frequencies. They also do not have predictable and consistent rates and levels of absorption that one can use to tune the room. It is difficult to tune a room using drapes. If you need more absorption in middle and high frequencies but no more room for drapes, you will be looking for other material types to solve the issues.
No Flat Surfaces
Guideline #9 discusses the issues involved with all flat surfaces in our rooms. It states that flat areas such as equipment surfaces and equipment racks contribute to high reflectivity especially where one does not need or want it. Reflections from flat surfaces interfere with the wanted, direct sound from our loudspeakers. Sound energy also takes on the characteristics of the surfaces that it strikes. If sound strikes glass, one gets glass sound. We have all heard this sound inside of our cars. It is not nice. Reflections off of metal equipment racks will take on a metal sound. Wood sound is the most favored.
Room Size Critical
The final guideline discusses room size and how not to choose room sizes that have room widths, heights, and lengths that are evenly divisible by some number. If you choose a room that has measurements of 12′ x 12′ x 8′, all of these numbers are divisible by 4. These are room dimensions to stay away from because they will have even distributions of room resonances. Room dimensions are critical if we are to control room resonances from the start. There are many approaches to this issue, but the best way is to run a room modal analysis on a proposed room size and see what those resonances are. One can also add footage or reduce footage and see what size works best with minimal resonances.
These ten guidelines in Home Recording Studio Basics are a good start. Guideline 10 is the most critical. Choosing a room that has the right dimensions to minimize low frequency resonances is paramount. If you don’t minimize room resonances in the beginning, you will be fighting an uphill battle against them forever.