First Things, First
There are basic acoustic rules that must be followed when one is dealing with setting up speakers for two channel listening or monitoring. Sound energy from our speakers must be dealt with in many ways that are the same regardless of the end use of the room. In a listening room we have rules and with a monitoring room we have similar acoustical principle that must be followed.
The first acoustic rule is choose a room size that has the proper ratio of width, height, and length. This is a priority because if one chooses the correct ratio of these three variables from the beginning we can minimize the number and amplitude of these pesky resonances. I know it is not possible to find larger rooms because larger rooms have less low frequency issues. Most of us have to make do with an existing room. However, we may be able to make the room smaller to sound better.
There are published ratios of room width, length, and height that lend themselves to a better acoustical environment. All of these “golden ratios” have one goal in common. They all use dimensions that when you run a frequency response measurement in that room with the proper dimensions, the lower frequency modal resonances are more evenly distributed throughout the room. This separation of room modes is acceptable if they are at least 10 Hz. apart in frequency.
Make Room Smaller
If you have a room that does not fit a golden ratio, then look to the next size room dimensions that will fit inside your existing room. A smaller room size can have a smoother frequency response. Find those smaller room dimensions that will fit into your existing room and change the room size to fit those smaller size. If you are moving only one wall to more conform to a golden ratio, make sure you use new construction materials with the same densities that the other walls are constructed with. Consistency in room surface densities is important when dealing with small room acoustic issues.
Sound is an energy wave that is generated by an electromechanical device, a speaker. The sound waves and rays produced by our speakers travel at a given speed. It is this speed that we deal with when we are trying to find that balance between direct and reflected energy off of our side walls. The direct sound from our speakers is the sound that travels in a straight line from the speakers to our ears. The reflected sound is the sound that leaves our speakers and strikes the side wall closest to that speaker. Our goal is to balance those two energies together into a direct/reflected ratio that works for the room’s intended use.
If we are tuning a control room, we know our objective at the mixing position is to have less room sound and more speaker sound. We want our reflections from the side walls, which represent the room sound, to be less than the amount of direct energy from the speakers. In fact, in most control rooms, we want only direct sound from our loudspeakers at our monitoring chair. No room sound in our mixes.
In hi-fi rooms where our acoustic goal is to recreate the original sound in the recording that the recording engineer put so much time into achieving, we want more of the room sound so that it more closely resembles the sound found as if live musicians are playing in your room. We need more of the room sound in our system presentations. Since we need more room sound for more realism, we will want more side wall reflections in our sonic signal at the listening position.
All Side Walls Are Equal
The only way to be able to effectively adjust this direct/reflected ratio of energy is to have side walls that are of equal distance from the left and right channels. Since sound travels at a given speed, we need surfaces that are of equal distance from the speaker, so that the speed of each reflection at the listening position is arriving at the same speed. This speed consistency from both side walls is much easier to manage than if one side wall is two feet farther away.
Low Frequency Resonances
Low frequency resonances in our rooms must be dealt with. All small room acoustical environments have some type of low frequency resonance that must be dealt with. Low frequency resonances smother and blur our instruments and vocals. Low frequency resonances can have amplitudes that are +20dB-30dB in strength. This is a huge unwanted guest in our rooms. It must be treated as a guest and asked to leave immediately. One must hunt each resonance down and treat the location within the room in which it occurs.
If our monitoring position or listening position is in one of these resonances, we will have to move. If we can not treat it by reducing its amplitude to a level that does not interfere with the task we are seeking at our monitoring or listening position. If we sit in a resonance or room mode, we can have certain frequency ranges smothered to the point we can not hear them. We can also have situations where the resonance exaggerates certain frequencies and makes them more predominant. A lower frequency resonance also has its first and second cousins in the form of fundamentals that we must also treat. The whole family of resonances must move out, so we do not have to.
Our first objective with any room that we are going to use for some sonic purpose is to choose a room size that has the proper ratios of room width, height, and length that will give us the smoothest, frequency response. We want to choose a room size that minimizes room resonances. We can even make our room smaller if those dimensions produce a smoother, frequency response and more evenly distributed room modes. Side wall to speaker distances must be equal. We must have consistency in distances to keep our reflections speeds the same at our listening or monitoring position. Low frequency resonances will only get worse as we interject sound energy into the room. We must treat them in the beginning with powerful low frequency absorption technology.