Free Space Sound
The best sound that we will ever hear is sound in free space. Free space sound is sound that is in free space. It is not locked up in a room with four walls, ceiling and floor. Take your stereo up in the mountains, elevation 8,000 feet and very quiet. Set up a simple two channel system, play music. No room, no reflections, no bass boom, just pure music.
Rooms Are Distortion
Our rooms produce distortions because they are rooms. They are a space created from nothing. If you raise walls from a floor and put a ceiling on it, one has created a room. The size of the room and its associated volume produces distortions due to the room’s dimensions. These distortions range from low frequency to low, middle frequencies. Low frequency issues are created by the room’s measurements. This low frequency distortion can result in +20 dB – 30 dB bumps in our room frequency response below 80 cycles.
Low Frequency Distortions
Low frequency energy issues produce many very severe distortions that minimizing them by choosing the combination of room width, height, and length from the beginning that produces the best room modal separation. There are room ratio guidelines available to assist one in determining these correct ratios. It may even be necessary to make the room physically smaller in order for it to have better low frequency response.
Box Of Pressure
When we build a room with floor, side walls, and a ceiling, we are building a box for sound pressure to be trapped in and surfaces for energy to bounce off of producing reflections. Reflections from room boundary surfaces take on a life of their own. They even disguise themselves with a new name. If a lot of reflections are concentrated in a room area, we call that a comb filter. It is called a “comb” filter because its sonic signature looks like the teeth of a comb.
If we take a normal room size dimension, we have three axis that room modal resonances can be located with. Well, located is used loosely. The axial mode is the modes that occur between the two parallel walls to the right and left of the listening position. These are also termed right angel reflections. A tangential mode is when we add in resonances that occur between the side and end walls in our room. If we add the floor and ceiling to the mix we now have 6 surfaces which are called oblique modes. These modes create pressure patterns that are strong on one side and weak on the other. Add in all the fundamentals that go with each frequency and you have a room that is talking to itself. It must be taken to a doctor.
Comb Filter Not For Hair
Behind this mass of “comb teeth” is hiding our real sound. Unfortunately, the comb filter effect is so pronounced that it will mask certain frequency ranges completely. Sounds and music found within this masked domain are not heard very well. The sound and music is there it is just that by the time some of it fights through the comb filter, one can not recognize the music.
Comb That Hides Music
Comb filters distort and hide the music from us. Comb filters can occur between the walls and listening or monitoring position. One can have a comb filter notch from the console to the monitoring position. Obviously, this is deadly if one’s goal is to produce a balanced mix. No amount of golden ears will be able to hear through that soup in the mid range and no one wants to fight that all day.
Furniture, Walls, And Us
Furniture and side walls can create a comb filter. Any two surfaces that can trap energy in some manner between each of their respective surfaces and cause time delays in the original signal, can produce a comb filter. Even our bodies could, if all other variables were addressed and in place. Once energy gets “trapped” between each surface, it gets angry and shows its teeth.
Speaker / Boundary Issues
The next issue of distortion is called the speaker boundary interference. These are issues that deal with low frequency pressure build up as a result of the speaker’s close proximity to the wall or as the name implies boundary. The direct sound from our loudspeaker is the sound that travels in a straight line from the speakers to our ears. The reflected sound from our room’s walls,floors, and ceilings produce their own set of difficulties.
Room Critical Band
However, sound also is radiated from our speaker sides and rear especially with lower frequencies and definitely with rear ported speakers. This lower frequency energy interacts with the room’s physical dimensions. Some of the room dimensions are a favorable ratio to low frequency waves and no resonances are produced. Other room dimensions can play havoc with certain frequency ranges from 30 Hz. – 200 Hz. This is not the frequency band that we want any resonances in. One could say that just as human hearing has critical bands that the brain processes information with and this 30 cycle to 200 cycles is a critical room band.
Stay Away From Our Vocals
Based on the size of most rooms that have less than 20′ in any direction, this is a critical low frequency band and the beginning of the low middle bands where are vocals both male and female begin to take form and shape. We do not want any resonances artificially boosting certain frequencies and smothering completely others when our vocals are directly impacted.
The reflections created by speaker/boundary impact can confuse the direct energy from our loudspeakers. Our total sound is the sound of our system interlaced with the room sound. One can not have quality room sound by adding different speakers or amplifiers. All you will get for the additional expense is more or less of what the room will permit you to have acoustically.
Room And System Balance
Quality two channel sound is always a balance between room and system. They both count 50% towards the total sound, so therefore with their 50% contribution to our final sound, we must take the room treatment just as seriously as we did our equipment.I see many good systems that are capable of so much more sound because of each components quality levels. The room is in the way.
Diffusion Brings Air
Lack of diffusion is viewed as a room distortion because room boundary surfaces that are properly treated with diffusion sound more natural and realistic. It is easy to place too much absorptive materials naturally in the room especially if the room is used for living and listening. Diffusion technologies properly positioned bring more “air” or realism to our musical environment. Using absorption is manage reflections, one runs the risk of absorbing energy to manage it. Once energy is absorbed it is converted to heat and thus is lost forever. Diffusion can minimize the negative effects of reflections without converting them to a different energy form.
Our rooms are “boxes” that contain sound pressure and reflections. All of these issues cause some type of distortion on our musical signal. Low frequency pressure causes bass boom and room modal resonances. The speaker and walls are the genesis of most of these issues. Lack of proper diffusion in our box makes for an unrealistic presentation with our music sources.
Reverberation times can be balanced throughout the church with proper treatments placed on the correct surface areas.
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