Room Dimensions, Modes & Modeling

(Why it Matters to the Bottom Line)

Did you know that the room dimensions of your home theater determine how accurate your bass response will be? The room actually has more influence on the signal at low frequencies than does any good speaker design. In the last segment we talked about theater size and how budget, number & type of seats, performance and noise control may influence the final size of the room and how those issues have to be personally prioritized. In this installment, we’ll talk about room dimensions in respect to performance.

How room dimensions can sound bad
Room dimensions dictate room modes, and how evenly those modes are distributed will dictate sound quality. Poor mode distribution means non-linear, unnatural bass response. Some notes are too loud and others are too soft. In its basic form, room modes are resonances between two parallel surfaces, for example the floor and ceiling, front and rear wall and sidewalls. When sound hits a large, hard surface like a floor, it reflects up to the ceiling and then bounces back to the floor, back to the ceiling, on and on until the energy is eventually absorbed. These axial modes are two sound waves traveling in opposite directions, parallel to one axis and striking two walls. As the sound reflects back and forth between the surfaces, certain frequencies reinforce themselves while others cancel (high and low pressure zones) depending on how they physically “fit” between these room boundaries. We have to get a little technical here in order to make some sense of this. Sound has frequencies (cycles per second called Hertz). These frequencies have wavelengths. You can determine wavelength by dividing the speed of sound, which is about 1,130 feet per second, by the frequency. For example, 50 Hz. turns out to be about 22.6’ long, while 500 Hz. is 2.26’ and 5kHz. is only 0.226’ long. For reference, middle C on the piano is 261 Hz. Room modes are most problematic at bass frequencies (up to around 300 Hz.) because of large audible energy gaps. Most of the keys on the piano are affected by room modes.

This may be repetitive, but I am trying to get my point across. Rooms have modes by default and they affect audio quality. When room modes are not dispersed evenly, modes can be too close and stack up on each other, causing certain frequencies to sound abnormally loud or soft depending upon whether speakers and listeners fall in their sound pressure “peak” or “valley”. They can also be too far apart, becoming isolated and draw attention. Sound congregates in corners. Try playing some music with a good bass track in it and walk to a corner of the room while paying attention to the bass energy. You will notice that the bass becomes heavy as you get closer to the corner. Even worse, in the eight tri-corners of a room you’ll find maximum pressure for all the room’s modes. As if cranking up a bass control knob, corners do not sound natural. You now understand one of the reasons why it is not a good idea to place listeners close to a wall, or worse, a corner. The same goes for speaker location. Placing a subwoofer in a corner may offer lots of bass energy for little amplifier power (because of acoustical reinforcement), but is not linear or natural sounding because it excites every mode in the room. Place the same subwoofer in the corner of two different size rooms and the result is two different bass responses, each dictated by the room’s dimensions. Evenly dispersed modes are important for linear bass response. As you might imagine that a square room would be a disaster because the same resonant modes would be generated for the length, width and height. The associated modes, and the train of harmonics, would be very loud where their pressure is high, and very quiet where their pressure is low. Dimensions that are divisible by each other are also bad, as in a room with a length of 30’, a width of 15’ and a height of 10’.

What to do
If we calculate every axial mode below around 300 Hz. for room dimensions that fit our requirements for theater size and distributes room modes evenly, we’ll have a great performing room. That’s a lot of math to do and it doesn’t include tangential modes (four traveling waves reflected by four walls), oblique modes (eight waves reflecting from all six room boundaries) or human perception. An Internet search will find recommended room ratios, these might get you in the ballpark, but ratio formulas are really not applicable here. There are also ample spreadsheets to do the math for you, but they don’t help you decipher the results. You can buy software, some of which offer little more. It really takes more than simple math. You must also have an understanding of how the modes relate to speaker/listener location and human perception (a.k.a. psychoacoustics). Acoustic treatments are available, but those designed for low frequencies are very large and obtrusive. Electronic equalization can also help smooth out the peaks, but cannot fill in the valleys, etc. This is where you have to call in outside help because it takes an acoustical consultant who is familiar with small room acoustics. To accomplish this, with the aid of computer modeling of room dimensions, speaker and listener locations and acoustic treatments, he/she can analyze the situation and make recommendations on how to improve it. A couple of inches in room dimensions might be the difference between wonderfully articulate bass or muddy bass, and your acoustical consultant can fix your problem with ease. This will ensure that the money you spent on your set up becomes money well spent.

Next installment: Speaker/listeners Locations - modeling, the boundary effect, soundstage, etc.

Norman Varney is the owner of A/V Room Service Ltd.