Small Room-Low Freq Control

Cool article on Modal mode in your small project studio.

It is no surprise that industry figures show that there might be over 100,000 "home / project" studios in existence today. This number will grow. In the last 10 years, an audio revolution has occurred, which allows "home studios" to rival the sounds of pro level studios. "Desk-top" audio, as I call it, is possible due to several factors:

1. reduced equipment prices;
2. integration with the ever more powerful home computer;
3. the digital revolution in general; and
4. an associated social change in the methodology of producing and distributing music.

While these technical changes have affected the way music is recorded, they have not affected the way music is played back (listened to). Songwriters, musicians and producers still listen to music in indoor environments and depend on some order of true acoustic response from a monitoring system in order to discern content and production values.

There are many acoustic and architectural issues that we will discuss in this column during the next year. Where do we begin? This is the challenge that TAXI has presented to me. The most common question I am asked is usually something like, "How big (or what size) should my room be?" During 34 years of designing studios, I have been asked this question thousands of times. It's a little bit like asking how big should a car be. (Of course the next question is how much will all this cost, which is a little bit like asking "How much does a car cost?" We'll get to that later!). This size question is the hardest. But a good answer will make the rest of the studio design and building process (or remodeling process) easier.

Most "home / project" studios are small. They often have less equipment than many larger commercial studios, plus the client accommodation factor is typically less or non-existent (you don't have to have a fancy lobby!). Small room design is not a black art, although certain acoustic issues are in fact more complicated in smaller rooms than larger ones.

Most studio acoustic issues can be reduced to two large areas of concern:

1. Sound isolation (prevention of sound transfer from one space to another)
2. Internal room acoustics (what happens to sound in a playback or recording environment)

It is the second issue we are concerned with today. (Regarding issue #1: A great number of "home / project" studios do not have serious isolation issues or elect to not deal with them for financial reasons).

Since music has a wide frequency range (as opposed to speech, for instance) and since the level of accuracy of affordable mid-size speakers has become amazing in the past few years, even our small room acoustically gets complicated very quickly. Rooms behave differently at different frequencies.

Simply presented, at mid and high frequencies (those for the most part above 300hz) individual reflection patterns will cause sound to be perceived cleanly or not at the listening position. At these frequencies, sound can be "viewed" a bit like rays of light. At lower frequencies, however, reflection control becomes less and less relevant (since wave lengths become larger) and what counts more is the ratio of raw room dimensions and the position of speakers and listener. Sound at these frequencies behaves more like waves. The overall dimensions of the room will effect the natural distribution of eigentones (fancy term for "standing waves").

A common misunderstanding is that standing waves are bad. This is ridiculous. That's like saying that wheels on a car are bad. Four different size wheels on a car are bad! Standing waves always exist in a closed environment. What we strive for is as even a spacing of these frequencies as possible. Think of these standing wave frequencies as the ability of the room to "ring out" or reinforce tones naturally. And so one can imagine that if the proportions of a room are chosen correctly, then there will be a more natural spacing of the tones and the room will tend to reinforce lower frequency tones more evenly. This is a good thing. The opposite, of course, would be harmful and tend to cause uneven response at the listening position. Not a good thing for audio playback.

So, the first step in room acoustic design (after making sure that all equipment, furniture, etc. fits) is to try to choose a room shape that has as good a chance of even low frequency eigentones spacing (organization) as possible. That is what we will discuss in the remainder of this article. (We'll look at high frequency reflection control in the next article).

By way of example, I recount this story. A former student called me up a couple of years ago and ask,

" . . . I have a 20 ft. by 20 -ft. basement room that I want to use as a control room for my home studio; what should I do to make it sound good?"

That's a big question. Half of me wanted to hang up, but half of me accepted the challenge of trying to give him a one-minute answer. After a minute of thinking, I answered,

"Build a closet."

He probably thought I was joking, but I still believe this answer was a good one. The square room (20 ft. by 20 ft.) is almost the worst shape you could have (only thing worse would be a 20 ft. cube). The width and length being the same dimensions would cause the lower frequency eigentones (standing wave frequencies) to be identical, thus causing harsh frequency anomalies -- pile up of energy -- as well as voids at other frequencies. These frequencies are not that hard to calculate, doing some very easy math. (My promise to TAXI was no math in this column, which is hard, since the language of acoustics is physics and one of the languages of physics is math!) By building a closet, the future TAXI driver would have possibly created a room that was 20 ft. wide and (more or less) 15.5 ft. deep -- much better room ratio. He also gets a closet for storage and possibly a good location for noisy equipment and other devices. Notice that I suggested in his new room that 20 ft. was the WIDTH of the room. By having the side walls further away from the listening position we help mid / high frequency reflection control (see article illustration 1). Again, we will discuss high frequency reflection control in next month's article. Choosing room ratios can also be easily analyzed by using a very well known industry "pictogram" of accepted room ratios (see article illustration 2).

Illustration 1
Before and after low frequency modal distributions for a 20' x 20' room and a revised design.

Note the improved room acoustics due to better modal distribution.

Ill. 1a - shows 20' x 20' room in plan

Ill.1b - shows newly created 16.5' x 20' room in plan

Illustration 2
Acceptable room ratio "pictogram" with basic user steps:
a. divide all room dimensions by the height (this will then set the height as 1)
b. plot width and length on horizontal and vertical scales
c. determine acceptability by noting whether plot is in or out of "the zone"

Ill. 2a - shows 12' x 24' x 36' room - poor low frequency room mode acceptability


Ill.2b - shows 21.5' x 15' x 9.5' room - good low frequency room mode acceptability

Reminder: the easiest way to begin to have good low frequency response in your room is start off with good room ratios. There are other acoustic "treatments" that can be added to the room if needed, particularly if circumstances do not allow good room ratios. We will look at them later.

Have fun!

PS. aaah...some of you will wonder what the hell is room modal and how to calculate your own space to what is the best room mode. That is what coming next....so come back!