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SMALL ROOMS -- LOW FREQUENCY CONTROL
By John Storyk

Several months ago, TAXI founder Michael Laskow, who I know personally and professionally for many years, suggested a series of articles on practical acoustic issues that would interest the many TAXI riders with "home / project" studios. 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).

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.

For the online version of this article (including illustrations), go to

http://www.wsdg.com
Click on "Resources," then on "TAXI Article 1"

Have fun!

ebcdivider

SMALL ROOMS -- HIGH FREQUENCY CONTROL
By John Storyk

In the last article (#1), we discussed low frequency control in small listening rooms -- the types of rooms that are typical for many TAXI passengers. (Quick review: The best low frequency response will usually take place in rooms that have good proportions, remember?)  Now we're going to cover the control of high frequencies (300-400Hz and up) in such rooms. The result we strive for is in an even distribution of sound in the primary listening position. The good news is that this is normally easier to understand, and deal with, than low frequency control! Again, the best solution starts with good geometry, which in turn will give us good reflection control.

Comb Filtering

When you listen to recording playback, you hear two different types of sounds: direct and reflected. "Direct" is what comes to you from the speakers, and "reflected" is what arrives at your ears after bouncing off one or more surfaces.

Many small studios (actually I would venture to say MOST small studios) use "near field" monitoring. The monitor speakers often end up on top of the console or very close to the console bridge.

"Near field" monitoring (where the listener sits quite close to the monitoring speakers) is often used to accomplish an important monitoring goal: to have the direct sound from the monitoring system arrive accurately at the listening position, and not be "clouded" with high level reflections that arrive too soon.

When this reflection confusion (or "clouding") happens, "comb filtering" takes place. In fact, poor reflection control will almost always cause some sort of "comb filtering."

The name "comb filtering" describes a pattern of frequency response that looks something like a comb: very high and low discrepancies in the frequency response. (see figure 1). This is the opposite of the smooth (and accurate) frequency response that we want to hear!

The comb filtering effect comes in many shapes and sizes and is certainly a subject a bit larger than we can tackle in a short article such as this. But following is one of the simplest examples (and one that is a common "mistake" in small production studio control room environments).

Test Your Studio

Want to find out if "comb filtering" is happening in your studio? Try this simple experiment:

In your primary mixing position, listen carefully to a recording, particularly in the vocal range of frequencies. Then move the speakers back about 12", and do the same critical listening. You will most likely notice three things:

1) a clarity in the frequency range;
2) better stereo separation; and
3) more accurate fidelity.

This is happening because there is no longer a conflict between the direct (accurate) response from the speaker, and a harsh first reflection: one that is arriving a few milliseconds behind the direct sound. The frequency response associated with each of these two positions is shown in the figures below. These are the frequency response graphs of what you hear! (See figures 1 and 2.)

Possible Causes

This same reflection control (or lack of reflection control) can happen with side room walls that are too close to the critical listening position. It can also happen with hard ceiling surfaces that are angled incorrectly (i.e. sloped downward from the front of a room towards the listener).

The technical term for this phenomenon is Time Delay Gap (tdG): the time between the arrival of the direct sound from a monitor (speaker) and the moment of the arrival of the first important reflection (see figure 3).

In small control room environments like the ones we are discussing, the tdG should be no less than 10 - 15 milliseconds. If sound is moving at roughly about one foot per second (it actually moves about 1.1 feet per second), then (again roughly) we can calculate 10 - 15 milliseconds to be about 10 - 15 feet.

Possible Solutions

You can see how solving this reflection control problem with good geometry is possible (i.e. angled walls, ceilings, etc.). For examples, see figure 4. When this is not feasible, then surface applied acoustic treatments provide the only remaining answers. We have two choices:

1) Absorption -- which does not change the reflection pattern but reduces the level of harsh reflection

2) Diffusion -- which does not change the level of the reflections, but changes the pattern.

In the next article we will discuss the pros and cons of these surface applied treatments. In most "existing" rooms -- the kind of spaces that many TAXI drivers will be using for their production spaces -- the only solution is applied surface treatments.

For the online version of this article (including illustrations), go to:
http://www.wsdg.com
Click on "Resources," then on "TAXI Article 2"
Have fun!

ebcdivider

This series of recording studio and sound engineering articles are being made available courtesy of our friends at Taxi.  The articles aim to help you make better sounding recordings in your home or project studio and will be available here or online at Taxi, complete with pictures using the link at the end of the article or better still - sign up to receive Taxi's Newsletters FREE and get them delivered straight to your inbox!

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