Calibrated Acoustic StringTM
One of the trickiest aspects of setting up studio control rooms or any critical listening environment is the challenge of getting rid of those pesky early reflections that damage the sound character and quality and muddy up the imaging of the sound field. Keeping the first 10 milliseconds reflection free is critical for true stereo imaging and maintaining the audio character (frequency response) of the signal.
You can make use of expensive and elaborate time domain measurement software/hardware, or you can hire a knowledgeable consulting firm (we're available) with the necessary equipment and design knowledge, or you can identify a significant portion of the problem with a length of Calibrated Acoustic Stringtm (CASTM).
CAS involves a length of string, preferably of minimal stretchiness, marked off in 1 millisecond increments. Sound travels about 13.2 to 13.5 inches (338mm) in one millisecond, varying depending on ambient temperature and air pressure. The string should be in the range of 15 feet longer than the distance between the loudspeaker to listening position. Using a felt pen, place marks on the string every 13.3 inches (338mm), leaving approximately 2 inches at the start as an anchor (this project may involve the use of a moderately dangerous substance - Duct Tape). Tape the anchor end of the string to the front of the loudspeaker (use some common sense here so you don't rip the tweeter out when you remove the tape - we accept no liability for the inept use of duct tape - any more than Red Green does). Now stretch the CAS to the listening position, maintaining a moderate tension on the string. Note the length of the string at the listening position. Add 10 milliseconds worth of string (132 inches or 3380mm) to the strength length at the listening position marker. Tape the end of the string down at the listening position, or have an assistant hold the string at the listening position, leaving the now-slack section free.
Now take the slack portion of the string, that extra 10 milliseconds worth, and try to touch various wall, floor, ceiling and equipment/console surfaces with it, as shown in the diagram above. Every reflective surface you can touch with the string is within the 10 millisecond reflection ellipse. This is another use of the concept of the TEF's TDS (Time Delay Spectrometry) measurement ellipse. The TDS ellipse bounds the reflection path length that determines which surfaces may contribute a reflection that will affect a measurement. The string defines the reflection path ellipse for the source and listener. Where the string is stretched taut when touching these surfaces, the angle formed by the string is such that the reflection path between the source and the reflective surface and the receive location are equal, which means it is likely a valid reflection path. You can also try this process in one millisecond increments by adding an extra 13.2 inches to the direct-source-to-listening-position string length and repeating the process. This will tell you which surfaces are problematic as well as the time offset of the reflecting surfaces, and how many reflections you have in each additional millisecond of arrival time window (the string is cheaper, not faster than using time domain measurement equipment). It would take dozens of these reflection-path/time interval ellipses to categorize the first 10 msec of most studio control rooms with the usual countertop clutter of outboard gear, coffee mugs and the console top, even before you get to the room walls and ceilings. Keep in mind that even round objects and the edges of equipment can act as efficient reflectors at some frequencies.
The other thing that the ellipse reminds you to do is check for reflections close to the listener; like the back wall, a producer's desk, an equipment rack near the back of the room. The reflections can occur at either the source or receive end and still arrive inside the critical time window.
Once the potential problem surfaces have been identified, you can set about applying some appropriate acoustical treatment to kill off those reflections, or diffuse them (or move the coffee cups). For help with that, you can either hire us, or buy all sorts of acoustical reference books if you're the DIY type.
Mc Squared System Design Group, Inc
323 - 901 West 3rd Street, North Vancouver, BC. V7P 3P9 Ph 604-986-8181