Today’s section of the AES paper dives into the technical background of why acoustic damping is a critical component of any high-end listening space. They say the best way to solve a problem is to prevent it from happening. In this case, the problem is a lack of musical clarity due to unwanted resonance (high Q). As you will learn in coming weeks, the preventive measure is the inclusion of damping in the system.

From the AES paper titled as above, by ASC Owner & President, Art Noxon, comes the first of many excerpts from this enlightening piece. Enjoy!

View the entire paper.

Room Acoustics and Low Frequency Damping

The quality, “Q,” of a resonant system identifies its response characteristic. High-Q systems are sharply resonant. They are easy to drive and have a strong response at the resonant frequency (Fo). Low-Q systems respond less strongly and over an extended frequency range. A flat response system has zero Q.

The frequency response curve of a speaker may be flat from 20-20,000 Hz in the test chamber, a room without reflections. Place the speaker in a real room with a microphone at the listening position. Measure again the response. A series of peaks and valleys are recorded. Move the speaker or mic and a different curve is developed. A room has many resonant frequencies. Which of them are stimulated is dependent on speaker placement. Each peak and null in the spectrum identifies a resonant condition.

Any physical resonance will have a pressure distribution in space. The microphone at a pressure peak will register a strong signal. Move the mic ¼ wavelength to a node and no signal is received. In either case resonance is evident.

Definitions of “Q”

The “Q” of a system can be measured from its frequency response curve. The ratio of the resonance center frequency to the bandwidth that accompanies the ½ power or 3 dB down point comprises one definition of the “Q” of a system.

Usually room response curves are presented dB vs. log frequency format. Resonances occur at different center frequencies. If the “Q” is the same, the response curve shape is the same no matter which center frequency is chosen. The “Q” of an average room lies between 10 and 40. The “Q” of a free piano string is 1000.

Resonant systems with slight resistance have High-Q responses. Add energy dissipations (resistance) to lower the “Q”. Another definition of “Q” is 2pi times the ratio of the energy of the system to the energy lost per cycle.

Decay Relations

Ordinary resonances decay out following an exponential curve in time. The time constant (T) of the decay is the time required for the system to drop to 1/e of the original energy level.

The exponential decay equation can be used to develop the definition of “Q” for the system. If the exponent is a small fraction, less than 1/10, then a simple approximation arises. “Q” equals 2pi times the resonant frequency times the decay constant.

The traditional presentation of decay measurements is the RT60; the time required for the energy to drop 60 dB. The exponential curve appears as a straight line in its dB vs. time plot.

By combining the dB level version of energy with the exponential version, the RT60 is resolved to be 13.8 times the decay constant.

“Q” and Decay Constants

The resonance response Q can be expressed in the traditional measure of decay, RT60. It is developed by combining the lightly damped Q relations with the RT60 decay constant relationship.

The result of the previous analysis is the linear relationship between the resonant frequency of a listening room and its “Q” for a fixed RT60. For example, a room may well have an RT60 of 1 second at a resonant frequency of 90 Hz. This means that the room has a “Q” of 50 for that resonance. A current spec for listening rooms is an RT60 of .5 seconds. If this applies to room resonance modes, their “Q” varies from 5 to 100 in the 20 to 400 Hz range.