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Low Frequency Audio in Small Rooms

June 29th Fact-of-the-Day

Audio buffs know high-fidelity sound systems nearly always produce more bass in large than in small rooms, but few know why. A loud-speaker couples maximum sound energy into a room where the mechanical load impedance on the diaphragm is the conjugate of the impedance looking back into the diaphragm (where there is a mechanical impedance match between the diaphragm and the air it drives). Speaker diaphragm impedance normally is high, so a normal speaker supplies maximum sound energy into a room at frequencies where the air impedance is high. By definition, high air impedance occurs where the ratio of air pressure to air particle velocity is high. That occurs at room acoustical resonance frequencies (and at extremely low frequencies if a room is sealed). The lowest acoustical resonant frequency is determined by room size. Below that frequency air impedance is low, resulting in a mechanical impedance mismatch. That mismatch makes good low-frequency response difficult to achieve in small rooms. ©2005 Martek, International. All rights reserved.


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