Don't Correct the Room, Build It Right
By Nick Colleran and John Gardner
The use of sophisticated (and expensive) equalization to attempt room correction made the rounds of sound reinforcement companies and recording studios in the 1960s and 70s. Now, this bad habit is back (times five or more) in home theater rooms. While equalization can make a good system sound even better in a good room, it does not re-write the laws of physics.
The room is an active environment. It will fight back in what we'll call an acoustic "zero sum" game. Simply put, increasing the power of an absent frequency also increases the level of the out-of-phase room reflection, perpetuating the "null" in the room. It may even be possibly broadening it. Remember, if the sum of the +3dB SPL original and -3 dB SPL reflection equals zero, so too will adding 3 dB for a + 6 dB original SPL and - 6 dB reflection SPL sum to zero.
This can be demonstrated effectively with a simple experiment. Using a single driver loudspeaker, a single frequency oscillator, and an amplifier, feed a 1000 Hz tone to the speaker through the amplifier to achieve a comfortable listening level. Reposition the speaker to face a hard, flat surface approximately six inches away. The sound will all but disappear. This is due to the distance being one half wavelength of 1000 Hz. The reflected energy is 180 degrees out-of-phase with the source. This is most easily shown graphically.
Sound travels at approximately 1,130 feet per second in air. The wavelength of 1000 Hz is 1,130 feet divided by 1,000 Hz, or 1.13 feet. Half wavelength equals 6.78 inches, which is 1.13 feet, divided by two times 12 inches per foot. This calculation will vary slightly for temperature, pressure (altitude) and humidity conditions.
As you can see from the diagram, increasing the source signal level will always cause a corresponding increase in the reflected signal level. In practice, the reflected signal is going to be slightly less, and some sound heard as signal strength will diminish over distance and absorption.