SUBWOOFERS

The Subwoofers should generally occupy the space between your main speaker systems. The reason for this preferred location is the so-called propagation delay of low frequency sound from the sub units.

All loudspeakers are natural electromechanical filters and as such, they produce a delay of some size between their input signal and their acoustical output response-there is no exception to the natural laws that cause this type of delay-despite what commercial manufacturers put on their literature.

In more technical terms, the amount of delay depends on the filter bandwidth and the amount of attendant phase shift inherent in the filter's band-limit or envelope shape and other characteristics. For example, a simple single-order or "single-pole" filter produces 90 degrees of phase shift at the frequency, where the signal passes through the half-power (-3 dB) frequency point. Thus a subwoofer whose high-pass characteristic is a single pole filter will exhibit a 6 dB per octave rolloff below its operating band. If we use a sealed-box with a 30 Hz lower band limit defined by the -3 dB or half power point, then we will have 90 degrees of phase shift or a delay of 8.3 milliseconds at 30 Hz. This delay is equivalent to roughly 9 feet of sound travel through air.

At the crossover frequency of 80 Hz we are imposing on this system, sound waves are 14.1 feet long. We will want to know what order our crossover will be. We will assume the subwoofer upper band limit is way beyond 80 Hz and will not add significant phase shift to the crossover, (in fact the 2242H driver described could be used as the low end of a three-way system up to 300-400 Hz with no special consideration except that its conversion efficiency is low). For the sake of example, I will use the common 18 dB/octave crossover slope and calculate the position correction estimate needed. First, each 6 dB/octave forms what we referred to as a "pole" for the calculation. Three poles gives us 270 degrees of phase shift or three-quarters of a sound wave at our 80 Hz crossover frequency. Three fourths of 14.1 works out to be 10.6 feet.

Fortunately for us, there is also delay in the column's low-frequency output due to the high-pass function and the built-in delays in the amplifier we have adjusted to get our acoustical alignment between cones and horn. This means we will not have to place the subwoofers 10.6 feet forward of the columns. In fact, because of the ear's forgiveness, you'll find there's a "window" of space for physical placement that allows a good deal of flexibility in setting the speakers into your listening space.

© 1997 Drew Daniels