Specifications

In such a case, an easy way to determine the best phase relationship is to lis-

ten to a low-frequency source while standing in front of the system and have

someone swap the phase relationship of the subwoofer from Normal to

Reverse while you listen. In one setting you should hear a distinct increase in

mid-bass, and in the other setting you should hear a distinct reduction in mid-

bass. It’s important to listen to the speaker system from the front, otherwise

you may be fooled into making an incorrect choice.

If you do not hear a change in the bass response, it’s probably because the

full-range and subwoofer systems are displaced by a distance of around 1/4

wavelength of the frequency region where they both provide the most out-

put. In such a case, neither polarity position will result in addition or cancel-

lation, hence the lack of change in response.

The solution is to either change the physical relationship of the two speaker

systems, or delay one of the two systems (whichever one is positioned clos-

er to the listeners) with a digital delay unit. A high-quality measurement sys-

tem that can read and depict phase response and/or impulse response would

be very useful in this situation. However, without such a system, you can

determine an effective delay time by trial and error. Simply increment the

delay time in small steps (1 ms) until the action of changing the Phase

Reverse Switch produces the maximum cancellation in one setting, and the

maximum addition in the opposite setting.With the switch set for maximum

acoustic addition, the two systems should be in time and in phase with one

another.You will have preserved optimum impulse and phase response and

can now filter out any objectionable mid-bass overlap with an equalizer.

(4) Absolute Phase

Quite a bit has been written about absolute phase, particularly in regard to

studio recording and hi-fi sound reproduction.The subject is, however, often

ignored in the field of sound reinforcement.

Figure 6

An APL-500 full-range speaker offset behind an APL-10 subwoofer