Conventional wisdom states that low frequencies, on the order usually reproduced by a subwoofer, are non-directional in nature. By extension that means that you can place a subwoofer anywhere in a room and expect to get good results because you really don’t perceive where the sound is coming from anyway. And that’s another example of conventional wisdom being wrong!
In nature, we know that if we are out in a thunderstorm, we have a relative sense of the directionality of the thunder. Since we can hear-and-feel a directionality of thunder, the rubric of non-directionality of low frequencies is not an absolute even in nature. In the home environment, this “theoretical” becomes even more muddled.
The fact that we listen with two ears that are separated is as important in understanding the directionality of a sound as the fact that we have two eyes – also separated — is crucial to our perception of space and depth. At high frequencies, the head shadows the source and it is easy to sense direction. In the midrange and upper lows, the sound reaches each ear at a different point in time, and that’s how we discern directionality. However, below a certain frequency the waves become so long as to reduce our perception of direction.
Up to a point…
Timing is everything. And with sound timing is geometry since it takes time for sound to travel – even an inch. Major problems come when both your main speakers and your subwoofer(s) are reproducing the same music. At the THX (and Dolby Digital) standard subwoofer crossover point of 80 Hz, the wave-length is 13.75 feet — the lowest note of a cello. Piano, organs, and many other musical instruments also produce music in this frequency range – and in some cases even lower. This frequency was chosen to keep all voice out of the subwoofer since we are most sensitive to voice problems. So, if the signal from the subwoofer arrives 6.25 milliseconds after (or before) the main speaker, they will cancel each other and that frequency rangewill be missing from the music. So, the biggest problem is not directionality; but truly losing some of the music.
Aha, you say, time correction will solve that problem. Except that time correction only works perfectly from one listening position. With two or more listeners, time correction can make things worse for one while improving for another. The ideal is having one subwoofer sitting under each main speaker; so at least they don’t cancel. A secondary advantage to this approach is that two subwoofers, playing the same music, also tend to even out the room peaks and dips.
However, if only one subwoofer is used, it is best centered between the two main speakers. Then, only the extreme left and right listening positions may have problems. If the main speakers are less than eight to ten feet apart and the subwoofer is centered between them, there will be no full cancellation throughout the listening area.
So, just moving your subwoofer can make a major improvement in music reproduction.
Enjoy and good listening,
John Strohbeen Author
John Strohbeen is the president of Ohm Speakers.