February 7, 2005

Q: What’s the best position for my subwoofer?

Jim Mueller

A: Human hearing is an extraordinarily sophisticated process whereby we are able to use our two ears and the slightly differing signals that reach them to locate sounds in three-dimensional space. Primary among these auditory cues is the minute difference in time of arrival of the same sound at each ear: whatever the relative loudnesses of the two arrivals, the sound will appear to come from somewhere nearer the ear that first received it. Such tiny differences require that the sounds themselves be made up of small events, and that means the treble and, to a certain extent, the midrange frequencies.

In the bottom two octaves of the audible range, however, sound is almost entirely omnidirectional. That’s because the wavelengths at those frequencies can be measured in tens of feet, rather than in fractions of an inch as in the treble frequencies. Because the ear-to-ear difference in level at any given moment of, say, a 40Hz signal is likely to be minimal, the human hearing system is not able to localize the sound. Thus, in theory, a system’s woofers could be placed anywhere in the room and still work well. And that’s half true. As long as the subwoofer produces only the lowest frequencies -- ideally, those below 80Hz -- its position is immaterial as far as localizing goes. It can be tucked away under a table.

But there’s another aspect to the long wavelengths of very low-frequency sounds. Chances are that each of the dimensions of a listening room will have a simple arithmetic relationship to the wavelengths of some of the frequencies being produced. If a sound has a wavelength of, say, 20’ and a room is 10’ wide, the sound wave will bounce back and forth, its peak levels reinforcing each other at the same physical point over and over, and its lowest levels, or nulls, similarly reinforcing themselves somewhere else. This is called a standing wave; depending on the frequency being reproduced and where you are in the room, the sound might be unbelievably boomy whenever that frequency occurs -- or nonexistent.

Because the different dimensions of the room will affect different frequencies, some frequencies may be boomy, some may disappear, and some may be about right. Move a few feet, however, and everything will change. The trick is to find positions for each speaker, and for the prime listening location, where none of these effects is too severe. That doesn’t mean the sound won’t be awful elsewhere in the room, but in many cases taming the primary listening position is about all you can hope for.

Things are simpler the more irregular a room’s shape. The enemies are parallel walls, which allow the sound to bounce back and forth regularly. Surfaces that are not parallel or at right angles to each other, such as cathedral ceilings, tend to randomize such awkward acoustic effects as standing waves -- effects that go under the general heading of "room modes" -- and permit quite flexible speaker positioning.

Very large rooms also tend to be fairly easy to handle -- their greater dimensions mean that their modes occur at frequencies below the range of most musical information. But most of us listen in rooms that are small rectangular boxes, and in such rooms acoustic problems abound.

Moving a subwoofer even a few inches can often have a dramatic effect on its output, so be prepared to spend some time tinkering and moving things about. Dolby Labs suggests you place the subwoofer temporarily in your prime listening seat, put on some music with lots of lows, then walk to the various places in the room that might be suitable locations for the sub. What you hear at each spot is pretty much what you’ll hear in your chair when the subwoofer is in that spot. Just pick the spot where it sounds best.

…Ian G. Masters