The Best Place To Put Subwoofers Is… ?
“Subwoofers should be located on the floor if at all possible.” This is one of those classic bits of audio “conventional wisdom.” I hear it from system operators on nearly every project.
But is it true?
If we look at the physics of sound, we learn that, as with most things in the world of audio, “it depends.” So let’s start by discussing why the floor might indeed be a good place to put subwoofers.
Placing any loudspeaker against a large plane (relative to wavelength) produces a half-space loading condition, meaning that all the energy radiating spherically (equal energy in all directions) from the loudspeaker is now being radiated into a hemisphere, or half the area. This results in a 6 dB increase in energy radiating into the hemisphere, the same increase that happens by doubling the number of subwoofers.
If the subs also happen to be very close to a vertical surface, say, a tall stage face, another quarter space loading is the result, producing as much as another 3 dB (assuming the stage face is a solid surface); in other words, again, another “doubling” of the number of subs.
For example, four subwoofers on the floor and close to a vertical surface could provide the equivalent output of as many as 16 subwoofers. This certainly seems to support the argument for putting them on the floor (and near a vertical surface), but what other issues need to be considered?
How about inverse square law? You know, the one that says for every doubling or halving of distance from the source, there is a 6 dB change in level (outdoors)?
At some live shows, listeners might be as little as 10 feet away from subs on the floor, while the farthest listeners might be 200 feet away. Let’s say your mix position is 100 feet from the subs. If the subs are hitting 100 dB SPL at the mix position, then the front row is getting about 120 dB while the last row is getting about 94 dB. That’s a 26 dB change in level from the front row to the last!
Of course, the difference wouldn’t be this great indoors, but it will still be uncomfortable for the first few rows of listeners.
Now let’s consider crossover points. With the subs on the floor, we have two sources (the mains and subs), reproducing the same content, that are separated by quite a large distance.
Let’s say the mains are flying at about 25 feet above the subs. What’s going on around the crossover point?
The first task is synchronizing the arrival of the subwoofer sound with the mid-high sound. For the front row, the subs need to be delayed about 13 milliseconds (ms) to be in sync with the output of the mains.
Yet at the mix position 100 feet away, so the physical offset is only about 3 feet (or 2.7 ms), and the back row offset is only 1.5 feet (or 1.3 ms).
So who gets the good sound?
This physical offset affects system alignment resulting in anomalies in frequency response around the crossover point as well. Any loudspeaker array in which there are two or more spaced sources reproducing the same sound produces a dip in the polar response in one or more directions, based upon the frequency and the physical spacing of the sources.