Zero would be a good number for a music only system with big full-range speakers. How big the speakers need to be is determined by the size of your listening room, how closely you sit to the speakers, the kinds of music you listen to and what kind of peak volume you want. For movies, zero is usually too few, as there is never enough bass to reproduce an atomic bomb or train crash. So, at least one is usually needed for the low frequency effects (LFE). This LFE information is only from 80 Hz and lower per the Dolby standards.
The Dolby standard also calls for just one channel of information for the subwoofer and LFE both in plain DVDs and Atmos Blu-ray DVDs. Dr. Floyd Toole, in his book Sound Reproduction: Loudspeakers and Rooms, devotes at least 18 pages to subwoofer number and placement and discusses experiments with up to 5000 subwoofers! His primary concern is smooth distribution of bass energy in the typical home listening rooms. Dr. Toole concludes that one sub is good, two is a substantial improvement and four is still better; but the improvement gets less and less each time you add another sub. We mostly agree.
In my blog about the critical importance of subwoofer placement for music reproduction, I explain why the location of the mains and subwoofers is so important. I conclude that if you have only one, the sub should be placed up front between the mains. If you have two subs, they should be each placed under or next to each main. This way there is no cancelation at the crossover frequency almost anywhere in the room due to time-to-travel phase differences. These shifts occur when a subwoofer is at a different distance to the listener than the main speaker. This can cause cancelations at various listening locations in the room.
Even music’s deep bass is usually played back in mono. In the days of records, this kept the needle from skipping from the up and down groove for stereo. Now it is true because of the near universal Dolby standards. This can cause problems. If you are sitting seven feet closer to the nearer speaker than the more distant speaker, you are going to lose most of the 80 Hz information from all mono sources – be it from mains or subwoofers. This is because the output from the more distant speaker cancels the output of the nearer speakers. It is best to move your chair away from the near speaker. These phase shifts can cause problems and every room has some. Having this mono information spread through multiple subwoofers also reduces cancelations in the very deep bass.
To a rather small amount. At 80 Hz, it is quite difficult to determine the source of sound. However, a sound event with this very low frequency information usually has some higher frequency sounds as well. If you can move your head around while trying to locate the source, it becomes like having many pairs of ears. Imaging clues come almost exclusively from the leading edge of the sonic event (this is why a continuous tone’s location is typically much harder to locate than a series of short events with some time between each event.) Therefore, imaging is easiest to perceive from different listening positions, when the front speakers with a coherent time, phase and amplitude signal first reproduce the sonic event.
For the best imaging when you have more than two subwoofers, the other subs should be placed so the listeners are equal or greater distance from the mains and at a somewhat lower sound pressure level. In my experience, a 30-50 millisecond delay works well to keep the image up front. Also, having all the extra subwoofers combined, play slightly lower in sound level than the main/front subs alone was the best for music.
Image Courtesy of Floyd Toole
Part of the scientific method is being able to repeat published experiments. I got a call the other day from a customer who had just added a second pair of subwoofers to his room and was raving about the huge improvement it made. Since we were skeptical about the effect of rear subwoofers on imaging, we decided to try it ourselves. We took a pair of MicroWalsh Shorts and four SBm-10 subwoofers and set them up in a small room.
Dimensions, 16’ (l) x 10’ (w) x 10’ (t); Construction, sheet rock with air space on all sides, brick wall behind one side wall and rear wall (but not touching) and the other two sides adjoining larger rooms; Air space beneath the floor; No soft surfaces. Not very live, but I could hear some ringing in the sound of my own voice.
One subwoofer in the corner. Not something that we recommend, but a very common placement that a lot of home theater receiver manufacturers recommend. The result was significant difference from side to side (not in terms of imaging, but seating position). The whole stereo image was a little off center in the direction of the subwoofer. The best position front to back was in the middle of the room. Moving farther back made the bass “ringier,” especially when approaching the back 1/8 part of the room or the front 1/4 of the room. Clearly the acoustic wall in the back was behind the actual wall, probably an effect of the unusual construction. Too much variation from side to side.
We moved the subwoofer half way in between the two speakers, which is what we normally recommend. Frequency balance was much smoother moving from side to side.
We added a second subwoofer in the same position behind us. The least effect was in the chair in the center of the room, but the bass was considerably cleaned up in that position, and to a greater degree in all other positions. Front to back frequency balance was much smoother.
We had two subwoofers in the two front corners. All the front to back ringing came back with a vengeance. Side to side was consistent, but consistently bad. Very noticeable excitation of the front to back room resonance.
We added two subwoofers in the back corners, making a total of four. It made the biggest difference when standing. Our ears were at the 50% location of the floor to ceiling resonance, whereas when seated, the floor to ceiling resonance was so loud that any effect from the rear subwoofers was less noticeable. The difference was subtle, but it was an improvement.
We made the front subwoofers into stands for the main speakers which were located 25% of the way from the side walls and placed the rear subwoofers exactly opposite them. This sounded very good when all of these were 25% of the way from the front and back walls, respectively; moving them against their respective walls raised the wall coupling frequency so high that adding the second pair of subwoofers didn’t really help solve the problem.
Leaving everything in the same place from the sixth test, we listened with the main speakers running full range versus restricted range. The restricted range speakers integrated much more easily with the subwoofers than the full range speakers. The mid bass was considerably cleaner. This was particularly noticeable on male voices.
In all cases, adding more subwoofers made the vibrations viscerally noticeable through our chairs, which always helps the impression of deep bass.
Moving from one subwoofer to two makes a very noticeable difference in the performance of the system. Going from two to four was always an improvement, sometimes for certain seating positions more than others. Often the improvement was so subtle, that it would probably not be worth doubling your budget for subwoofers. However, our room was not especially live, so it is conceivable that in a room with a bad standing wave problem the second pair of subwoofers could be part of the solution. At the point we had reached, there would have been a much more appreciable improvement if we had added some absorptive material such as large overstuffed furniture. One thing we did not notice was that the rear subwoofers made the image any less focused in the front. The most important thing that we found to affect imaging was that the arrangement be symmetrical from side to side and having the subwoofers as stands for the mains.
See you in two weeks, until then,
Enjoy & Good Listening,