Ratio Of Subs To Main Speakers
The subwoofer is usually required to put out more power than the main loudspeakers, so you might need more of them than you had guessed.
You’re not going to get much bass if you’re using 20 full-range loudspeakers at full power and only one or two subwoofers of the same power rating as the main loudspeakers.
Picture a 3-way home stereo loudspeaker. It has a 4-inch midrange driver and a tweeter. What size bass driver would you expect that one loudspeaker would need to keep up with the midrange and highs? You’re probably picturing an 8-inch driver, or even a 10- or 12-inch one. That’s to balance a single 4-inch midrange!
Now imagine a sound system with twenty 4-inch midrange drivers. You’re going to need several 8-inch subs, or a couple of 12-inch drivers, plus a lot more power handling capability. Note too that the full crossover mode requires more subwoofers because the subs are carrying the low-frequency load by themselves; but, again, it results in the best overall sound quality.
The ratios in Table 1 are no more than rough guidelines intended as starting points. The system designer needs to compute the SPL capability and determine that it will meet the user-expectations for the application.
Note that the chart assumes the loudspeakers are all installed in the ceiling away from wall and corner boundary surfaces (not getting the bass reinforcement from these boundary surfaces) and that if they are 70-volt/100-volt models, that both the mains and the subwoofers are tapped at their highest settings. You can scale up or down from there.
For example, if you’re tapping the main loudspeakers down two taps (usually this means they’re down 6 dB) then you can reduce the number of subwoofers from what is suggested. Also, if you’re placing the subwoofers in or near corners (around a 6 dB increase in sensitivity) you can reduce the number of subwoofers.
In addition, we’re assuming certain sensitivity and power handling capabilities that may need to be changed for your installation.
The subwoofer module in JBL’s DSD Distributed System Design software asks a bunch of these questions and then computes the proper number of subwoofers for the system.
If you are using in-ceiling subwoofers, remember (from Part 1 of this article) that the subwoofer coverage projected onto the listening plane only covers 120 degrees. If you’re concerned about having even coverage throughout the room, determine how many subwoofers you need based on approximate coverage of 120 degrees per subwoofer.
As noted above earlier, there are several computer programs that can help you with the design process. JBL’s free Distributed System Design utility does the polar-to-listening-plane conversions, computes how far apart to place the loudspeakers, computes how loud the system can get with music or speech (taking into account the overlap factors), tells you what the sound level variation will be throughout the room based on the layout pattern and density, calculates how much amplifier power the system needs, and calculates the number of subwoofers (if subs are being used).
While this particular program is only set up for JBL loudspeakers, there are utilities out there that handle other models.
But even the snazziest software won’t help until you determine your goals — and your client’s goals — for the system. That will tell you whether you’ve succeeded. Success is completely within your reach.
A well-designed, properly installed, wide-bandwidth, low-distortion business sound system can create an outstanding space that customers will enjoy and that will improve the business’ image and ultimately their sales.
Rick Kamlet is senior market director, commercial sound, JBL Professional. Published with permission by JBL.