Real Coverage, Part 4
Dec 1, 2001 12:00 PM, By Rick Kamlet
PART 4: SUBWOOFERS
This is the fourth and final installment of your own personal business music systems handbook. With these articles in your library, you will be well-equipped to install a system that will please your clients and garner enthusiastic referrals. Although you may already know how to install business music systems, some of the tips and hints you'll find here will make good systems great.
In March 2001, Part 1 led into the topic by discussing the increased awareness and higher standards of today's music-listening public — a public that includes your clients. People are no longer content with shoddy or shallow-sounding music. The public's ear has gotten more sophisticated. So beginning your project by asking some specific questions, spelled out in Part 1, will make the first attempt to fulfill your client's needs much more likely to succeed. That first installment went on to discuss objectives for power amplifiers and accessories, and we began to look at the concept of the listening plane. Recall that the polar coverage specification for speakers does not accurately reflect what people will hear in the actual listening field.
Part 2 (May 2001) focused on sound pressure levels and how to compute the maximum sound capability of a proposed speech or music system. Part 2 also discussed the right way to equalize a ceiling speaker system. Part 3 (August 2001) outlined the options for speaker patterns and density and delved into the benefits of each possible set-up. Equations were given to help you determine the variance in SPL that comes with each possible layout pattern of ceiling speakers.
Now, let's turn our attention to another element that can mark the difference between good and great business music systems: subwoofers. Remember that sound quality — naturalness and clarity — is what will earn you a reputation as the installer to hire.
Subwoofers are an important part of an outstanding business music system. Light background or foreground music might not require subwoofers; however, even in systems where the bass doesn't need to be a dominant factor, having clear low frequencies can make a big difference in the customers' enjoyment of the music.
The number of subwoofers to use, where to position them, how to set the taps (on 70V/100V subs) and how loud to run them vary depending on the characteristics of each installation. Criteria such as speaker placement, boundary loading (are speakers placed close to a wall or in a corner?), size of the room, coupling of multiple speakers/subwoofers, reverberance of the room, the type of music, the type of activity and the expectations of the listeners all come into play. The following guidelines are given, therefore, in very general terms.
CROSSOVERS
The four ways to cross over to a subwoofer are: passive crossover, which is usually built into the subwoofer; acoustic crossover, such as a bandpass box that is acoustically filtered not to reproduce high frequencies; active crossover, which may be a separate electronic device or can be built into the subwoofer or a controller; and a combination of these, such as using a bandpass box with an active crossover.
In addition, there are two main topographies for crossing over: overlap crossover, where the main speakers are run full range and the subwoofers are just added to them; and full crossover, where the subwoofer covers the subwoofer frequencies and the main speakers are high-passed to cover the rest. (See Figure 1.)
You need to decide on a system topography — the way you're going to cross over the system — before you can figure out the quantity of subwoofers needed. Let's talk for a moment about the options.
OVERLAP CROSSOVER
In an overlap crossover, the main speakers are run full range (as full as they cover), and the subwoofers just add to the bass frequencies. An overlap crossover can be accomplished either with a built-in passive crossover or with an active crossover. The advantage of using an overlap is that it sometimes allows you to use fewer subwoofers. The big downside of this topography is that the main speakers usually only go down to 80 Hz or so, and the subwoofers often have a response as high as 160 or 200 Hz. (Hopefully, the subwoofers are internally low-passed with a passive crossover or they're limited by being a bandpass design.) Even if the subwoofer only goes up to 120 Hz, you're often in trouble.
The problem is the overlap band. Between 80 Hz and, let's say, 160 Hz, both the mains and the subwoofers are reproducing, whereas below that range it's only subs, and above it's only mains. Thus, you get much higher sensitivity in this low-to-midrange band. You end up with a big bump in this mid-bass range, which is often perceived as muddiness.
“Don't mistake loudness for fidelity,” is a good adage here. An overlap-crossover system might get loud but fall far short on the fidelity scale. The business might comment that the subwoofers don't seem to get very low because you're emphasizing the mid-bass range. You can add more subwoofers until the cows come home, but it will only get muddier.
To compensate for this effect, you need to include a good EQ to notch out the bump. A single parametric band can often do it, but, unfortunately, most business music sound systems don't include a parametric EQ. It's difficult with a graphic EQ of less than 31 bands. Even a 15-band graphic can take out too much good stuff along with the bad, unless the frequency and bandwidth happen to match your bump precisely. Certainly, 7-band EQs are of little use with this kind of overlap bump. While an overlap crossover might allow you to use fewer subwoofers, unless you're including a really good EQ, an overlap tuning in a business application is probably not advisable.
When using an active crossover on the subwoofer, you can slide down the low-pass frequency to reduce the mid-bass bump. While that will help a lot, it's still often difficult to match the electronic low-pass characteristics of the subwoofer band with the acoustic low-frequency roll-off of your main speakers. Adjusting the low-pass frequency is usually a big improvement from the passive overlap bump, but you can still end up with some abnormalities at and below the crossover point.
FULL CROSSOVER
A full crossover high-passes the main speakers and low-passes the subwoofers. The result is a nice, smooth transition. A full crossover almost always sounds better than an overlap, but since the subwoofers have to cover the bass frequencies all by themselves, you will probably need to use more of them. You can accomplish a full crossover either passively or actively.
Full Passive Crossover
Passive systems usually use crossovers built into the subwoofer. The full-range amplified sound goes to the subwoofer, where the lows are sent to the subwoofer driver. The main speakers are connected to the satellite output, which sends them mids and highs (with the bass removed). This works alright, but the crossover components need to be large (to handle the low frequencies), and they eat up some of your power. The crossover slope is usually not very steep, around a 12 dB per octave low-pass to the sub and a 6 dB per octave slope to the mains, or satellite speakers. Steeper high-pass slopes are typically avoided because they can self-resonate or cause strange impedances to the amp if a satellite speaker doesn't get connected to it or if the satellite speaker blows during use. However, with a first-order (6 dB per octave) passive high-pass crossover (for the satellite speakers), the crossover frequency changes with the impedance that is connected. The higher the impedance load, the lower the crossover frequency. An output that works properly with a 4-ohm load — as with two 8-ohm speakers — will be too low in frequency if you only connect a single 8-ohm speaker.
The biggest potential problem of a full passive crossover is that you're at the mercy of the sensitivities of the subwoofer speakers versus that of the mains and the need to properly balance the volume of each. The subwoofer might have a sensitivity of 89 dB, while the full-range speaker might have a sensitivity of 92 dB. We will talk more about this later, but in business applications, because of the low volumes, the bass often needs to be between 6 dB and 10 dB louder, not quieter nor even equal in volume, to the mains in order for the sound to be balanced. In passively crossed systems, the subs are often softer than the mains, and that's a problem.
The fix? Well…one fix is our friend the high-resolution EQ that can pinpoint and boost the exact frequency where the volume drops. A standard bass control (shelving type) is usually not a good solution. The chance that it matches with the exact frequency, slope and shelving characteristics that are needed by any particular system are slim.
It is difficult enough for engineers to design a good passive crossover when they know the exact characteristics of every component in a single cabinet. With a business music system, you've got so many variables — sensitivities, roll-off characteristics, number of speakers, placement, boundary loading effects, etc. — that it is difficult to get a passive crossover to work well. Although a well-done passive full crossover can sound quite good, it's much easier to wind up with one that sounds pretty bad.
Full Active Crossover
Full active crossover is the most reliable way to get a good subwoofer sound. This means using an active crossover and a separate power amplifier for the subwoofer(s). The subwoofer gets low-passed using a steep slope, usually 24 dB per octave, and the mains get high-passed with a steep slope. They interact in predictable ways. There is virtually no overlap between the subs and the main speakers. There is no booming overlap bump like you get with the overlap crossover. You get independent control over the bass volume so you can easily balance it with the mains by ear, or via an SPL meter. If the customer doesn't like the balance, you can easily adjust it.
If you have a choice between an active crossover or an EQ, you're probably better off adding the crossover instead of the EQ. In business applications, you may not need to do much room equalization like you would with a center cluster in a sound-reinforcement system. With the proper crossover, you minimize the need to equalize at the subwoofer crossover point. There are business music controllers that include a subwoofer crossover built right into them, like JBL's Soundzone controllers and some from other companies that are starting to hit the market.
SPL AND FREQUENCY RESPONSE
Typically the goal for subwoofers is to be somewhere between 2 and 10 dB louder than the main speakers in the system. Low- and medium-level music requires subs to be a little louder than the main speakers because at low sound levels the human ear needs more bass for a perception of well-balanced sound. Given the same music, higher levels of music can sound well balanced with less relative bass increase. There are also some music types and applications, like dance music in upscale fashion retailers, that may require even a little more than +10 dB of bass. Setting the ratio of subs to main speakers somewhere between 0 dB and +10 dB, as measured by a flat SPL meter, is usually a good starting point, though.
As for subwoofer frequency response, especially for business applications, you probably don't want the subwoofer to go much below 45 Hz. Below that, you end up with bothersome rumble, which can build up in corners or at room-mode nodes (function of a room's dimensions). Rumble can annoy customers. They might not be conscious of it, but when they're standing near the corner of the room, looking at clothing on a rack, low-frequency rumbling can make them uncomfortable enough to chase them out of the store.
If a customer says they don't want subwoofers because they heard subs at the XYZ Store and they were bothersome, then there is a good chance that XYZ Store's system installer made the mistake of putting in too much rumble below 45 Hz, or left in a mid-bass bump because of an overlap crossover, or used a passive crossover that doesn't work correctly with the selected product mix. The list of pitfalls goes on, but if it's well-implemented by a skilled contractor, a business music system with subwoofers can sound absolutely wonderful!
SUBWOOFER POSITIONING
So how many will you use, where will you put them, how will you arrange them, and how will they relate to the mains?
Effect on SPL
Hanging a subwoofer in the middle of a room results in the lowest possible output from the subwoofer. Placing a subwoofer at the ceiling, wall or floor increases its output. Placing it within a few feet of a 2-boundary junction (like a ceiling/wall junction or a wall/wall junction) increases its output further. Placement within 3 feet of a corner increases its output still more. In these cases, there is both an increase in sensitivity (output per watt of input) and in maximum total SPL capability. This can help in getting as much sound as possible from a few subwoofers. However, there is a potential pitfall in placing a subwoofer in a corner: You can wind up with uneven bass coverage in the room.
Achieving Even Coverage
In most installations, there are a lot more satellite speakers than there are subwoofers. Because there are often so few subwoofers (maybe only one), you can have a problem getting even coverage of the space. People sitting or standing very close to the subwoofer are going to get blown away with lows while people who are father away might not be getting enough. How do you make the subwoofer coverage as even as possible?
As you move farther from the sub, the volume drops off, typically at 6 dB per doubling of distance. Then, when you reach a certain distance, the subwoofer level stops dropping off at such a high rate. This is called the critical distance, which is where the reverberant field within the room equals the direct sound from the subwoofer. (See Figure 2.) The critical distance depends on how reverberant the room is. As you get farther past critical distance, even though the level of the subwoofer doesn't drop off nearly as quickly, the quality of the subwoofer sound might not be as good. But even though this may happen, it's sometimes acceptable for subwoofers in business music applications.
One way to make the subwoofer coverage as even as possible is to use more than one. It's a myth that all you ever need is one subwoofer. In many places, it's a good idea to add a second subwoofer, or more. Even if you don't need additional subwoofers for volume reasons, you might want to consider them just for evenness of subwoofer coverage. If I absolutely have to use just one subwoofer, my personal preference is to sacrifice the sensitivity increase and place the subwoofer for most even coverage, as long as I can achieve the SPL goals.
Placing Two Subs
Subwoofer placement is an art. In systems with two subs, it is often best to place them asymmetrically within the room. In other words, if one sub is in the middle of a wall, try to avoid placing the second sub in the center of the opposite wall. Small-room acoustics can cause interactions between subwoofers to create places where the bass builds up and other places where the bass cancels out and disappears. The topic of room modes is an article in itself, but for now it's important to realize that, while there is not much you can do about room modes, you can minimize their effect with conscious subwoofer placement.
If you place both subs symmetrically (on opposite walls), they will excite the same room modes in the same way, making disparities worse. If you place the second sub in a different position, it will tend to excite the room modes in a different way, and this is usually better. It's also good to know that placing the sub in the corner, while it does excite room modes, usually results in fewer mode bumps as compared to mid-wall placement. My experience when using two subs has been that one goes in the corner and the other goes close to, but not in, the opposite corner, about 10 feet along one of the walls.
Ratio of Subwoofers to Main Speakers
The subwoofer is usually putting out more power than the main speakers, 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 speakers at full power and only one or two subwoofers of the same power rating as the mains. Picture a three-way home stereo speaker in your mind. It has a 4-inch midrange driver and a tweeter. What size bass driver would you expect that one speaker 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 speakers 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 70V/100V models, then 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 speakers 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 6dB 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.
Distributed Subwoofers
If you are using in-ceiling subwoofers, remember (from Part 1 of this series) that the subwoofer coverage projected onto the listening plane only covers 120°. If you're concerned about having even coverage throughout the room, determine how many subwoofers you need based on approximate coverage of 120° per subwoofer.
If you're using subwoofers, I recommend selecting a full active crossover system and using enough subwoofers to handle the SPL requirement and cover the room evenly.
CONGRATULATIONS!
If you've made it this far, you can consider yourself an expert on business music system design!
As I mentioned earlier in the series, there are several computer programs that can help you with the design process. JBL's free utility called “Distributed System Design” does the polar-to-listening-plane conversions, computes how far apart to place the speakers, 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, and calculates how much amplifier power the system needs. (While this particular program is only set up for JBL speakers, 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. Only that will tell you how many subwoofers to use and 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.
Rick Kamlet is the senior director for installed sound at JBL Professional. His prior roles included senior product manager, director of engineering and national sales manager for a number of professional audio manufacturers.
For the other installments in this series, use the search feature at the top of this page and look for "Kamlet."
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