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 personalbusiness music systems handbook. With these articles in your library,you will be well-equipped to install a system that will please yourclients and garner enthusiastic referrals. Although you may alreadyknow how to install business music systems, some of the tips and hintsyou’ll find here will make good systems great.
In March 2001, Part 1 led into the topic by discussing the increasedawareness and higher standards of today’s music-listening public— a public that includes your clients. People are no longercontent with shoddy or shallow-sounding music. The public’s ear hasgotten more sophisticated. So beginning your project by asking somespecific questions, spelled out in Part 1, will make the first attemptto fulfill your client’s needs much more likely to succeed. That firstinstallment went on to discuss objectives for power amplifiers andaccessories, and we began to look at the concept of the listeningplane. Recall that the polar coverage specification for speakers doesnot accurately reflect what people will hear in the actual listeningfield.
Part 2 (May 2001) focused on sound pressure levels and how tocompute the maximum sound capability of a proposed speech or musicsystem. Part 2 also discussed the right way to equalize a ceilingspeaker system. Part 3 (August 2001) outlined the options for speakerpatterns and density and delved into the benefits of each possibleset-up. Equations were given to help you determine the variance in SPLthat comes with each possible layout pattern of ceiling speakers.
Now, let’s turn our attention to another element that can mark thedifference between good and great business music systems: subwoofers.Remember that sound quality — naturalness and clarity — iswhat will earn you a reputation as the installer to hire.
Subwoofers are an important part of an outstanding business musicsystem. Light background or foreground music might not requiresubwoofers; however, even in systems where the bass doesn’t need to bea dominant factor, having clear low frequencies can make a bigdifference in the customers’ enjoyment of the music.
The number of subwoofers to use, where to position them, how to setthe taps (on 70V/100V subs) and how loud to run them vary depending onthe characteristics of each installation. Criteria such as speakerplacement, boundary loading (are speakers placed close to a wall or ina corner?), size of the room, coupling of multiple speakers/subwoofers,reverberance of the room, the type of music, the type of activity andthe expectations of the listeners all come into play. The followingguidelines are given, therefore, in very general terms.
The four ways to cross over to a subwoofer are: passivecrossover, which is usually built into the subwoofer; acousticcrossover, such as a bandpass box that is acoustically filtered notto reproduce high frequencies; active crossover, which may be aseparate electronic device or can be built into the subwoofer or acontroller; and a combination of these, such as using a bandpass boxwith an active crossover.
In addition, there are two main topographies for crossing over:overlap crossover, where the main speakers are run full rangeand the subwoofers are just added to them; and full crossover,where the subwoofer covers the subwoofer frequencies and the mainspeakers are high-passed to cover the rest. (See Figure 1.)
You need to decide on a system topography — the way you’regoing to cross over the system — before you can figure out thequantity of subwoofers needed. Let’s talk for a moment about theoptions.
In an overlap crossover, the main speakers are run full range (asfull as they cover), and the subwoofers just add to the bassfrequencies. An overlap crossover can be accomplished either with abuilt-in passive crossover or with an active crossover. The advantageof using an overlap is that it sometimes allows you to use fewersubwoofers. The big downside of this topography is that the mainspeakers usually only go down to 80 Hz or so, and the subwoofers oftenhave a response as high as 160 or 200 Hz. (Hopefully, the subwoofersare internally low-passed with a passive crossover or they’re limitedby being a bandpass design.) Even if the subwoofer only goes up to 120Hz, you’re often in trouble.
The problem is the overlap band. Between 80 Hz and, let’s say, 160Hz, both the mains and the subwoofers are reproducing, whereas belowthat range it’s only subs, and above it’s only mains. Thus, you getmuch higher sensitivity in this low-to-midrange band. You end up with abig bump in this mid-bass range, which is often perceived asmuddiness.
“Don’t mistake loudness for fidelity,” is a good adagehere. An overlap-crossover system might get loud but fall far short onthe fidelity scale. The business might comment that the subwoofersdon’t seem to get very low because you’re emphasizing the mid-bassrange. You can add more subwoofers until the cows come home, but itwill only get muddier.
To compensate for this effect, you need to include a good EQ tonotch out the bump. A single parametric band can often do it, but,unfortunately, most business music sound systems don’t include aparametric 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 thebad, unless the frequency and bandwidth happen to match your bumpprecisely. Certainly, 7-band EQs are of little use with this kind ofoverlap bump. While an overlap crossover might allow you to use fewersubwoofers, unless you’re including a really good EQ, an overlap tuningin a business application is probably not advisable.
When using an active crossover on the subwoofer, you can slide downthe low-pass frequency to reduce the mid-bass bump. While that willhelp a lot, it’s still often difficult to match the electronic low-passcharacteristics of the subwoofer band with the acoustic low-frequencyroll-off of your main speakers. Adjusting the low-pass frequency isusually a big improvement from the passive overlap bump, but you canstill end up with some abnormalities at and below the crossoverpoint.
A full crossover high-passes the main speakers and low-passes thesubwoofers. The result is a nice, smooth transition. A full crossoveralmost always sounds better than an overlap, but since the subwoofershave to cover the bass frequencies all by themselves, you will probablyneed to use more of them. You can accomplish a full crossover eitherpassively or actively.
Full Passive Crossover
Passive systems usually use crossovers built into the subwoofer. Thefull-range amplified sound goes to the subwoofer, where the lows aresent to the subwoofer driver. The main speakers are connected to thesatellite output, which sends them mids and highs (with the bassremoved). This works alright, but the crossover components need to belarge (to handle the low frequencies), and they eat up some of yourpower. The crossover slope is usually not very steep, around a 12 dBper octave low-pass to the sub and a 6 dB per octave slope to themains, or satellite speakers. Steeper high-pass slopes are typicallyavoided because they can self-resonate or cause strange impedances tothe amp if a satellite speaker doesn’t get connected to it or if thesatellite speaker blows during use. However, with a first-order (6 dBper 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. Anoutput that works properly with a 4-ohm load — as with two 8-ohmspeakers — will be too low in frequency if you only connect asingle 8-ohm speaker.
The biggest potential problem of a full passive crossover is thatyou’re at the mercy of the sensitivities of the subwoofer speakersversus that of the mains and the need to properly balance the volume ofeach. The subwoofer might have a sensitivity of 89 dB, while thefull-range speaker might have a sensitivity of 92 dB. We will talk moreabout this later, but in business applications, because of the lowvolumes, the bass often needs to be between 6 dB and 10 dB louder, notquieter nor even equal in volume, to the mains in order for the soundto be balanced. In passively crossed systems, the subs are often softerthan the mains, and that’s a problem.
The fix? Well…one fix is our friend the high-resolution EQthat 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 shelvingcharacteristics that are needed by any particular system are slim.
It is difficult enough for engineers to design a good passivecrossover when they know the exact characteristics of every componentin a single cabinet. With a business music system, you’ve got so manyvariables — sensitivities, roll-off characteristics, number ofspeakers, placement, boundary loading effects, etc. — that it isdifficult to get a passive crossover to work well. Although a well-donepassive full crossover can sound quite good, it’s much easier to windup with one that sounds pretty bad.
Full Active Crossover
Full active crossover is the most reliable way to get a goodsubwoofer sound. This means using an active crossover and a separatepower amplifier for the subwoofer(s). The subwoofer gets low-passedusing a steep slope, usually 24 dB per octave, and the mains gethigh-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 overlapcrossover. You get independent control over the bass volume so you caneasily balance it with the mains by ear, or via an SPL meter. If thecustomer doesn’t like the balance, you can easily adjust it.
If you have a choice between an active crossover or an EQ, you’reprobably better off adding the crossover instead of the EQ. In businessapplications, you may not need to do much room equalization like youwould with a center cluster in a sound-reinforcement system. With theproper crossover, you minimize the need to equalize at the subwoofercrossover point. There are business music controllers that include asubwoofer crossover built right into them, like JBL’s Soundzonecontrollers and some from other companies that are starting to hit themarket.
SPL AND FREQUENCY RESPONSE
Typically the goal for subwoofers is to be somewhere between 2 and10 dB louder than the main speakers in the system. Low- andmedium-level music requires subs to be a little louder than the mainspeakers because at low sound levels the human ear needs more bass fora perception of well-balanced sound. Given the same music, higherlevels of music can sound well balanced with less relative bassincrease. There are also some music types and applications, like dancemusic in upscale fashion retailers, that may require even a little morethan +10 dB of bass. Setting the ratio of subs to main speakerssomewhere between 0 dB and +10 dB, as measured by a flat SPL meter, isusually a good starting point, though.
As for subwoofer frequency response, especially for businessapplications, you probably don’t want the subwoofer to go much below 45Hz. Below that, you end up with bothersome rumble, which can build upin corners or at room-mode nodes (function of a room’s dimensions).Rumble can annoy customers. They might not be conscious of it, but whenthey’re standing near the corner of the room, looking at clothing on arack, low-frequency rumbling can make them uncomfortable enough tochase them out of the store.
If a customer says they don’t want subwoofers because they heardsubs at the XYZ Store and they were bothersome, then there is a goodchance that XYZ Store’s system installer made the mistake of putting intoo much rumble below 45 Hz, or left in a mid-bass bump because of anoverlap crossover, or used a passive crossover that doesn’t workcorrectly with the selected product mix. The list of pitfalls goes on,but if it’s well-implemented by a skilled contractor, a business musicsystem with subwoofers can sound absolutely wonderful!
So how many will you use, where will you put them, how will youarrange them, and how will they relate to the mains?
Effect on SPL
Hanging a subwoofer in the middle of a room results in the lowestpossible output from the subwoofer. Placing a subwoofer at the ceiling,wall or floor increases its output. Placing it within a few feet of a2-boundary junction (like a ceiling/wall junction or a wall/walljunction) increases its output further. Placement within 3 feet of acorner increases its output still more. In these cases, there is bothan increase in sensitivity (output per watt of input) and in maximumtotal SPL capability. This can help in getting as much sound aspossible from a few subwoofers. However, there is a potential pitfallin placing a subwoofer in a corner: You can wind up with uneven basscoverage in the room.
Achieving Even Coverage
In most installations, there are a lot more satellite speakers thanthere are subwoofers. Because there are often so few subwoofers (maybeonly one), you can have a problem getting even coverage of the space.People sitting or standing very close to the subwoofer are going to getblown away with lows while people who are father away might not begetting enough. How do you make the subwoofer coverage as even aspossible?
As you move farther from the sub, the volume drops off, typically at6 dB per doubling of distance. Then, when you reach a certain distance,the subwoofer level stops dropping off at such a high rate. This iscalled the critical distance, which is where the reverberantfield within the room equals the direct sound from the subwoofer. (SeeFigure 2.) The critical distance depends on how reverberant theroom is. As you get farther past critical distance, even though thelevel of the subwoofer doesn’t drop off nearly as quickly, the qualityof the subwoofer sound might not be as good. But even though this mayhappen, it’s sometimes acceptable for subwoofers in business musicapplications.
One way to make the subwoofer coverage as even as possible is to usemore than one. It’s a myth that all you ever need is one subwoofer. Inmany places, it’s a good idea to add a second subwoofer, or more. Evenif you don’t need additional subwoofers for volume reasons, you mightwant to consider them just for evenness of subwoofer coverage. If Iabsolutely have to use just one subwoofer, my personal preference is tosacrifice the sensitivity increase and place the subwoofer for mosteven 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 oftenbest to place them asymmetrically within the room. In other words, ifone sub is in the middle of a wall, try to avoid placing the second subin the center of the opposite wall. Small-room acoustics can causeinteractions between subwoofers to create places where the bass buildsup and other places where the bass cancels out and disappears. Thetopic of room modes is an article in itself, but for now it’s importantto 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 willexcite the same room modes in the same way, making disparities worse.If you place the second sub in a different position, it will tend toexcite 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 itdoes excite room modes, usually results in fewer mode bumps as comparedto mid-wall placement. My experience when using two subs has been thatone goes in the corner and the other goes close to, but not in, theopposite 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 mainspeakers, so you might need more of them than you had guessed. You’renot going to get much bass if you’re using 20 full-range speakers atfull power and only one or two subwoofers of the same power rating asthe mains. Picture a three-way home stereo speaker in your mind. It hasa 4-inch midrange driver and a tweeter. What size bass driver would youexpect that one speaker would need to keep up with the midrange andhighs? You’re probably picturing an 8-inch driver, or even a 10- or12-inch one. That’s to balance a single 4-inch midrange! Now imagine asound system with twenty 4-inch midrange drivers. You’re goingto need several 8-inch subs, or a couple of 12-inch drivers, plus a lotmore power handling capability. Note too that the full crossover moderequires more subwoofers because the subs are carrying thelow-frequency load by themselves; but, again, it results in the bestoverall sound quality.
The ratios in Table 1 are no more than rough guidelinesintended as starting points. The system designer needs to compute theSPL capability and determine that it will meet the user-expectationsfor the application.
Note that the chart assumes the speakers are all installed in theceiling away from wall and corner boundary surfaces (not getting thebass reinforcement from these boundary surfaces) and that if they are70V/100V models, then both the mains and the subwoofers are tapped attheir highest settings. You can scale up or down from there. Forexample, if you’re tapping the main speakers down two taps (usuallythis means they’re down 6 dB) then you can reduce the number ofsubwoofers from what is suggested. Also, if you’re placing thesubwoofers in or near corners (around a 6dB increase in sensitivity)you can reduce the number of subwoofers. In addition, we’re assumingcertain sensitivity and power handling capabilities that may need to bechanged for your installation.
If you are using in-ceiling subwoofers, remember (from Part 1 ofthis series) that the subwoofer coverage projected onto the listeningplane only covers 120°. If you’re concerned about having evencoverage throughout the room, determine how many subwoofers you needbased on approximate coverage of 120° per subwoofer.
If you’re using subwoofers, I recommend selecting a full activecrossover system and using enough subwoofers to handle the SPLrequirement and cover the room evenly.
If you’ve made it this far, you can consider yourself an expert onbusiness music system design!
As I mentioned earlier in the series, there are several computerprograms that can help you with the design process. JBL’s free utilitycalled “Distributed System Design” does thepolar-to-listening-plane conversions, computes how far apart to placethe speakers, computes how loud the system can get with music or speech(taking into account the overlap factors), tells you what the soundlevel variation will be throughout the room based on the layout patternand density, and calculates how much amplifier power the system needs.(While this particular program is only set up for JBL speakers, thereare utilities out there that handle other models.)
But even the snazziest software won’t help until you determine yourgoals — and your client’s goals — for the system. Only thatwill tell you how many subwoofers to use and whether you’ve succeeded.Success is completely within your reach. A well-designed, properlyinstalled, wide-bandwidth, low-distortion business sound system cancreate an outstanding space that customers will enjoy.
Rick Kamlet is the senior director for installed sound at JBLProfessional. His prior roles included senior product manager, directorof engineering and national sales manager for a number of professionalaudio manufacturers.
For the other installments in this series, use the search featureat the top of this page and look for “Kamlet.”