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One is not enough
While just one subwoofer is enough to enjoy movie sound in 5.1 or 7.1 (or casual music on a Bose SoundDock), two subwoofers (or full-range loudspeakers) have always been required for serious Hi-Fi reproduction of stereo music.
While low frequencies are less directional, and the direction of a low-frequency sine-wave point-source may be less perceptible, critical to fine music reproduction are low-frequency interchannel phase differences present in true stereo recordings. When reproduced properly with stereo subwoofers (or two full-range speakers), these phase differences are critical to reproducing the sound space of the original concert hall.
Critical even to casual listeners is that if you sum the low frequencies of true stereo recordings into mono by using only one subwoofer, low-frequency interchannel phase differences partially or completely cancel each other, reducing or eliminating low-frequency output levels!
Try jamming a stereo bass signal through one mono subwoofer and you lose a lot of hall ambience as well as bass level.
True stereo recordings are usually classical and other proper acoustic recordings made with pairs of microphones. Most "stereo" pop and jazz recordings are really just multi-miked mono, with each mono mic "panned" someplace between left and right. Pop recordings rarely have low-frequency interchannel phase differences, although they usually have bass information panned one way or another that also should be reproduced through stereo subwoofers if you demand the most accurate reproduction.
Putting the math aside, the best way to demonstrate this is to swap between one and two subwoofers with a real stereo recording, and it will be obvious.
Music reproduction is quite different from motion picture sound. This article addresses the reproduction of fine music at home. I will touch a bit on motion picture sound because, while a mono subwoofer works well for casual movie sound, only two will do for serious music reproduction.
While low frequencies may radiate less directionally from a source, this is completely different from our ability to recognize the direction from which a low-frequency sound comes. Stars also radiate light nondirectionally, and we certainly can see their direction. Even if we might not be able to localize a very low fundamental, I'll show why we usually can hear exactly where a mono subwoofer is placed.
Even for mono recordings, doubling-up on subwoofers more than halves distortion and usually evens-out room response.
I'll first explain how mono subwoofers wrongly got popular for stereo music today, then I'll explain the details and specifics of why high-quality stereo music reproduction demands two subwoofers and lastly I'll go over how to hook up your own stereo subwoofers in your system.
In the old days before stereo, music was most often reproduced at home through one large speaker.
In the 1920s the speaker was often a large full-range horn.
In the 1940s these single horns evolved into one large DC-electromagnet woofer in a very large cabinet, with a large high-frequency horn tweeter on top.
The 1950s brought us 15" horn-loaded permanent-magnet woofers with huge multicell horn tweeters, epitomized by Altec's A7 "Voice of the Theater."
Some music lovers brought these home, while most serious home hi-fi systems made due with big coaxial speakers in huge boxes.
Stereo became new next big new thing in the late 1950s, and music lovers had to go buy a second matching huge speaker for stereo.
If it wasn't a horn tweeter, it was electrostatic. When I was born, my dad had a pair of 15" horn-loaded woofers with huge electrostatic panels sitting on top.
The 1960s and 1970s brought us smaller acoustic-suspension speakers, with only a 12" woofer in a small 2' x 1' x 1' box that could get bass as deep as the 15" woofers of before.
These speakers, like the KLH 6 and AR-3, weren't as efficient as the huge reflex boxes, but more amplifier power was available now, so no problem. A pair of KLH 6s or AR-3s got down to 32 cps ("Hz") clean and strong with no problem. Good times!
In the 1970s, not everyone could afford or had the room for a pair of AR-3axs, and other people wanted solid response down to 20 cps or below, not just 30 cps or 35 cps.
To answer this, subwoofers first became popular in the 1970s, exemplified by pioneers Miller & Kreisel (M&K) with their self-powered subwoofers.
A subwoofer is a specialized woofer that operates below the range of a regular woofer. A subwoofer crosses-over to your full-range speakers at around 50 Hz to 100 Hz, and has response down to 20 Hz or lower. If it can't go as deep as 30 Hz or 20 Hz cleanly, it's just a regular woofer, not a subwoofer.
When subwoofers first became popular to lovers of organ music in the 1970s, they were exotic and expensive. In order to sell these expensive oddities, subwoofer makers convinced music lovers that they could get by with just one subwoofer. This was usually the case in the 1970s, since most people's recorded music at home came from LP stereo records.
On LPs, out-of-phase or stereo information makes the needle jump up and down vertically, which causes problems at low frequencies since the needle can jump out of the groove! To solve this, most LPs had been mixed with the lowest frequencies mixed to the center (mono). Summing the bass to mono can be done later in mastering, but it's better if it's done in mixing.
Turntable rumble is also strongest in the vertical direction, so reproducing an LP with the low bass summed to mono eliminates most of the rumble and loses little or nothing that was on the LP in the first place, so single subwoofers were great in the 1970s. Again, good times for the recordings available to most people for reproduction at home.
LPs aren't happy reproducing extremely low frequencies, even in mono, because they also tend to make needles jump left or right out of the groove. The large wiggles require more space so they reduce the playing time on each side. Rumble leaves very little signal-to-noise ratio at very low frequencies.
Even if one bought prerecorded 7½ IPS reel-to-reel tapes, or had one's own original 15 IPS masters, the analog tape from which LPs are cut could never record much lower than about 20 Hz to 30 Hz anyway. At the higher tape speeds used professionally, magnetic effects called "head bumps" limited the ability to play back very low audio frequencies. Analog tape has limited maximum output level at very low frequencies, and even if not, modulation noise could get nasty. Regardless of if you could reproduce it at home or not, and regardless of if you could record it on an LP, recorded sound wasn't happening below about 20 or 30 Hz with analog tape recordings or masters.
Digital recording changed everything.
Digital cheerfully can record straight down to DC, and the new digital "Compact Discs" (CDs) allowed anyone who could afford a CD player to have perfect sound forever.
In practice, if the analog path is up to it, actual digital recorders and home CD players usually have distortion-free and flat response down to 2 Hz or less.
For the first time in 100 years of electronic sound reproduction, consumers have access to recordings with unlimited deep bass. Since there is no point in mixing or summing the bass to mono, bass has been in stereo in our homes since the introduction of the CD.
Even if your old recordings had their bass mixed to the center on the original LP, even the silliest old original recordings usually have their bass in stereo. When released again today, the bass again is in glorious stereo. Even something silly like Burl Ives' "Have a Holly Jolly Christmas" from 1965 has the bass panned hard-right most of the time. Go ahead: you can hear this right now over Amazon's preview if you have a stereo bass system or headphones connected to your computer, iPad or whatever.
As of the 1980s, everyone has needed two big speakers for stereo as always, or two smaller ones with two subwoofers for stereo.
But then home video came along.
Some time around 1990, Tom Holman invented the 5.1 channel system for film sound with its "0.1" Low-Frequency Effects channel.
As you can read at Bass Management, the 5.1 system adds one subwoofer purely for very loud "booms" as a special effect for telling stories as a part of a motion picture. Each of the other five channels also has low bass, and movies today have a lot of multichannel bass information if your system can reproduce it.
Video and movies replaced music as the predominant form of recorded home entertainment in the 1990s. Everyone brought home some sort of "surround sound" system. These systems make do with only one subwoofer. Even the most elaborate home A/V systems rarely have five huge speakers, and instead have five puny ones and just one subwoofer.
Women love it: men's big stereo speakers are gone, and only tiny little Bose speakers hide in the corners with one subwoofer hiding someplace else. Women have no idea how lucky they are; what passes for a "subwoofer" is usually only a wimpy replacement for one regular woofer, a fraction of the size of just one real speaker or subwoofer. Even the most elaborate 5.1 A/V systems rarely have anything other than their subwoofer with any more than an 8" woofer.
For most people, movies, DVD, Blu-Ray, Roku, Netflix and Tivo are the main event, while music has been relegated to background ambiance.
As of today, only serious music lovers (and the ocasional confused audiophile) have dedicated music listening systems.
Music lovers and audiophiles have forgotten the proper ways of music reproduction, honestly mistaking movie-soundtrack-inspired 7.1, 5.1 or 2.1 systems as the best solutions for formal stereo music reproduction.
Stereo Music Reproduction
For casual music enjoyment, 5.1 systems sound great playing stereo (2-channel) music. Even serious listeners can can enjoy stereo music playback on 5.1 systems immensely, even if 5.1 is a film sound, and not a music, playback format. It's always about the music, and never about the equipment, but using only one subwoofer and small speakers is never optimal for serious music reproduction — even if 2.1 systems are ubiquitous even with audiophiles.
You need two subwoofers for stereo bass. It's got to be at least 2.2, 5.2 or 7.2, not 2.1, 5.1 or 7.1. (For music reproduction, Tom Holman has advocated a 10.2 system, with some of these speakers above the listeners to reproduce reverberation.)
Summing the bass to mono is not optimum for stereo music reproduction, unless you have a recording that already has had its bass summed into or mixed as mono. Most music recordings have been released with stereo bass since 1982.
Also as you can read at Bass Management, while 5.1 SACDs may have 5.1 channels, all of those five main channels are full-range, and each deserves a big speaker — or use five + one = six subwoofers! That's how it's monitored in Hollywood.
Why We Need Two Subwoofers
Bass has been released in stereo ever since 1982 on CDs, DVD-As, SACDs, and even in 5.1 movies, and we can hear the difference.
I checked with a friend of mine who's one of the top lacquer mastering engineers in the country (he probably cut your favorite audiophile LP) to ask what's up with stereo bass on today's LPs, and he confirms that, "Hell yes, everyone should have stereo subwoofers for LPs!" Unlike the days of old when most bass was mixed to the middle for LP release and turntables rumbled like crazy, today the LPs he's cutting are loaded with stereo information in the lowest bass, and turntables have much less rumble.
Even if we can't hear the direction of the lowest 32 Hz fundamentals themselves, we very much can hear the difference in phase between the two channels, and in stereo recordings, out-of-phase bass information would be summed to zero in a single subwoofer. With stereo subwoofers, any out-of-phase bass information in a true stereo (acoustic) recording is reproduced properly at full level, adding immensely to the perceived width and depth of the room in which the recording was made.
If you sum the bass to mono and try to squeeze it through just one subwoofer, all the out-of-phase information cancels, and reduces the level.
While a pure 40 Hz sine wave can't be localized, subwoofers almost always produce some higher-frequency distortion products that can. Subwoofers generate plenty of harmonic distortion (higher frequencies) which are easy to localize, and vented subwoofers have higher-frequency wind noise as they are pushed harder.
Even if your subwoofers are perfect, your crossover probably isn't. A 12 db/octave crossover at 80 Hz doesn't stop sound above 80 Hz; it only merely starts attenuating it above 80 Hz. A 12 db/octave 80 Hz crossover attenuates 160 Hz by only 12 dB, and attenuates 320 Hz, which is a very localizable E above middle C, by only 24 dB. 24 dB is equivalent to turning your volume only halfway down, and you can hear that very well.
In my own tests on myself, in order not to be able to localize a subwoofer, I can cross it over at no more than 60 Hz at 18 dB/octave. I can hear exactly where it is when crossed over at 80 Hz at 18 dB/octave.
In order truly not to be able to hear the location of a non-distorting subwoofer, cross-over at 60 Hz or below or 18 dB/octave or more.
At 80 Hz with a steeper slope of 24 dB/octave, a THX standard, ought to prevent localization, but you still have the problem of only getting mono bass and losing all the hall ambiance. Bass wavefronts propagate all over a hall, and even with the basses on the right, you'll hear bass on the left and all around as it bounces around. You want to reproduce all of this accurately and not merely sum it into one confused-sounding mono woofer.
Not being able to locate the subwoofer is good, but summing low bass into mono is always a bad thing for true stereo recordings.
This all applies to true stereo recordings, which means most classical recordings. With popular music, there is rarely a concert hall involved, and having stereo bass is mostly a matter of where the bass effects are panned.
Even with recordings with mono bass, having two subwoofers offers the benefits of less than half of the distortion, since each subwoofer needs to output only half the power. Woofer distortion more than halves every time you halve the power output. As you approach the limits, distortion climbs quickly. For the same distortion, you of course get twice the output with two subwoofers.
With two woofers, even if they are reproducing the same signal, their different positions will help even-out frequency response in a real room. Each will have somewhat different response in the room, and the net effects are smoother response.
Summary and Recommendations
When quality matters, crossover at the lowest frequency you can. It's always best to run as much of the music through the main loudspeakers and to keep the deleterious effects of the crossover (phase and frequency abnormalities) as far away from most of the music as possible. With a dedicated crossover and good speakers, you can usually crossover around 40~50Hz instead of the more common 80~100Hz. For public address and concert sound where maximum power is more important, people crossover higher to put more of the mid-bass into the ample subs and away from the mains, but for home hi-fi, crossover as low as you can.
A trick for nearfield audio is to crossover to the subwoofers at a frequency below room resonances. In my small office, there are horrible peaks at 50 Hz, so using big desktop speakers that make it to 40 Hz let me drive the subwoofers only below 40 Hz, so the 50 Hz room modes aren't excited!
I got my first M&K subwoofer in the 1970s, and upgraded to a stereo pair of 18" JBL B460s in the 1980s. Stereo subwoofers, or two full-range speakers, make a huge difference in soundstage.
While a mono subwoofer in an x.1 system can sound great for pop music, they cannot reproduce the ambience and out-of-phase bass information in true stereo acoustic recordings necessary for doing a great job of reproducing the actual sound of the concert hall.
Even most pop recordings and movies have significant stereo bass information, so if you genuinely desire to reproduce as much of it as possible, you owe it to yourself to run two subwoofers for stereo, or at least two large full-range loudspeakers. Sorry, two puny B&W Diamond 805s on stands with just one mono JL Fathom f113 isn't high-fidelity, unless you're only listening to mono recordings. Speaker makers love selling you these little speakers, since you're getting a fraction of the speaker for not much less money than full-size speakers.
Better than one subwoofer is two big full-range speakers!
I haven't covered how difficult it is to get a crossover to work well with the bizarre realities of how real speakers perform in the frequency range over which we cross them over. We potentially lose a lot attempting to cross over in such a delicate region if its not done correctly. A pair of big speakers like B&W 801 is a far better idea for music than using one subwoofer, or even two subwoofers if you don't do it right.
If you only have a common 5.1 home stereo A/V receiver with one subwoofer output, you may need something better, or a dedicated crossover, to accomplish bass management appropriate for stereo subwoofers. A/V receivers, designed for movies with music capability provided only as an afterthought are rarely are setup for stereo subwoofers. No worries, most active subwoofers today have built-in crossovers.
Personally, the wife has a Bose system for watching movies, and for my hi-fi I have a dedicated stereo system with an active crossover feeding my subwoofers. I don't bother with an A/V receiver for music; everything is dedicated 2-channel stereo.
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