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| April 15, 2003 Sound and Music: Three-Way Loudspeakers In last month’s feature I described a relatively new type of loudspeaker that sits squarely between the two-way and the three-way: the "two-and-a-half-way" loudspeaker. It’s essentially a two-way, but with an additional drive unit providing extra impetus through the bass region -- or you could see it as a two-way with a built-in passive subwoofer.But I was intrigued to encounter the phrase "tapered three-way" recently, which leading Canadian brand Energy was using to describe one of its two-and-a-half-way models. This is obvious marketing-speak, to emphasize the third driver. One crucial difference between the two- and the three-way, though, is that the three-way completely separates bass, midrange, and treble, so that the midrange unit is protected from the substantial fore-and-aft excursion required to deliver the bass end of things. It’s an important distinction, exemplified in B&W’s upmarket Nautilus three-ways (800, 801, 802, and 803), all of which use a very radical FST driver that is expressly designed for midrange duties, and quite unsuitable for bass reproduction. Acknowledging there’s no need to provide excursion, the cone diaphragm has no conventional surround, but rather its edge butts up against a special synthetic-rubber gasket whose prime task is to absorb -- very efficiently -- the vibrations as they reach the edge of the cone. Another type of dedicated midrange driver is the large (usually 3") fabric dome, a little like a giant tweeter, which has long been favored for professional-audio monitoring. These have found their way into the hi-fi arena via brands such as ATC and PMC, which have feet planted firmly in both camps. In the vast majority of three-ways, however, the midrange driver tends to be just a scaled-down version of the bass driver. Typically, midrange duties are handled by a conventional 5.25" cone driver, mounted in its own sub-enclosure, operating above an 8", 10", or maybe twin 6.5" bass drivers. That same 5.25" driver will probably also find its way into a small two-way miniature loudspeaker, where it will undertake both bass and midrange duties. Whatever the type of midrange -- or other -- driver used, a three-way has both advantages and disadvantages compared to a two-way, both of which deserve examination. Let’s start with the pluses. Power handling is dramatically increased simply because the amplifier power is shared between three rather than two drivers. Using separate drivers for bass and midrange allows each to be optimized for its task, and goes some way towards countering the dispersion difficulties discussed below. The downside is that the extra driver means extra complexity. It’s now necessary to integrate the acoustic output of three drivers (and probably a port, too). And the crossover network now requires four separate filters rather than two, with the extra two operating at awkwardly low frequencies. A bass driver has to move substantial amounts of air, either through its cone movement or via its associated port. It’s therefore obvious that a large-diameter driver will have an easier time of it than a small one; it simply won’t have to move as far, fore-and-aft, to move the same amount of air. Trouble is, what’s best for the bass is not so good for midrange or treble, at which point its necessary to bring in a bit of physics to describe the size of sound waves and the way they are generated. The lowest audible bass sounds (around 20Hz) have wavelengths of more than 50 feet, while 20kHz, at the extreme top-end of audibility, has a wavelength of just two-thirds of an inch. Those facts matter because of the way sound waves travel out from a vibrating diaphragm. Provided the wavelength is larger than the diaphragm, the sound waves are propagated in every direction. When the diaphragm is larger than the wavelength, however, the sound is beamed forward like the light from a torch or headlight. Although a single driver can cover the whole audio range, one of the key problems is that the lower frequencies will be generated in every direction, while the higher frequencies will form a narrow pencil beam. This wouldn’t be a problem if we all listened outdoors. However, because our speakers operate in rooms, the sound we hear is a mixture of the direct sound from the speaker and the sound that reaches us after reflecting off major surfaces like walls, floor, and ceiling. If we want a speaker to deliver an even sound balance at the listening seat, both the off-axis as well as the on-axis sounds need to be as smooth, even, and flat as possible.
Although it’s never entirely possible to achieve this, it is possible to get much closer with a multi-driver system than with a single-driver or even a two-way speaker. In fact, the ideal target here is a speaker diaphragm that becomes smaller as frequency rises, which is just what the Quad ESL-988 cleverly does. But that famous design is a very special case -- an electrostatic doublet, rather than a three-way -- that will be discussed more fully in a future chapter. Ultimately, of course, the three-way ought to have the edge, as it definitely has certain clear technical bonuses, though in practice the extra complexity can easily negate all the benefits. Indeed, in my experience low-cost three-ways are often better avoided because a three-way built down to a price may well involve compromises that significantly prejudice the sound quality. The three-way can be made to work very well indeed, but achieving that doesn’t come cheap. At modest prices, the two-way is quite likely to be the better bet. One three-way that does work well is Dynaudio’s Audience 82 that costs £1460 here in the UK ($2640 USD, right). That’s not exactly cheap, but this substantial 55-pound floorstander really kicks, and is a great choice for those who like their music heavy and loud, without sacrificing true hi-fi subtlety. However, three-ways really come into their own when one moves up into the true Ultra Audio sector. I’ve already mentioned B&W’s Nautilus models; another classy high-end range is the Utopias from the French company Focal-JMlab. These two brands are fiercely competing all round the world, and this seems to be leading to substantial performance improvements for both marques. When I tried B&W’s newish Nautilus 800, introduced in 2001, I was amazed at how much of an improvement it represented over 1998’s 801. Similarly, JMlab’s 2003 (four-way) Grande Utopia Be represents a remarkable performance upgrade over its 1996 non-Be predecessor -- much more so than that provided by its well-publicized use of a beryllium tweeter diaphragm. It turns out that in both cases much of the improvement is due to changes made in their complex crossover networks, and especially the components used therein. That these two top brands can achieve such dramatic improvements over a relatively short time span simply through refining their crossovers shows how little we know about this vital element. And also maybe just how much more there may be to come, not so much amongst high-end three-ways, but particularly at the more affordable end of the market. The future for three-way loudspeakers is likely to be very interesting. ...Paul Messenger
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