Controlling Laws of Physics
June 3, 2005
You don't have to read too far in any publication dealing with stereo and/or Home Theater equipment to discover that ads for loudspeakers usually outnumber all the others by a wide margin — and that they generally contain even more superlatives than movie ads. What's underneath all the adjectival bloat? What are the factors that really determine speaker performance? Some of the answers are sprinkled elsewhere through these Web pages, but here is a rundown of some basics that hopefully will help you evaluate the disparate claims for speakers.
The most basic of basics is that sound is air in motion. The originating source of a sound, whether it's the movement of a cricket's hind legs or the combined efforts of a hundred musicians in a symphony orchestra, produces waves in the gas we call our air — in somewhat the same way as water, in response to various stimuli, moves in waves through your nearest body of water. The goal of a loudspeaker is to re-activate soundwaves into motion, in accurate reproduction of the original sound that was recorded or broadcast your way.
The electro-mechanics of this process in almost all speakers involve routing the electrical signal from an amplifier through a coil of wire that's suspended in the gap of a cylindrical magnet. The up-and-down electrical waves that comprise the electrical signal react with the magnet, creating an opposing force that makes the "voice coil" of wire move back and forth according to the signals' ups and downs. The coil in turn pushes and pulls a diaphragm that's attached to it. Generally that diaphragm is either a relatively big and heavy cone for low frequencies (which require a lot of air to be set in motion in relatively slow and long waves) or a quite small, ultra-light dome for highs (which call for very rapid re-creation of very short wavelengths). The broad range of midrange frequencies between the lows and highs can be reproduced alternatively by the inner portion of a bass speaker's cone, or by a separate midrange speaker with a medium-sized (3-5-inch) cone, or, in some cases, by the high-frequency speaker.
A lot of demanding work is involved in this process. A speaker system's diaphragm must move back and forth very quickly in accordance with the signal's demands. Even the "slow" movement of low frequencies means moving back and forth thirty or more times a second, and the highest frequencies may demand 20,000 cycles back and forth per second. (Specs express these cycles per second from the lowest bass to the highest highs as "Hz," for Mr. Hertz, whose name was given the dubious honor of replacing the previous, pretty straightforward "cps".) Besides going back and forth in response to the signal triggering its motion, the diaphragm also has to move varying distances in both directions, depending on how loud a sound it's being asked to produce. When you have the subtle overtones of a flute occurring at the same time as the loud burr of brass, not to mention both the fundamental notes of various instruments, their overtones, and maybe the sound of kettle drums and bass drums, there are a LOT of back-and-forths of all kinds going in a speaker system.
Let's look at the major factors that determine how a speaker system manages all that:
Bass, Efficiency and Enclosure Size
You have to move plenty of air to reproduce really low frequencies (60 Hz and below), and the louder you want to play those low frequencies, the more air you have to move. The most obvious expedient for moving the most air is a BIG speaker cone (say 15 inches in diameter), which obviously takes a bigger bite of air than a small one. But there are penalties involved, especially if you're talking about a full-range system. One is that the cabinet involved must be of an appropriately large size — which to a lot of people means an inappropriately large size for use in a normal living room. Another is that the behavior of the speaker at frequencies above the lowest one begins pretty quickly to leave something to be desired. You can overcome that by crossing over to a midrange speaker, which dictates at least a three-speaker system, since you'll need a really small diaphragm for good highs. That may be fine in a big, expensive speaker, but if you want to do what most people can afford and accommodate, you had better stick to a smaller woofer that will also handle midrange frequencies, so that you can use just one other speaker (the good old "tweeter") for the rest of the range.
So what almost all speakers designed to be heard in rational living spaces do to produce bass involves manipulation of four factors: the size of the cone, the mass of the moving system (mainly cone and voice coil), the strength of the magnetic "motor" driving the voice coil, and the volume of air enclosed in the cabinet used. Increase or decrease any one of those and you affect what's required of the other three for a given good level of bass performance. Increase speaker size and you need a bigger enclosure for a the same low-frequency limit at the same efficiency. Increase magnet size and you'd better increase the mass of the moving system to prevent a peak in response from happening at or near that limit. Increase the mass of the moving system and you'll have to increase the weight of the magnetic motor for the same efficiency and the same "Q." ("Q" is the ratio of the energy stored in a system to the energy lost by the system; for good, unpeaky performance of a woofer at its resonant frequency, you need a "Q" of 1 [relative equality] for a sealed system, and of about 0.7 for a vented system like most of ours.)
What illustrations like those are supposed to do (besides confuse) is to indicate that the easy catch-word improvements you see touted in advertising may not mean anything, or actually may signal decreased performance. A system, for instance, that offers a bigger woofer in the same size cabinet as one with a smaller woofer may have less bottom rather than more, though what output it has above the lowest frequencies may be at higher volume than the smaller one's. An advertisement boasting increased efficiency from a larger magnet may also mean less bass, depending on the other factors. And an advertised bass response secured by a smaller magnet that underdamps a speaker will mean boomy and/or undefined bass. In physics, as elsewhere, there is no free lunch.
As you go up the performance scale further and further toward the "ultimate," you may leave cost factors behind. But you don't leave behind the acceptable size of a speaker enclosure, so designers use expedients such as the one we favor in some of our systems, which is to use two identical drivers of moderate size, so that their combined operating area becomes pretty large while their individual diameters stay small enough for good performance up into the midrange.
One way or another, you must move a lot of air to reproduce frequencies below 60Hz at anything like their actual presence in music or Home Theater material, and the louder you want those lows to be, the more prodigious the demands on a speaker. There is no way around that, and, one way or another, it costs. On most music, the demands for low frequencies are enough within reason that a pair of our remarkable Alphas — which are plus or minus 3 dB down to 68 Hz, and have an effective (-10 dB) cutoff of 50 Hz — will sound very convincing at low cost in cabinets of very compact size. But if you want the lowest, room-shaking notes of an organ at full strength, or the deepest subterranean rumblings of Home Theater videotapes and discs, you'll have to pay more than twice as much — often far more — either for an ambitious full-range system (and an amplifier with good reserve power) or a separate powered subwoofer.
One of the bonuses of a good, musical-sounding Home Theater system is that the subwoofer used for those movie rumbles will kick in very nicely on music. (We hear regularly from people who have put together all-Alpha systems with a SubZeroi sub, who are delighted with what happens on music as well as Home Theater.)
Placement vs Bass and Other Wavelengths
A good way to represent the "fluid" nature of sonic radiation is with the illustration below, which depicts how the radiation of sound waves from different basic points in an enclosed space affects its sonic output, especially at lower frequencies. As you can see, sonic output is increasingly reinforced as a sound-source such as a speaker moves away from a central position in a space and toward boundaries. The greatest focusing occurs in a room corner and the greatest effect happens with low-frequency waves. Relationship to boundaries has a profound effect on the sound of a speaker, and we'll have quite a bit to say about this in the near future when we discuss speaker placement as a separate topic.
Moving Up the Scale
Above the bass region, the physics of sound involves less brute force and more subtlety. One of the big considerations is making sure various wavefronts arrive at your normal listening position together, without cancellations and additions. The other main — and interrelated — one is how high-frequency power response and dispersion, and the matching of speakers off the production line, affect definition and imaging.
If the first of those issues sounds a little dry, it's actually anything but. Besides the sound that reaches your ears directly from a speaker, there is a jumble of sounds reflected in many different ways. Near-by reflections from cabinets affect response with tiny "echoes" at higher frequencies (like TV ghosts caused by reflections off buildings near-by a TV antenna). Close-by reflections from walls affect response by in-phase additions and out-of-phase cancellations. You probably will notice these more — or at least identify them more clearly — in their effects on bass frequencies, but midrange and high-frequency effects are very important. Because they're subtler, they tend to be less identifiable by most of us, and the effects we do hear and dislike are more likely to be confused with speaker inadequacies.
Good speaker designers give enormous consideration to minimizing diffraction effects on high frequencies on the front surfaces of their speaker enclosures. What happens with sonic reflections after the designers get through depends mainly on you. If you're among the many speaker owners (let's say half the total to be generous) whose first placement of speakers in a listening room sounds so satisfying that the thought of repositioning them never occurs to you, wonderful. But if things don't sound quite right, either at the beginning or later on, very small changes in speaker placements — sometimes only a few inches — may make a major difference. We'll shortly be posting our advice about speaker placement, and if the sound you're experiencing is not what you had hoped for, take a look at our advice before deciding your speakers or amplifier or cables are failing you. In the meantime, we urge anyone who's discontented, and whose speakers are movable without too much strain, to experiment with very small changes in positioning.
If room and cabinet reflections are as important as we say they are (and they are), you might see why we have some reservations about bi-polar speaker designs that radiate sound in two (or more) directions at once for an increased sense of spaciousness. They make perfect sense for surround speakers in a Home Theater system, which are meant to create a diffuse soundfield, but we find them undesirable for use in the main speakers of a music or HT system. Besides a plethora of high-frequency in-and-out-of-phase effects, we don't like the loss in imaging and overall definition that we've experienced from those designs. The artificial spaciousness produced by deliberately bouncing sound around also tends to produce effects, like the ten-foot-wide soprano singing from everywhere in sight, that we find a bit disconcerting. Real spaciousness depends not only on the ambiance of the original recording and the positive or negative nature of your listening room, but on extreme precision in the factors discussed under the next sub-head. And besides the midrange and high-frequency difficulties, we're also not fond of the fact that placing full-range bi-polars even a trifle off from the suggested relation to the rear wall tends to bring bass boominess.
Musical Definition and Imaging
If you've gotten this far, you're probably the kind of the person who knows why musical instruments sound like themselves. But just in case, this seems the time to say that the sonic character of a musical instrument only begins with the notes it generates. What gives each instrument its definition is the range of overtones (harmonic frequencies above the main ones) and undertones produced by factors such as how the sound is produced — by blowing, bowing, plucking, hitting, or whatever — and the nature and materials both of the main vibrating surface, such as a string, and the supporting structure, such as the soundbox of a guitar or violin.
To reproduce the way musical instruments really sound, you need excellent, well-dispersed high-frequency response that also keeps the phase relationships of sounds at various frequencies intact. When trying to reproduce instruments with intense high-frequency energy, such as cymbals and snares, at "live" volume, you also need a lot of high-frequency power output. Which is why, all other things being equal, an expensive tweeter with greater power capabilities will sound more "real" than a good bargain one.
In other segments of these Web pages, we try to describe the sound of our speakers. But the reason they sound the way they do is that they meet the above requirements, not just through the quality of their drivers' construction and the resulting power response and dispersion, but through very painstaking system design that uses carefully calculated, high-quality crossover networks to divide the frequency range between speakers, and takes the most simple, direct route to system performance to help things cohere at the listener's ears. The more complexity you add to a system, the greater the chance to blur phase relationships, whether in too many or too-hastily-designed crossovers or in too many speakers pointing in too many directions. Our most complex speaker, the Stratus Gold, is a three-way system — with all of the demanding phase-matching that this design requires carefully worked out. But the furthest we go in our other systems is "two-and-a-half-way" design, in which one of the two woofers used rolls off at a fairly low frequency while the other continues into the midrange. That's more complex than a simple two-way system, but it keeps the crossover low enough (as with a subwoofer) to avoid audible difficulties, and also preserves phase relationships more easily than a three-way design, by having a single speaker rather than a pair operating in the midrange.
Not only the definition of musical instruments but the true "spaciousness" of reproduced sound — that is, the "air" around instruments — requires great dispersion and in-phase wavefronts arriving at your listening spot. When those two factors are present, you also may get another of the qualities so many listeners value: the precise "imaging" that places instruments distinctly across the soundstage as they were in the original performance. But for imaging to be precise, you also need a precise manufacturing match between the speakers on each channel. If the speakers aren't very closely matched, the result is anything from indistinct imaging to audible "wander" of instruments across the soundstage.
The reason our compact, highly affordable speakers can sound so much like our very best a great deal of the time is that we do our best in the design of each to meet the requirements we've outlined here. Apart from the greater power handling of our most expensive systems, the improvements are relatively small ones that are increasingly costly to make. And there is a point — represented by the Platinum T8— where potential improvements are so vanishingly minimal and the added costs so high, and where the potential exists for actually worsening performance through complexity that fuzzes over the very qualities that define truly realistic sound, that we know it's right to stop. So we do, and we hope we've gone to just the right lengths in these words as well.
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