UNDERSTANDING LOUDSPEAKER DIFFERENCES
By George McKechnie
Most people are wired to respond emotionally to sound and music. Whether it’s a nostalgic song evoking earlier times, the tense sonic build-up to a pivotal movie scene, or the excitement of hearing a favorite band, so much depends on what we hear. So getting the best sound that the overall design goals, the physical constraints of the room, and the budget will allow is critical to achieving an immersive entertainment experience in the home. And no other component impacts emotional response as much as the speakers do.
Blaming speakers for poor sound quality is a time-honored tradition. Until recently, variability in the quality of recorded materials and playback components—as well as amplifiers—also played significant roles in determining final sound quality; so pointing the finger at the speakers was not always justified. Today things are different. Now even modestly-priced amplifiers deliver reasonably good sound quality. And basic music sources like iTUNES and other downloaded content—while perhaps not meeting the needs of audiophiles—bring satisfaction to many music lovers.
Paying close attention to both speaker quality and appropriate product application are essential to achieve satisfying results—within the constraints of space, aesthetics, and budget. In this Insights report, we’ll explore the speakers themselves—configurations, sizes, and technologies. In a companion report, we’ll examine the all-important issues involved in Matching Speakers to Your Room.
Speaker Configurations. Before the days of surround sound (which requires a minimum of five speakers plus a subwoofer), a home music system needed only a single pair of speakers, as long as the enclosures were large enough to hold woofers that could reproduce with conviction the lowest notes on a recording. This meant floor-standing speakers. With the advent of surround sound came the practical need to reduce the size of the enclosures, so that a room wouldn’t be visually overwhelmed with five (or more) large objects strewn about.
If the low frequencies from all five channels were all allocated to a single subwoofer, then the three front and two surround speakers could be made much smaller—without compromising the overall low-frequency performance of the system. And since humans can’t hear spatial effects much below Middle C (due to the small distance between our ears relative to the wave lengths of the lowest notes we can hear), nothing would be lost.
Today, few people want floor-standing speakers in surround sound systems— although this format is still popular in high-performance stereo systems. If the room is large (and the system will be called upon to play very loud) left and right floor-standing front speakers may be the best choice—especially if the system is used mostly to play music. But for the center and surround channels, there’s little advantage in using floor-standing speakers, since the surround sound protocol cuts off the lowest frequencies to these channels anyway. Also worth noting are the aesthetic and practical disadvantages of placing a floor-standing speaker in front of the TV screen. Instead, bookshelf-style speakers, in-wall or in-ceiling speakers, or sound bars have become the go-to choices for most surround sound systems.
Bookshelf speakers have been around for more than half a century, and today’s versions include some models that achieve very high-performance, using advanced drivers (woofers and tweeters) and construction techniques. When combined with a well-matched and carefully tuned subwoofer and properly configured surround receiver, they can provide superb sound, provided they are correctly positioned.
High-quality in-wall and in-ceiling speakers can likewise provide excellent sound—with several important caveats. First, many built-in speaker models are not high quality, and they’ve given the category a bad reputation. Upwards of a thousand different “house brands” of in-wall/in-ceiling speakers are available in the US today. Most of them are of similar or identical design, produced in a few Pacific Rim factories, and then private-labeled. They all sound pretty much alike: not great.
To add to the confusion, many of the best speaker brands have moved their own in-wall/in-ceiling manufacturing facilities to Pacific Rim countries, allowing them to sell their superior products at competitive prices. Their built-in models include woofers, tweeters, and crossover networks that are often equal in quality to their prestigious bookshelf and floor-standing products—just without the fancy cabinets.
Within these quality brands, the uniformity of voicing and performance across formats makes it possible to combine bookshelf, in-wall/in-ceiling, and—if needed—floor-standing products into a single system. This hybrid approach typically consists of bookshelf (or floor-standing) models in the front, in-ceiling units for the surrounds, and in-walls for the rears (in a 7.1. surround sound system). For some installations, using all built-in speakers may make the most sense practically, as it eliminates the need for furniture, shelving, or speaker stands to hold the bookshelf models up off the floor.
Most in-wall or in-ceiling speakers are two-way designs. This means that they employ a woofer to reproduce the low frequencies (typically up to 2000 Hz), a tweeter (to handle frequencies above 2000 Hz), plus a cross-over network to direct the lows to the woofer and the highs to the tweeter. Some manufacturers also offer three-way speaker designs (consisting of a woofer, midrange, tweeter, and crossover) in their built-in versions—often borrowing technology from their bookshelf and floor-standing models. In this approach, the additional driver handles the middle frequencies (typically from 200 to 2000 Hz), providing clearer and more harmonically accurate midrange sounds—especially valuable for vocals and dialog.
Specialized center and surround speaker versions are available from some manufacturers in both bookshelf and in-wall/in-ceiling formats. For the dedicated center speaker, small woofers are positioned to the left and right of a single tweeter, resulting in a bookshelf or in-wall product that is relatively flat and wide. Known technically as a d’Appolito configuration (after the inventor), this design provides wider and more symmetrical horizontal dispersion than a conventional “tweeter over woofer” design—especially in the mid-frequencies that are critical to center speaker performance. Special box and built-in surround models, known as effects speakers, are likewise available. These use twin tweeters mounted side by side to maximize the dispersion of high frequencies, to optimize surround effects.
In-wall speakers are traditionally rectangular in shape—with the tweeter positioned above (or below) the woofer—and almost always less than 12 inches wide and 4 inches deep, so that they will fit into a conventional stud wall. In-ceiling speakers are usually round, with the tweeter suspended in front of the woofer (but behind the grill). This concentric design makes in-ceiling speakers deeper than in-wall units (sometimes as much as 8 or 10 inches deep), and is the reason why round speakers are intended for ceiling use.
In-wall speakers can be installed in ceilings (they usually fit physically), if they won’t conflict aesthetically with recessed lighting cans (which are typically round). Likewise, in-ceiling models can be installed in walls—if the space inside the wall is deep enough to accommodate the speaker. In fact, when a wall-mounted speaker is needed for a room with a vaulted ceiling, a round model may look less awkward than a rectangular one in the triangular space that is created where the wall joins the ceiling—but only if the wall is deep enough to accommodate it!
Some final thoughts on built-in speakers. Although most in-wall/in-ceiling speakers available today can be mounted directly into a wall or ceiling by cutting a hole into the sheetrock, plaster, or wood paneling, they will sound better—sometimes much better—if they are mounted into a back box, an enclosure of wood or metal specifically designed for the speaker model, which is pre-installed during the construction process.
This box optimizes the acoustical environment for the back side of the speaker driver elements, and also limits sound leakage from the back of the speaker into adjacent rooms. For retrofit work, a back box may not be practical, as it involves removing a section of wall material large enough to allow installation of the back box (which is often much larger than the speaker cutout itself), then replacing the material and patching.
With the popularity of flat-panel TVs has come the introduction of the speaker bar. It consists of a single wide, short enclosure that sits above or below the TV, and contains the three front speakers (left, center, and right). Some manufactures offer speaker bars custom built to the exact width of the TV, creating a very clean, seamless look. For aesthetic reasons, speaker bars are seldom more than 4 or 5 inches high.
This means that the woofers used are by necessity very small (more like midrange units, actually), significantly limiting their low frequency output. Because of this limitation, such speaker bars should always be matched to a subwoofer (even an 8 inch unit will help a lot, even if it cannot reproduce the lowest frequencies). Without this additional low-frequency support, bass instruments and male voices will lack their normal “weight” when reproduced on most speaker bars.
New wireless speakers are now available, using Wi-Fi to send the signal to the speaker. This format eliminates the need to run in-wall signal wiring from the equipment to the speakers, an advantage in retrofit applications. But they still need 110 volt power to operate. So wireless speakers are practical only in the bookshelf format, where the wireless receiver and amplifier can be tucked into the speaker box itself. The unit is then plugged into a power outlet—limiting its use to locations where 110 volt power is available and, (if important aesthetically) easy to conceal.
Speaker Size vs Room Volume. Just as you wouldn’t use a motorcycle to tow a large trailer, you shouldn’t attempt to fill a large room to convincing sound levels with tiny speakers. They’re simply not designed for the job. If you try, the sound will be distorted—and if you persist, they’ll likely burn out!
When selecting speakers for a sound system, the first consideration is the room volume. If the room opens to a kitchen or dining room, the entire volume must be included, as the sound energy will fill the entire space; it doesn’t stop where the listening room ends on a floor plan. Some manufacturers provide coverage specs for their speakers. Example: one pair of two-way in-ceiling speakers (with 8 inch woofers), rated to deliver moderate (background) level music in a room having a volume of 2,500 cubic feet. This is equivalent to a medium-sized bedroom (measuring 15 feet wide x 16.6 feet long x 10 feet high), which does not open up to adjacent rooms.
Specifying the right speaker type (and number) for a home theater application can be complex, as it is based on both user factors and technical specifications. Important user factors:
- How will the room be furnished—heavy carpeting and lots of stuffed furniture—or minimalist?
- How loud will the system be played—moderate, loud, or DEAFENING?
- What kind of sound—action movies; solo guitar; symphonies; or Pink Floyd?
Technical specifications that matter:
- The sensitivity (or efficiency) of the speaker model
- It’s power handling capacity
- The number of speakers used (a stereo pair vs 5.1, 7.1, even 9.1 surround sound)
- The type and number of subwoofers included, and the crossover point between main speakers and subwoofers; and
- The power output of the surround receiver or amplifier.
The goal is to tailor the system to your room, your listening preferences, and your budget. In the hands of an expert, all of these factors will be used to help choose the right size, model, number of speakers, and configuration.
Speaker Technologies. Nearly a century of creative effort has gone into the development of modern speaker technologies. Most current designs are point source direct radiator models. Point source means that the sound energy is created by a tightly arrayed set of drivers (point source), usually consisting of woofer and tweeter (but maybe also a midrange). Direct radiators means that the energy from these drivers radiates directly into the room from the front of the speaker.
All but a few of the thousands of speaker models available today are electro-dynamic, meaning that they use magnetic force to convert electrical energy into acoustical energy. The others are a rare design known as electro-static. Electro-dynamic drivers consist of:
- a moving voice coil (an electromagnet) which receives the electrical signal from the amplifier
- a stationary permanent magnet, against which the voice coil pushes and pulls
- a diaphragm (a cone or dome) attached to and driven by the voice coil, which propagates the sound energy into the room, and
- a suspension system, which centers the voice coil within the magnetic field and guides the diaphragm
The acoustical energy produced by a point source speaker disperses rapidly in both the vertical and horizontal directions as it leaves the speaker box and enters the room. Because of this dispersion pattern, the sound volume level (loudness) decreases significantly the further the acoustic waves travel into the room. [For those of you who are mathematically inclined, the rate of decrease in energy is equal to 1 divided by distance squared.]
In practice, this decrease in volume level with distance is moderated by sound waves that reflect back from the walls, ceiling, and floor, becoming part of the total mix of sound that the listener hears. So this decrease in volume is usually not a problem—except in very large rooms, like churches and auditoriums. But in heavily furnished residential spaces where the waves are heavily damped (and reflections are few), this decrease in loudness can be quite noticeable.
A line source is a speaker that uses a single elongated driver (or multiple drivers, stacked up to four or five feet high) to pre-disperse the sound vertically in the room. For this kind of direct radiator, the rate of falloff for the sound level entering the room is less than for a point source speaker. [It is equal to 1 divided by distance.] Relatively few full line source models are available today, although partial vertical arrays are not uncommon.
A third design type, the planar speaker, uses a thin plastic membrane stretched on a large frame to create a very large diaphragm (up to about 2 ft wide by 6 ft tall), which pre-disperses the sound in both horizontal and vertical directions. As a result, the image created by the speaker is very large, and the sound falloff with distance is minimal. And, because the membrane is extremely light in weight relative to traditional woofers and tweeters, the transient response of planar speakers (their ability to reproduce dynamic detail) is excellent.
The back wave produced by the diaphragm of most planar speakers is not contained within a box or the wall (as it is for direct radiator speaker). Instead, this speaker type radiates sound into the room from both the front and back surfaces (it’s called a dipole radiator). Because the back wave could partially cancel out the front wave at low frequencies if improperly placed relative to walls, a dipole radiator is harder to position in a room than is a point or line source type.
But careful positioning of dipole radiators can be very rewarding. Properly used, the back wave can help create a wonderful sense of ambience—even from a stereo speaker pair—which can easily rival the surround sound experience produced by a direct radiating 5.1 system. This accounts for their continued popularity among audio enthusiasts, in spite of their large size. Because of the size issue, planar speakers are typically used in stereo systems—although center and surround products (even subwoofers) are available for those who desire a planar surround sound system.