Volume 1, Number 1, 1994
Revised July, 2003Speaker Driver Design Speakers consist of one or more driver units in a box. The driver is constructed of a metal frame to which is attached a cone, made of paper or plastic and occasionally metal. At the rear end of the cone is attached a coil of wire (the "voice coil") wound around an extension of the cone, called a "former". The two ends of the voice coil are connected to the crossover network, and the crossover network is connected to the speaker binding posts on the rear of the speaker enclosure. The voice coil is suspended inside a permanent magnet so that it lies in a narrow gap between the magnet pole pieces and the front plate. The voice coil is kept centered by a "spider" that is attached to the frame and to the voice coil. In some speakers, a rear vent allows air to get into the back of the driver when the cone is moving, but a dust cap on the cone keeps air from getting in through the front. In others, the dust cap is the vent, made of a permeable material such as cloth. A rubber, foam, or sometimes cloth surround at the outer edge of the cone allows for flexible movement. In the case of a tweeter, the cone is very light, perhaps made of silk or a thin sheet of metal, and is glued directly to the voice coil and the frame, forming both the diaphragm and suspension. Most tweeters don't move enough air per cycle to require a vent, though some are mounted with damped chambers, or open transmission lines which "vent" the rear wave to reduce acoustic resonance behind the tweeter for smoother response. When the musical electrical signal from the amplifier passes through the voice coil, the voice coil turns into an electromagnet. Depending on which way the current is traveling in the voice coil, the north and south pole of the magnetic field will be at one end of the voice coil or the other. The permanent magnet has a north and south pole as well. Its magnetic field will push the coil (and the attached speaker cone) outward if the north and south poles of the two magnetic fields are lined up together (north to north, and south to south), or pull the voice coil inward if they are lined up oppositely (north to south, and south to north). So, as the electrical signal from the amplifier, which is a representation of the original musical waveform, passes through the voice coil, and changes direction with the musical waveform, the voice coil and attached speaker cone are driven outward and pulled inward with the music. The speaker cone pushes or pulls air in the room, which translates to increases or decreases in air pressure at your eardrums, and there you have it: music. A speaker with just one type of driver is called a "one way". If it has two drivers, such as a "tweeter" to handle high frequencies and a "woofer" to handle the mid and low frequencies, then it is called a "two way". Separate high frequency drivers (tweeters), midrange drivers, and low frequency drivers (woofers) are found in "three way" speakers. You might find a one way speaker in a pocket radio, but home theater and hi-fi speakers are generally two way or three way in design. Speaker Enclosures The various combinations of drivers are placed in boxes (enclosures or cabinets) of two basic configurations being the most common. The sealed box is the simplest configuration, in which the woofer is simply mounted in a box with no other openings. This provides two functions. First, it keeps the output from the back of the driver, which is out of phase from the output of the front of the driver, from the outside of the box, so that the bass doesn't cancel out. Secondly, it provides some additional stiffness for the suspension of the driver, as the internal air acts as a spring when the driver moves back and forth, changing the internal air pressure. Enclosures which provide more stiffness from the internal air than the driver's own suspension are called "acoustic suspension." These are typically small sealed boxes. Enclosures that contribute less stiffness through the internal air pressure than the driver's own mechanical suspension are referred to as "infinite baffle," and are usually on the large side. Most commercially available sealed box designs available today are acoustic suspension, primarily due to the smaller enclosure size. The second configuration, which is quite common in most mass market products, as well as some high-performance products, is the bass-reflex design. While a sealed box is a single resonant system, a bass-reflex system is two resonant systems, the active driver, and the reflex system. The reflex system works by sucking acoustic output from the rear of the active driver, and then rebounding that energy back out of the cabinet, either through a "port" (a tube) or a passive radiator (has the frame and cone but no voice coil or magnet). These systems can generally deliver more bass extension in terms of their -3 dB cutoff limit everything else being equal, but they also have their drawbacks. The output from the port or passive radiator becomes greatest at the system's tuned frequency, determined by the port dimensions, passive radiators dimensions and weight, and the enclosure volume. Above the tuned frequency, the port contributes relatively little to the total output, moving more or less in phase with the active driver. As the frequency reproduced moves down to the tuned frequency, the reflex system starts to dominate the output, as it's sucking energy so efficiently from the active driver that while the active driver barely moves, the output from the port or passive radiator may be tremendous. Below the tuned frequency, the output of the port or passive radiator becomes more and more out of phase with the active driver, and more and more equal, so that at lower frequencies, while the driver and the port may be huffing away, it results in very little real SPL, resulting in a much sharper decline in response (24 dB/octave) below the low-frequency cut off than a sealed box (12 dB/octave.) In other words, a bass reflex system can improve bass extension and output to a point, after which it makes it worse. The difference between a port and a passive radiator in terms of performance, generally speaking, are that a port doesn't have any output limitations relating to limited air volume, compared to a passive radiator with a fixed displacement limit. On the other hand, a passive radiator cannot suffer from turbulence noise which afflicts many ported designs. A variation used in some cases is called "push pull", uses two active (electrically connected to the amplifier) drivers for the pushing and pulling. One driver is mounted inside the non vented box facing outward, and another is mounted outside, facing inward, with the two drivers electrically connected out of phase so that when the cone of one is being driven outward, away from the magnet, the other is being pulled inward, towards the magnet, both being actively driven by the amplifier. Because they're electrically out of phase, AND mechanically out of phase, they're acoustically in-phase. The primary benefit of this arrangement is that even ordered harmonic distortion from each driver is acoustically out of phase, due to the inverted mechanical orientation, and so cancels out, resulting in lower distortion. A secondary benefit, due to the presence of a second driver working in tandem, is an increase in efficiency of roughly 3 dB, assuming the drivers are fairly close to each other, so that two drivers have the potential for four times the output of a single driver. "Push-pull" arrangements can be used with either sealed or bass-reflex designs. Another trick is to use a "compound pressure" system, sometimes called "isobaric." In essence, this stacks two drivers front to front <>, back to back ><, or back to front >>, essentially using two loudspeakers to operate as one. The outside driver radiates sound, the inside driver pushes on the internal air, and both are coupled to each other with the air between them. The benefits of this are primarily that you can use an internal enclosure of half the size required for a single driver, though the cost is that the efficiency drops to half as well. This is because the two drivers are essentially operating as one, but using just as much power individually. Twice the moving mass, twice the suspension, twice the motor capacity, same surface area and displacement. Bottom line, two drivers, half the box, same output capability, twice the power requirement for the same SPL. However, if the driver are face to face or back to back, and therefore wired out of phase to work at all, the resulting arrangement will be pretty much the same as a push-pull situation in terms of reducing even-order distortion. Sometimes loudspeaker designs will incorporate a "servo" system, which provides feedback and correction to a driver to lower distortion and assure flat frequency response, much in the same way many amplifier circuits incorporate negative feedback circuits to correct their output by comparing it to their input. Typically, an accelerometer will sit on the voice coil of such a woofer, measure the net force applied to the speaker, which results in motion, subtract the difference from what it's "supposed" to be doing compared to the input signal, invert the difference, and then apply it back to the input signal to offset the error. It does this continually, fairly quickly, so that there isn't much in terms of lag, particularly in the case of subwoofers. Because the accelerometer measures the output of the driver directly, this technique will only work with sealed systems. The direction that the speakers radiate their sound depends on how the drivers are lined up in the enclosure. If the speaker has all drivers in the front, and radiates sound primarily forward, it is called a "direct radiating" speaker. If there are some drivers on the back, and those drivers are in phase with the front drivers, it is called a "bipolar" radiator. If there is substantial output from the rear of the speaker, and it is out of phase with the output from the front of the speaker, either due to out of phase drivers, or simply because the output is from the rear of the same speaker of the front, such as the case if Ribbon or Electrostatic panels, it is a "dipolar" radiator.
Accuracy, Soundstage, and Ambience The three basic characteristics of what you hear when you use your high fidelity sound system are accuracy, soundstage, and ambience. (One can find dictionaries which have dozens of words to describe the audio experience, but let's keep things simple.) Accuracy is the ability of the system to reproduce sounds precisely so a violin sounds like a violin, a trumpet like a trumpet, a piano like a piano, and so on. Secondly, it allows you to distinguish such subtleties as the fact that two violins are playing a melodic line, rather than just one violin. With good accuracy, you can hear the mallet striking the skin of the drum, and not just the boom of the drum. A guitar pick touching the string is heard, and not just the note being played. One speaker can produce this characteristic, so stereo is not necessary. The soundstage is the left to right, and front to back (depth) placement of the instruments or voices. At least two speakers and separately recorded channels (stereo) are necessary for a soundstage (unless you can be satisfied with a soundstage as wide as only one speaker, if you only have just one). The left to right concept is easy enough to understand, since the sound is being reproduced through at least two microphones, one each for the stereo channels. Front to back is a little more complicated. This phenomenon (front to back soundstage) is a result of how close the microphones were to the instruments, and the relative volume (loudness) that different frequencies are reproduced by the speakers. If the speakers have a strong midrange, then instruments in that frequency range (clarinets) would sound "forward", whereas if the speakers are very strong at the high end of the spectrum, piccolos and cymbals would sound forward. If the midrange frequencies are not as strongly reproduced, then the clarinets would sound "recessed" into the background. Imaging is a term used to describe the placement of the instruments on the soundstage. The third characteristic, ambience, is a factor not only of the speakers, but the room in which they are placed. Ambience is essentially made of echoes (reflections or reverberations), and it gives you a feeling that "you are there". If you listen to a concert live, the ambience is a result of the sound of the instruments bouncing off the ceiling, walls, floors, other people, and anything else in the symphony hall. This goes onto the recording along with the original sound of the instruments. In your home, the original ambience comes through, filtered somewhat by the fact that you are listening to the music through two windows (stereo speakers), but careful listening will reveal the reverberant characteristics in many recordings, telling us something either about the environment where the recording was made, or the studio techniques used to fake an environment. The ceiling, walls, floor, doors, windows, and furniture in your listening room add to this ambience, which can make up for some of the loss caused by having only two speakers. Ambience is also emphasized in surround sound. In fact, that is one of its main functions. You can get this ambience in a Dolby Surround system, and you can also achieve it to a certain degree by simply adding two speakers in the rear that duplicate the front stereo speakers, without using any surround processor, although this compromises the ability of a system to create a "focused" center image in front, as it will wrap sound around you homogenously. Further ambience is achieved by using dipolar or bipolar speakers (enclosure has drivers on opposite sides) which aim the sound in several directions, bouncing it off numerous surfaces, totally enveloping you with sound. Many rear channel surround speakers are of the dipolar or bipolar design, and you should audition some of them. Most of these methods for increasing ambiance from two-channel sources aren't accurate in terms of reproducing the recording, but it can make the experience more pleasant, and in some cases, for some people, more subjectively realistic. Digital surround sound, such as Dolby Digital (DD, AC-3) and DTS, have the potential for tremendous, more accurate ambience, since each channel is discrete from the others and can offer a more detailed and deliberate surround experience. Once you have purchased the speakers, experiment with their placement before you mount them permanently. Consider how your particular speaker's radiation pattern interacts with your room. Typically, you need to sit directly in front of them (on axis) to hear all the frequencies clearly. Many "home theater" speakers made for the front channels are designed with wide dispersion patterns, so that you can sit anywhere in the room to watch a movie, and hear the sound clearly from all the speakers, even if you are off to one side. However, because time arrivals play a key role in our perception of depth and image, there will only be one ideal listening location in any room, so make sure you have it. Try to match speakers as closely as possible in a surround sound setup, paying particular attention to the center channel, where an identical match (with the left and right) would be ideal. Impedance Every speaker model has an electrical specification called the "nominal impedance". Usually, it is 8 Ohms, which means that, not including the speaker cable, the amplifier is presented with 8 Ohms of resistance (impedance) in passing its electrical signal through the speaker. The word nominal means that 8 Ohms is the average impedance. Actually, it varies depending on the frequency of the music, and may range from 3 Ohms up to 20 Ohms across the auditory spectrum. Some speakers have a nominal impedance of 4 Ohms, and others, 6. This is important, because, as the impedance drops, the demand for current to be delivered by the amplifier increases for a specific output level. While the current goes up, the voltage stays the same, and the power requirements at the same signal level go up. This is defined by "Ohm's Law" which states that E = I R, or Voltage (Volts) = Current (Amps) x Resistance (Ohms), and the Power Function which states that P = E I, or Power (Watts) = Voltage (Volts) x Current (Amps). You can see from Ohm's Law that, for a specific amount of voltage, the current will go up if the resistance drops. So, if you change from an 8 Ohm speaker to a 4 Ohm speaker, the current doubles when the voltage is held constant. Then, by the power function, if you want to deliver 100 watts to the 4 Ohm speakers as you did when you had 8 Ohm speakers, the voltage would drop by a factor of Voltage/1.414, while the current would increase by a factor of Current x 1.414, in order for the power to remain at 100 watts. Therefore, if you choose a low impedance speaker (4 Ohms nominal), make sure that your amplifier can supply the current that will be necessary to drive the speaker at high listening levels. On a side note, a since a 4 ohm speaker will draw twice as much power as an 8 ohm speaker at the same signal (voltage) level, even if the efficiency is the same, the sensitivity increases by a factor of 3 dB, so that in a back to back comparison, the 4 ohm speaker may seem more efficient, even though it's simply just drawing more power. Damping Factor The impedance of the speaker will also affect what is known as the "damping factor". This is defined as the ratio of the impedance of the speaker to the output impedance of the amplifier. Thus, if the speaker impedance is 8 Ohms, and the amplifier output impedance is 0.05 Ohms, then the damping factor is 8 divided by 0.05 = 160. High damping factors usually mean that the bass response will be well defined ("tight"), whereas a low damping factor will result in a loose sounding bass. Tight or loose bass is from an amplifier is not a matter of preference. A low damping factor on an amplifier implies a high output impedance, which means that the frequency response of the amp can change by reacting to the changing impedance of the speaker it drives, making results unreliable. The bass is just one area that might suffer from a low damping factor, as it can also adversely affect other frequencies where a speaker's impedance isn't flat. Some tube amplifiers may have low damping factors, for example, 10, compared to solid state, which may contribute to their typically loose bass response (tube amplifiers are often described in terms of "warmth" or "looseness", and it can be a very pleasant effect). Such an effect may be very pleasurable, but it's also a good guarantee that you're not hearing what the loudspeaker designer created. In any case, the specification sheet for the amplifier will sometimes list the damping factor, but so long as it's above 70 or so, it's not really a real world issue. Subwoofers If you are not using full range speakers, (floor-standing does not necessarily mean full-range) then you will need a subwoofer. Surround processors often have a subwoofer output jack, or alternatively, a "mono out", to connect such a speaker. Subwoofers are designed to reproduce only the lowest frequencies in the music or movie sound track. There are 10 octaves in the range of human hearing. Each octave doubles the frequency. So, octave 1 is 20 Hz - 40 Hz, octave 2 is 40 Hz - 80 Hz, octave 3 is 80 Hz - 160 Hz, and so on, up to 20 kHz. Subwoofers reproduce the lowest 2 octaves, or 20 - 80Hz, sometimes slightly higher. Most of the speakers designed for surround sound will reproduce frequencies down to about 80 Hz, so this is where the subwoofer comes into play. One should note that in most modern surround sound receivers or processors, the setup is critical to ensure that the subwoofers get the range that they handle best, as opposed to passing low frequency information directly onto speakers incapable of reproducing the full range of bass. If in doubt, set up each channel (Left, Center, Right, and both surround channels) to "small" so as to redirect bass information to the subwoofer. Many types of subwoofers are available, in fact, just about every speaker manufacturer has at least one powered subwoofer in their lineup. The drivers typically range from 8" up to 18" in diameter. Most have built in power amplifiers, and a few are just the driver in the enclosure (you need a separate power amplifier to use them). For those readers who need an inconspicuous, yet powerful subwoofer, there are several designs with small enclosures, but very long throw drivers and large amplifiers to compensate, like the one shown on the right, which has a 10" driver. The example on the left is a 12" push-pull subwoofer. Depending on which type of subwoofer you choose, and you decide on one of the power amplifiers that has several channels of amplification (3 - 6) all in one chassis, you will need to consider how many amplifier channels to purchase. If you are not going to use the center channel (in which case, you are using the "phantom" mode), and the subwoofer is self powered, then you only need a four channel power amplifier. If you use the center channel and a self powered subwoofer, then you need a five channel power amplifier, and if you use the center channel and a subwoofer that has no amplifier of its own, then a six channel power amplifier is in order. Remember to make your choices up front. The majority of commercial subwoofers are self powered. Most "Digital" surround receivers have enough amplifiers for all channels except the subwoofer. Interestingly, subjectively higher volume will be heard from higher distortion subwoofers at a given SPL because the harmonic distortion is higher in frequency than the ultra-low fundamentals, and therefore more audible. However, this distortion is not as irritating at low frequencies as it is at higher frequencies, so, if you want a subwoofer with additional "punch", you might consider a subwoofer with lots of distortion. As always, listen and compare. Auditioning Speakers When auditioning speakers, listen primarily for detail without artificial accents. An extra zing and zap may get one's attention on first listen, but it can also get really irritating after many listening hours. It's very difficult to compare speakers from store to store packed into the demonstration rooms with each other, so reserve the option to return your final choices, or arrange for an in-home audition for your final contestants. As a quick qualifier, rap your knuckles on the speaker cabinet. If it sounds like you are rapping against a piece of granite, this means the cabinet is well damped from vibrating on its own, and will suffer less from coloration due to cabinet resonance. If rapping your knuckles against the cabinet results in a drum-like sound, the cabinet may not be well damped, and could very well obscure sonic detail in the midrange and mid-bass areas. A good cabinet isn't a guarantee of good performance, but it's a start. Click here to listen to a wav file that was recorded when rapping on a speaker that has excellent damping (the speakers shown on the left). Most speakers have a vinyl finish (usually black) with a wood grained appearance. Some of these look like real wood, and it is difficult to tell that it is vinyl. If you want real wood veneers, many designs offer this as an option. Exotic woods add several hundred dollars to the final cost. Keep in mind that that's money not directly contributing to real performance, even though it may very well make a customer happier with their purchase. The speakers on the left have a rosewood veneer that is exquisite. Other beautiful and unusual woods include jarrah and bubinga. Move around the room during the speaker demonstration. Listen for how the sound changes as you move side to side, or from a sitting to a standing position. Ambience will be difficult to test in the dealer's demonstration room, since your own room at home will be much different. However, a certain degree of ambience can be heard with different surround systems, such as direct, bipole, and dipole. Careful listening pays off here. You should take a more than a few of your favorite CDs with you to audition equipment. Include music with a wide variety of instruments and dynamics (loudness variations) such as an orchestral symphony, as well as sharp transient sounds (like steel string guitar), piano music, and solo singing voices (especially female). Several brands carry models specifically made for home theater. Keep in mind that it's difficult to tell what's the recording, and what's the speaker, without some reference. At low volume, even the least expensive speakers can sound very good. Be sure to audition the speakers at the maximum volume you would be listening to them at home. At some loudness point, every speaker begins to produce significant amounts of harmonic distortion. A fundamental sin wave in the low midrange, such as 500 Hz, can have many harmonics that are audible, i.e., 1 kHz, 1.5 kHz, 2 kHz, 2.5 kHz, etc. This makes the sound mushy or sometimes harsh. The diagram on the right shows a sin wave in blue. When there is significant harmonic distortion, the waveform looks like the outline in red (like a square wave). The red area is distortion. So, if the speakers you are considering sound mushy at the loudest volume you like to listen, try a different model or brand. If the speaker doesn't distort at louder levels, it won't "sound" louder than when listening at lower levels, it just will be. Keep in mind that digital surround is full range in frequency, even in the rear surround channels. So, if you purchase small, limited range speakers, unless you have a good subwoofer, and a proper setup in the receiver or surround processor, you're missing a lot. Also, when using a subwoofer, be sure to calibrate it's level with an SPL meter (Radio Shack analog model set to "C" weighting and slow response) and test tones, usually generated by the receiver, or perhaps a test DVD, such as Ovation Software's Avia test disc, to match the other speakers. For that matter, make sure all speakers are calibrated in relation to each other in terms of level and distance. Otherwise, with a subwoofer, particularly with higher crossover points, not only will the frequency response be unnaturally bass heavy, but the subwoofer will become easy to localize, ruining the illusion of a solid soundscape. If the other channels levels aren't matched, you won't hear the proportion or directional balance that the director or recording engineer intended. A mediocre, well-calibrated system will far outperform a "premium quality" poorly calibrated system.