Loudspeaker Basics

By Chuck Hawks

Klipsch RF-7 II
Klipsch RF-7 II in cherry veneer, a 2-way system using a 1200 Hz horn
and dual 10" woofers. Illustration courtesy of Klipsch Group, Inc.

The loudspeakers are, by far, the most important component in any stereo system. If you are building a component stereo system from scratch, you should plan to spend about half of your entire budget on the loudspeakers. Speakers, after all, are where the sound comes from.

Choosing the right loudspeakers for a home stereo (music) system is not easy. The specifications of most Hi-Fi components, including CD players, turntables/cartridges, tuners, pre-amps and power amps are relatively well defined and easy to compare; they give the informed buyer a good idea of what the unit should be able do. A simple physical inspection, without even turning the component on, can give a reasonable idea of how well it is made. However, this is less true of loudspeakers, whose most crucial elements, the drivers, are hidden inside the enclosure. All you can see, even with the grill cloth removed, is the front of the cones and what is behind the cones is what matters most.

Loudspeakers are the least accurate of our components. They are the typical stereo system's biggest source of distortion, coloration, un-natural resonances and other problems. Unlike the electronic components, they interact with the listening room in which they are used, as well as the listener's ears. No two people hear sounds, especially music, identically and we play different kinds of music at different listening levels. All of this complicates speaker selection.

Loudspeakers with high fidelity pretensions are usually provided with at least some specifications, but what and how much is revealed varies with the manufacturer. Usually, the more detailed the specifications, the better. It tends to indicate that the manufacturer has less to hide. Common loudspeaker specifications include the type and number of drivers, nominal system impedance (usually 4, 6, 8 or 16 ohms), system frequency response, efficiency, power handling, crossover points, physical dimensions and weight.

The impedance of a loudspeaker system varies with the frequency it is reproducing. The impedance specified is usually an "average low"; the system should not go much below that specified. Eight ohm systems are the most common and almost all home audio amplifiers can handle eight ohm speakers. Four ohm speakers, especially if they dip below three ohms at some point, can be a problem, as the amplifier may begin to see them as a short; this can send an amp up in smoke. 16 ohm home speaker systems are no longer common. Today, these are usually professional speaker systems. However, most amplifiers that can handle eight ohm speakers can also handle 16 ohm speakers.

The speaker system's frequency response is important, particularly at the low end. The most typically quoted frequency range for perfect human hearing is 20Hz to 20,000 Hz. However, our hearing response is such that there is not much practical difference between a tweeter that extends to 17,000 Hz and one that goes all the way to 20,000 Hz or above. On the other hand, most people will definitely notice the difference between a speaker system that can reach 50 Hz and one that can reach 40 Hz or, even better, 30 Hz. 10 cycles per second difference is meaningless at the high end, but very audible at the low end.

To be meaningful, frequency response specifications must be accompanied by an indication of the allowable variation in loudness, expressed in decibels (db). Typical variation ranges are quoted as +/- 3 db, +/- 4 db, or +/- 10 db. The smaller the specified variation the better. The decibel scale is logarithmic, so a +3 db change indicates twice as loud, a -3 db change indicates half as loud and -6 db is of the reference level. In other words, decibels add up fast! Frequency response specs that do not include variation limits in decibels are meaningless and should be viewed with great suspicion; the company is trying to fool the unwary.

Efficiency or sensitivity indicates the speaker system's acoustic output in decibels (usually at around 500-1000 Hz, on axis, in an anechoic chamber) for a given amplifier input signal at a given distance. This is usually measured with one watt (2.83 volts into 8 ohms) input at a distance of one meter (39"). This is important, as it indicates how much amplifier power you will need and how loud the speaker will play with a given amount of amplifier power. Typical specifications might be "85 db at 1 watt / 1 meter," or "101 db @ 1w / 1M." Remember that decibels are logarithmic, so 101 db is many times louder than 85 db.

Power handling is relatively unimportant, although it tends to impress the uninformed. Speakers do not produce watts, amplifiers do. A 400 watt speaker is NOT better than a 100 watt speaker and does not necessarily play louder; in fact, it probably does not play as loud. The speaker system's efficiency indicates how loud it will play with a given input. Power handling just indicates how much power it takes to overdrive the speaker, which you don't want to do.

The number of drivers and crossover points gives some insight into the manufacturers design priorities, as does the enclosure design. The crossover from woofer to midrange/tweeter is crucial and the lower it is the better. A lower frequency crossover means that the woofer has less of the midrange to reproduce and can therefore be larger and more specialized to reproduce low frequencies. 500 Hz or less from woofer to midrange/tweeter is excellent.

As we will see later, size matters in loudspeaker systems. Bigger is usually better and heavier tends to indicate more robust drivers and a solidly built cabinet. Be aware, however, that unscrupulous manufacturers have been known to weight speaker boxes inside with lead or iron, simply to make them feel heavier and more substantial.

Looking backward up the audio component chain, speakers must be a reasonable match for the power amplifier with which they will driven in terms of impedance, efficiency, power handling, required volume at the listening position, etc. The listener's sonic preferences also come into play, as does the type of music to which he or she primarily listens, since some music requires higher volume levels, greater dynamic range, wider frequency response and so forth than other types. Acoustic jazz, folk, new age and chamber music are less demanding in these areas than orchestral or symphonic music, amplified rock and roll and new country music. (Guess why retailers demonstrating bookshelf and pedestal speakers like to use jazz and new age recordings!) Even the listener's sex plays a part in speaker preferences, as women are typically more sensitive to high frequencies than men. (Note that female voices are, on average, pitched higher than male voices, so this makes sense.)

The optimum loudspeaker system would be a one-way (single driver) system, but I know of no single driver that can effectively emit the entire range of frequencies required for high fidelity music reproduction at similar levels, similar fidelity and across an acceptably wide sound stage. Thus, two or more drivers are required for a full range loudspeaker system, each handling part of the audible frequency spectrum.

Electrical circuits called "crossovers" (usually capacitors and coils in parallel circuits) feed the appropriate section of frequencies to the appropriate driver. A 12 db per octave turnover/roll-off slope has proven about optimum for this purpose, in order to minimize driver duplication without creating an audible hole in the system's overall frequency response curve. Simpler, cheaper and inferior crossovers may have six db or even three db per octave slopes and should be avoided.

Every crossover point creates distortion and frequency response irregularities, so it is wise to minimize them. You don't want two dissimilar drivers reproducing the same frequency. The best solution to this dilemma is the two-way system (one crossover point), which means a specialized "woofer" for low frequencies (for example, 20-500 Hz) and some sort of midrange/treble driver (tweeter) for the higher frequencies (500-20,000 Hz in this case). Drivers with such a wide frequency range capability are expensive and usually large, so a more common solution is the three-way system, typically with a woofer (for example, a 10" diameter cone), midrange speaker (4" cone) and tweeter (1" dome). A four-way system adds a "super tweeter" for the highest frequencies. These multi-way systems allow the use of progressively less capable (and therefore cheaper) drivers at any given price point.

I admit to a long standing bias in favor of horn-loaded compression drivers for the midrange and high frequencies, as they project more direct sound to the listening position and are usually more efficient than direct radiating drivers. I also prefer two-way (one crossover point) systems over three or four-way loudspeakers with multiple electrical crossovers. The best two-way systems have often used a 12" or 15" woofer combined with a 500 Hz or 800 Hz horn with compression driver for the mid/high range. Such systems, if good quality, are expensive! They are also widely used as studio monitors and in professional sound reinforcement applications, such as live concerts.

The compression drivers that power mid/high frequency horns are quite similar to the ubiquitous dome tweeter. In fact, the dome tweeter was originally adapted from a compression driver detached from its horn. Horns very efficiently direct their output over a defined, moderately wide, listening area. (Horns pretty much throw like they look.) Domes are less direct and efficient, but their sound distribution, not constrained by the horn bell, is considerably wider. A small cone speaker (say 1.75" in diameter) can also accurately reproduce high frequencies, but they tend to be very "beamy," meaning that you must sit right in the path of the speaker's axis to get the proper effect. Most upscale home speakers today use dome or horn tweeters.

Most horn drivers and dome tweeters are self-contained and sealed on the back side, meaning they do not need to be isolated inside the main cabinet. Typical cone midrange speakers (usually in the range of 3" to 6" diameter) can perform well in small enclosures; in large enclosures with powerful woofers, the midrange driver is sometimes isolated from the effect of the woofer's back radiation.

Most speaker cabinets are primarily designed to complement the performance of the woofer, the biggest and most expensive driver and the one with the most difficult job. Low frequencies are difficult to reproduce, due to their free air physical wavelength (some 38 feet for a 30 Hz sine wave) and various types of enclosures have been designed to house woofers. The most common types of speaker enclosures are infinite baffle (also sometimes called "air suspension"), horn loading and bass reflex.

A true "infinite baffle" is just a very large, flat panel with a speaker mounted in the middle. It must be large enough so that the longest sound waves from the back of the woofer cannot get around the board to cancel the sound waves emanating from the front of the woofer. The result is a baffle board extending for maybe 50 feet in all directions around the edge of the woofer for a speaker with a 30 Hz free air resonance. A form of true infinite baffle would be a wall mounted speaker front firing into a room with the back of the speaker radiating its sound into the great outdoors. One advantage of a true infinite baffle is that it allows the woofer to achieve its full excursion in free air. The overriding disadvantages are that the woofer's rearward radiation is wasted and, of course, the baffle's immense size. True infinite baffles are impractical for most applications.

An infinite baffle enclosure folds the infinite baffle into a box that contains the woofer's back radiation, thus preventing cancellation of the front radiation. This works, but the disadvantages are that the back radiation is wasted and the air trapped in the box must be compressed by the woofer as it pumps back and forth, reducing the woofer's excursion and efficiency. A very large enclosure, as was used in the top of the line Bozak loudspeakers, is the best type of infinite baffle cabinet.

Air suspension is a variation of the infinite baffle enclosure popularized by Acoustic Research (AR) in their loudspeakers. This uses a woofer designed with a very loose suspension in a smaller box. The idea is that the air trapped in the box effectively serves as part of the woofer's suspension. AR speakers were known for their low bass and Altec-Lansing used this principle, on a grand scale, for the 15" woofer enclosure in their big and powerful Barcelona home speaker.

Dipole speaker panels, such as full range electrostatics and the planar-magnetic speakers produced by Magneplanar, could be considered a sort of driven "infinite baffle" that isn't infinite. The whole freestanding Magnepan panel is driven magnetically to create sound pressure waves equally from its front and back surfaces. The sound of panel speakers is often described (subjectively) as "light and airy, but lacking in bass." Panel type speakers are often quite good at reproducing midrange frequencies, but low frequency sound pressure waves are physically much larger than the overall size of the panel speaker. Panel speakers are therefore finite (as opposed to infinite) baffles and suffer from the endemic problem of having their back radiation cancel their front radiation at lower frequencies. These dipole panels must usually be augmented by conventional woofers to reproduce the lower musical octaves.

Magnepans are also inherently deficient in high frequency output, because the entire Mylar panel with its imbedded wires has too much mass to be good at reproducing very short wave lengths. The current Magnepans incorporate special high frequency "quasi ribbon" tweeter panels to address this problem, creating a two-way panel speaker. Upscale models also incorporate quasi ribbon super tweeter panels, making them three-way systems. How or at what frequencies these different panels crossover is not given in the specifications supplied on the Magneplanar web site.

Panel speakers also tend to be very inefficient. The top of the line Magneplanar MG 20.1 is a four ohm system whose output is specified at 85 db at one meter with 2.83v input. That is fully 16 db lower than the eight ohm Klipsch RF-7 II tower speakers pictured at the top of this article! Naturally, the bigger a panel speaker is the better. The MG 20.1 measures over 6.5' tall and 29" wide. (For reference, the RF-7 II is four feet tall and 11-5/8" wide.)

Horn loaded woofers are very efficient, but to be effective a low frequency horn must be very large. Anyone who ever attended a "Sensurround" movie in the 1970's (Midway, Earthquake, and Rollercoaster being the prime examples) knows what I mean. Those horns were powered by 18" Cerwin-Vega woofers and were so large that whole sections of seats in the theaters had to be removed to make space for the horns!

A more practical solution is the corner horn, popularized by Paul Klipsch with his famous Klipschorn, still produced today. This highly efficient design (105 db SPL at 1 watt/1 meter) uses a 15" woofer radiating into a folded wooden horn that uses the 90-degree corner walls of the listening room as the outer sides of the horn bell. The low frequency Klipschorn can reach 33 Hz at -4 db. The wooden bass horn is augmented by midrange and tweeter horns with compression drivers. The crossover points are 450 Hz and 4500 Hz. The system frequency response is 33-17,000 Hz +/- 4 db. Unfortunately, Klipschorns are still very large, floor standing speaker systems that weigh 175 pounds and many listening rooms do not have two solid corners on the same wall. Klipschorns are often incompatible with the "open" floor plans popular in many modern houses.

More versatile, but more restricted in low frequency response, are free standing speakers with folded horns that radiate directly to the front, such as the Klipsch La Scala. This floor standing system uses the same 15" driver as the Klipschorn, but due to its shorter folded bass horn, its frequency response specification is 51-17,000 Hz +/- 4 db. The La Scala uses the same mid and high frequency horns as the Klipschorn. The substantial La Scala also weighs 175 pounds.

The famous Altec-Lansing Voice of the Theater (VOTT) two-way speaker system typically used a 500-20,000 Hz exponential, cast aluminum, mid/high frequency horn and compression driver. (Somewhat smaller and less expensive 800-20,000 Hz horns in otherwise identical VOTT systems were also available.) This was combined in a single system with a 15" woofer front loaded into a straight, exponential wooden horn that puts the low frequency and high frequency drivers' voice coils in the same vertical plane, thus eliminating the phase discrepancies endemic to folded horns. Another very large floor standing loudspeaker system, the VOTT's short bass horn is only effective down to about 120 Hz. The VOTT is actually a hybrid system, as the enclosure is also a bass reflex type with the mid/high frequency horn mounted in a front firing port that delivers the bulk of the system's bass energy between 35-120 Hz from the woofer's back radiation. This made the VOTT part horn and part bass reflex. Altec built VOTT systems into the early 1990's.

Bass reflex enclosures use a port to let the back radiation of the woofer into the room. Using both the front and back pressure waves created by the woofer significantly increases the total bass output and raises efficiency by substantially reducing the effect of air compression inside the box. Of course, the internal volume of the enclosure, the area of its reflex port and the free air resonance of the woofer must all be designed for each other. The back woofer radiation must be in phase with the front radiation or the two will not augment, but cancel, each other. The Altec Enclosure Guide pamphlet had an excellent section on the design and benefits of bass reflex enclosures, which have become the most popular type used in high-fidelity loudspeaker systems.

Speaker cabinets of all types must be very solidly constructed and internally braced to reduce unwanted resonances. They are usually lined with sound absorbent material. Conventional drivers (speakers with cones, voice coils and magnetic structures) should have cast frames of great rigidity. The suspension should be adequately compliant for the intended purpose and not deteriorate with age. (Rolled foam cone surrounds should be avoided for this reason.) Voice coils should be large in diameter and formed of copper wire. Magnetic structures should be large so that they can generate the high gauss required to control cone movement for crisp transient response. (The 15" woofer in Altec VOTT systems had a 3" diameter voice coil of edge-wound copper wire and an Alnico magnetic structure weighing about 10 pounds.) Manufacturing tolerances are very tight. None of this comes cheap, so loudspeaker systems that use high quality drivers are invariably expensive to build and, consequently, to buy.

Incidentally, most loudspeaker companies, even those who design and build their own enclosures in-house, out-source their drivers. Sometimes these drivers are proprietary designs, but more commonly, they are off the shelf items. Altec-Lansing and JBL (at least before they were acquired by corporate conglomerates), were exceptions that actually built their own drivers. Klipsch, as this is written, designs their own drivers and has them produced overseas in factories directly supervised by Klipsch technicians and engineers. Lower line Klipsch speaker systems are now assembled in China to Klipsch specifications; these can be identified by their "wood grain vinyl veneer" cabinets. Upscale Klipsch loudspeaker systems, those with real wood veneer cabinets, are built in Hope, Arkansas, U.S.A.

Size matters in speaker system design, with bigger enclosures being better. Big woofers, especially those 10" and larger, need big enclosures to function efficiently. A large enclosure requires fewer design compromises in other areas. Unfortunately, very large speaker boxes, such as the Altec Voice of the Theater, JBL S8R, Electro-Voice Patrician, Bozak Concert Grand and Klipschorn, take up a lot of floor space and may not be suitable for small listening rooms. Of these, only the Klipschorn is still made today.

Low frequency drivers must move a lot of air to be audible to human ears and require large enclosures to function efficiently. It is not difficult to make a small driver (say a 4" diameter cone) resonate at a low frequency (say 30 Hz) by making its cone heavy, but its output is inadequate, because it does not move enough air to significantly impact your eardrums. Remember that a 30 Hz sound wave is over 38 feet in physical length. Therefore, large woofers are better than small ones. Here are the surface areas of some common diameter drivers: 6" = 28.3 sq. in., 6.5" = 33.2 sq. in., 7" = 38.5 sq. in., 8" = 50.2 sq. in., 10" = 78.5 sq. in., 12" = 113.1 sq. in., 14" = 153.9 sq. in., 15" =176.7 sq. in.

As you can see, one 8" driver is worth almost two 6" drivers. It takes three 6" drivers to approximate the surface area of one 10" woofer and four 6" drivers to equal one 12" woofer. Moving up in size, two 8" woofers move less air than one 12" and two 10" woofers equal a single 14". Full-range tower speakers, with their typically narrow enclosures, cannot accommodate large woofers, so they generally use multiple smaller woofers to move the requisite amount of air. Unfortunately, whenever you have two drivers reproducing the same frequency, it can cause problems (beat frequencies, etc.). When multiple woofers are used, as in most modern tower speakers, two larger woofers are theoretically better than three smaller ones, other things being equal.

According to my research, the great majority of home stereo speakers, regardless of brand name, are now produced in Red China. I find this distressing. During the formative part of my audiophile life, North American and British companies were at the forefront of high fidelity loudspeaker design and manufacture. Sadly, many of the great U.S. speaker manufacturers of the past have either disappeared, quit producing home stereo speakers, or been purchased by conglomerates who have merely exploited the brand name and reputation.

For example, Bozak is long gone. Altec-Lansing has been bought and sold several times since their glory days and the present owners seem to be primarily interested marketing a line of cheap, imported computer and i-pod speakers with the Altec name on them. This once great American company no longer manufacturers any high fidelity home loudspeakers. JBL has also fallen on hard times and is now owned by Harmon International, as is Infinity. Neither offers the wide range of high quality home stereo speakers upon which their reputations were built, although JBL still offers some formidable professional speaker systems. Electro-Voice is out of the home speaker market, but they still offer a wide range of professional (concert hall and stadium) sound reinforcement systems.

Klipsch still maintains a strong position in the upscale home speaker market, but a proposed buy-out of Klipsch by AudioVox, infamous for their acquisition and subsequent gutting of other reputable audio companies, was announced in January 2011. Since its founding by Paul Klipsch, the Company has been a family owned business. The acquisition of Klipsch by AudioVox, purveyors of plastic Chinese junk through "big box" stores, does not bode well for the future of the upscale Klipsch loudspeakers that are (as of this writing) still assembled in Arkansas.

What makes a good loudspeaker? My definition of good Hi-Fi loudspeaker performance is a speaker system that can accurately reproduce the musical information it is fed. If it is fed a low quality, distorted, compressed recording, it should produce a low fidelity, distorted and compressed sound. If fed a high fidelity signal with wide dynamic range and frequency response, that is the sound that should come out of the speaker system.

In other words, contrary to the verbiage used in audio magazine reviews, a loudspeaker should not intrinsically sound "musical." (Should a speaker sound "musical" when it is reproducing a narrator's voice?) Nor should a speaker system make recorded music sound like a live performance, unless it is playing a recording of a live performance. Most music is recorded in a studio and should sound like a studio recording when played through a good speaker system.

A loudspeaker is a transducer, converting an electrical input signal into sound waves we can hear. It should do this without adding or subtracting from the information it was fed. Of course, no loudspeaker does this perfectly or even very close to perfectly. Loudspeakers are the least accurate link in our component chain, so to minimize aberrations, buy the best you can afford. Finally, a high-end speaker system should be made to last of top quality materials and components. Its service life should be measured in decades.




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Copyright 2011, 2013 by Chuck Hawks. All rights reserved.


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