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發表於 2012-7-21 08:34
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SPEAKERS
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. |
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發表於 2012-7-21 08:18
