Speaker specifications - Speaker specifications can be difficult to meaningfully interpret because there are many variables in the way speakers can be measured. The best way to evaluate a speaker is to listen to it but there are those of us who insist on thoroughly examining the specs and if you're going to read the specifications, make sure you know what they mean .

Frequency response specifications - Interpretation of frequency response specifications often is difficult because too little data is supplied. The real problem with frequency response specifications is the number of variables involved. The distance at which response is measured, the size of the room, the axis of measurement, the nature of the test signal, the power level used for the test… all of these factors, and more, affect the measured frequency response. As with other speaker ratings, there is no standard and to be meaningful, the manufacturer should give the frequency response and the maximum number of dB that the response varies, relative to a reference frequency. The reference frequency is usually assumed to be 1kHz, unless otherwise noted.

For an accurate specification, a rated speaker might have a frequency response of “20Hz to 20,000Hz, +-4dB”. This specification is reasonable, informative, and likely to be realistic. Specifications of "20Hz to 20,000Hz” imply no more than 1dB variation between the level at 1,000Hz (the common “0dB” reference frequency) and the level of any frequency from 20Hz to 20,000Hz. Actually, a speaker that performance with only 4dB of variation from 20Hz to 20,000Hz is considered very good. A good speaker specification will include a graph of frequency response. A graph will show the maximum deviations from a perfectly “flat” response, and where the deviations occur.

"Flat" response is in the ear and eye of the beholder; the hypothetical curve above can be considered flat. While it is not uncommon for manufacturers to “smooth” a graph, the overall response is still easier to see than with no graph at all. If the graph is very jagged-looking, it may indicate that the manufacturer is being a little more honest. Instead of assuming that a smooth curve represents a better speaker, look for overall shape; most small dips and bumps in response curves tend not to be audible, and even the finest speakers will have some response irregularities. It is the wide dips or large bumps (peaks) that impair the sound quality and power handling capacity of the speaker. Also, look at the vertical scale (relative level); if there are large numerical jumps between calibration marks, a “flat” curve may actually fall off considerably.

Amplifier & Speaker power rating - Amplifier power ratings are rated in "RMS". PMPO, which stands for peak music power should not be confused as the real amplifier power . PMPO is quoted for gear like mini system and ghetto-blasters to make them look more powerful;"10,000 Watts PMPO!" merely indicates how loud a system can sound for a brief moments in order to recreate sudden, high-energy transients. RMS (Root Mean Square) is the average continuous power output an amplifier is capable of producing; power output an amplifier can produce consistently over extended lengths of time. A good amplifier (solid state) should give at least 20 Watts RMS. Ideally, you should follow the recommended amplifier power specified by the speaker manufacturer. However, an amplifier with a higher power rating than the recommended specification will not necessarily damage the speakers.

How can you evaluate the power capacity of various speakers? The speaker system specification sheet may contain statement like this: Recommended amplifier power....10 - 70 Watts RMS and you are taking a risk by using larger amplifier than 70 Watts RMS. The speaker specification also recommends a minimum power but such minimum power ratings are not firm, they do tend to be indicative of the power needed for reasonably loud reproduction, especially for good bass output. Generally speaking, the "power" rating of a speaker system is its ability to absorb audio power and translate it into sound and heat, whereas the “power” rating of an amplifier is its ability to generate audio power and deliver it to a load . These power ratings really describe two different characteristics, so the practice of using the power ratings developed for amplifiers to describe speaker power capacity is probably invalid and can misleading.

Speakers' power ratings are also measured as "peak" or "continuous" power and you should be aware of both terms. "Peak" power means that's the maximum they can handle before the smoke starts pouring out, while "Continuous" means just that: they'll hum along happily at that power rating. Most amplifiers' power rating are "Continuous" Watts per channel (which means per speaker), so if you match the amplifier's continuous rating to the speaker's, you should be okay. Amplifier ratings often include "peak" power as well, which is helpful. Remember that quite often a store will list an amplifier's power as, say, 200 Watts when what they're really selling is a 100 Watts per channel amp.

Advantage of large amplifiers - The limiting factor in matching an amplifier to a speaker is that too much power can cause the voice coil to move too far, which might permanently deform or tear the cone, dome or diaphragm, particularly with a woofer. The truth is that sometimes an amplifier rated at higher power than a speaker’s “power capacity” may be used with good results. While you might think a larger amplifier would "burn out" a speaker even faster, this is not always true. In some instances, a larger amplifier may actually be safer for your speakers. When you try to obtain high listening levels from a small amplifier, the amplifier may reach its power limits and begin to “clip”. When this happens, the tops of the amplifier’s electrical waveforms (its power output) are clipped or squared-off, and severe distortion occurs. Instead of moving continuously, the voice coil of a speaker which is driven by an amplifier that is “into clipping” will move into one position and stay there, move back and stay there, and so forth.

Under these conditions, the speaker draws more electrical power, yet converts less of that power to acoustical power than during normal (unclipped) operation. Since the power fed to the voice coil has to go somewhere, it turns into heat…heat which can “burn out” the speaker. A larger amplifier could deliver higher listening levels before it clips, and even though more peak power would be fed to the voice coil, the energy would be dissipated as sound (air motion) instead of heat. Also, it is unlikely that the speakers will ever see a fraction of that high power unless you play at maximum level all the time. It only takes a few watts to get decent level from the speakers; the rest is there to handle the dynamics of music.

Efficiency - Efficiency of a speaker system is a measure of how well the speaker is able to convert electrical energy into sound (acoustic energy). Speaker efficiency varies widely; typical consumer speakers have an efficiency rated at 1% to 3%. The highest efficiency consumer speaker systems are about 20% efficient; 4% can be considered high efficiency. Efficiency has no direct relationship to the quality of sound reproduction, and many lower efficiency speakers have excellent frequency response, low distortion, etc. Some high efficiency speakers, while they may be louder than lower efficiency speakers (given the same power), may not sound as good. Efficiency is difficult to measure, hard to evaluate, and seldom specified by numerical value so a related value is specified instead…sensitivity.

Sensitivity - Sensitivity is measured in terms of dB/watt/meter or dB/2.83 volts/meter which means that a certain loudness (1 dB) will be achieved with a standard signal level (2.83 volts or 1 watt @ 8 ohms) at a standard distance from the speaker (1 meter). Sensitivity is related to efficiency, but there is an important difference; efficiency compares the total amount of sound power (including sound coming from the back, top, bottom, and sides of the speaker) to the electrical input power, whereas sensitivity measures only the sound pressure level along the front-center axis of the speaker. Since it describes which speaker will be louder at the central listening position, the sensitivity rating is more useful and sensitivity is measured with a specified input signal. While the sensitivity rating is useful, you should realize that, like power ratings, sensitivity measurement methods can and do vary. If two speakers’ sensitivities are measured similarly, the speaker of higher sensitivity will be louder. On the other hand, the lower-sensitivity speaker having a high power rating may be able to produce more sound than a higher sensitivity speaker with a low power rating. Sensitivity ratings give only an indication of the relative loudness that can be produced, and have little to do with sound reproduction quality.

Dispersion & polar response - The polar diagram is a loudness curve drawn on a 360-degree circular grid, where the distance from the center of the circle indicates relative level. Representative frequencies are measured and charted as the speaker is rotated, giving series of "curves" that represent the relative frequency response at various angular positions. In simple terms, polar diagrams are like views of a speaker, with imaginary lines that show how well different frequencies are "projected" in two perpendicular planes. Since dispersion and polar response are very closely related, they are sometimes used interchangeably. Another name for dispersion and polar response is "off-axis response". A speaker specification might read, "dispersion 120-degrees"; this suggests that if you move along an arc anywhere within 60 degrees on either side of the speaker's center axis, the frequency content will remain essentially the same as it is along the center axis. A better specification would give the maximum number of dB that the response varies (not the loudness change, but the change in frequency response). For example, a speaker might be specified as having "120-degree dispersion, +-6dB between 40Hz and 16kHz". An even better method to specify dispersion is with "polar response" diagrams.

Polar diagrams reveal what sound the listener hears while moving around the speaker. Problems that are easily seen on polar diagrams are “beaming” (a narrowing of dispersion at higher frequencies), and “lobing” (a series of severe fluctuations in level as the listener moves across an arc in front of the speaker). Horizontal dispersion perhaps is more significant than vertical dispersion, though both should be wide. Remember, if the speaker is operated horizontally, as a bookshelf speaker on its side, the vertical dispersion becomes the horizontal dispersion.

At the sides, most speakers will “fall off” somewhat in overall level, particularly at higher frequencies. Speakers with wider dispersion are likely to sound better, not only to listeners on the sides, but to those in front of the speakers. Also, wide dispersion helps by keeping sound more “even” when sitting or standing. Due to construction and physics, the horizontal dispersion and vertical dispersion hardly ever match, but this makes little difference to the speaker’s performance. Most small speaker systems have wide dispersion at low frequencies, but narrow beaming dispersion at high frequencies. An ideal speaker system will have even dispersion at all frequencies.

Impedance specifications - Every speaker model has an electrical specification called the "nominal impedance". Usually, it is 8 Ohms, which means that, not including the speaker wire, 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 . Impedance has nothing to do with quality . 4, 6, 8 or 16-ohm speakers can be used with almost any amplifier available for home use.

If several speakers are wired in parallel, amplifiers may distort, blow a fuse, or overheat and even burn out. Two 4-ohm speakers or four 8-ohm speakers, wired in parallel, constitute a 2-ohm load, which is about the same as placing ten feet or wire across the output of your amp… a near short circuit!. If you plan to use several pairs of speakers, or if your speakers are rated at 4-ohms or less, be sure the amplifier can handle the load. Sometimes series or series/parallel wiring to the speakers will provide the needed minimum impedance. With any multi speaker load, some degradation of performance may be inevitable due to interaction so for best results, all speakers should be identical.

Nominal impedance equals the actual impedance, over a very limited portion of the speaker’s frequency range because impedance changes with frequency. To give a better indication of the actual impedance, a graph of impedance versus frequency sometimes is listed with the speaker specifications. It is true that the lower the impedance, the more power a speaker will tend to draw, but the fact that impedance changes with frequency does not mean that frequency response will be uneven. This is because the efficiency of the drivers also varies with the frequency, and because the combination of enclosure resonance with crossover network tends to compensate for the changes in power draw that might be suggested by typical impedance curves.

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 a matter of preference; one is not necessarily better than the other. Tube amplifiers often 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. In any case, the specification sheet for the amplifier will sometimes list the damping factor, so if you choose a low impedance speaker, a higher damping factor on the amplifier may be necessary if you like the "tight" bass sound. Dropping the speaker impedance from 8 to 4 Ohms would reduce the damping factor by 50% for any given amplifier.

Distortion specifications - Distortion in speaker systems may be caused by many different factors, is very complex and is hard to fully describe. While harmonic distortion, usually specified as T.H.D. or "total harmonic distortion" is a common specification, inter-modulation distortion (usually specified as I.M.) is seldom specified. Many other factors, such as non-linearity at low listening levels, might be considered “distortion” but few such items are ever listed on specification sheets. The fact that rigorous distortion specifications are omitted is no fault of the manufacturer as there is a genuine difficulty deciding how to measure many subtle distortions in a way that can be meaningfully interpreted by the buyer, that is acceptable to other manufacturers, and that is repeatable in other testing situations. Even the measurement of inter-modulation distortion, a simple task for power amplifier specification, is very difficult for speakers. Intermodulation often results from the "droppler effect". A speaker reproducing a high and low frequency at the same time will generate unwanted "sidebands" (new frequencies that were not present in the input signal, and that are non-musical to the ear). Intermodulation in an amplifier may be specified at many frequencies; 60Hz and 6kHz, 70kHz, and so forth, all of which have validity.

However, if a two-way speaker is measured for intermodulation at 70Hz and 7kHz, the chances are that these frequencies are being reproduced by two different drivers (70Hz by the woofer and 7kHz by the tweeter), so intermodulation is not likely. If the same speaker were measured at 40Hz and 300Hz, both of which frequencies are fed to the woofer, severe intermodulation could occur, with audibly distressing result. Thus, even if an intermodulation rating is provided on a speaker, it may or may not be meaningful . Harmonic distortion may be the result of overdriving the speaker (too much power), of “rippling” speaker cones, or both. The harmonic distortion specification is useful because it can be meaningfully compared on various specification sheets. Phase distortion often is talked about and seldom defined. It may be a partial signal cancellation caused by differences in listening distance relative to two drivers, by phase shifts in crossover networks, or by other factors. Few authorities suggest we can hear “absolute” phase differences, but many believe we can hear such things as a harmonic being “out of phase” with respect to a fundamental, a severe phase shift in one portion of the frequency spectrum, or a “fuzzy” midrange. Wherever the truth lies, phase distortion is another quality unlikely to be meaningfully specified.

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