The human ear is normally capable of hearing frequencies from 20 hertz (cycles per second) to 20,000 hertz. This is quite an amazing range. Most speakers cannot reproduce this wide range of frequencies with a single driver (a single speaker). This is why most speaker cabinets will have multiple drivers. A large driver (woofer) to reproduce the low frequency sounds (20-150 hertz). Medium sized drivers (midrange drivers) to reproduce the mid frequencies (150-5000 heartz), and small drivers (tweeters) to reproduce the high frequency sounds (5000-20,000 hertz.

A Tweeter (high frequency driver) has a small cone, and thus a low reciprocating mass, and is much better suited to reproduce the high frequency electrical signals. Tweeter cones are not well suited for the low frequencies because they cannot move the large amounts of air that good bass reproduction demands. However, they are well suited for high frequency reproduction. High frequency sounds disipate easier in common carpet, clothing furniture, etc., and are quite directional. Aiming the tweeters in the right direction is important in order for the listener to hear them properly. Normally, the low frequency signals are filtered out with an inline capacitor, so that the tweeter only sees the high frequency analog signals over 5000 hertz.

The coils and capacitors mentioned above are called "crossovers". The function of the crossover is to separate low, mid and high frequencies according to the number of drive units in the loudspeaker. A crossover can be passive or active. A passive crossover is generally internal to the cabinet and consists of a network of capacitors, inductors and resistors. Having no active components, it doesn't need to be powered. An active crossover on the other hand does contain transistors or ICs and requires mains power. It sits between the output of the mixing console and a number of power amplifiers - one for each division of the frequency band. A system with a three-band active crossover would require three power amplifiers.

As it happens, a slope of 6 dB per octave is useless. High frequencies would be sent to the woofer at sufficient level that there would be audible distortion due to break up. Low frequencies would be sent to the tweeter that could damage it. 12 dB/octave is workable, but most systems these days use 18 dB/octave or 24 dB/octave. The four most important aspects of any speaker are OHMS, Decibels, Enclosures and Sensitivity

OHMS
Ohms, electrical resistance of speaker drivers.
Electricity is the flow of electrons across a conductor, like copper wire. How well the electrons travel from negative to positive depends upon the resistance in the conductor. The electrons can be thought of as water, in a pipe. If there is a lot of rust, bumps, and otherwise resistance in a water pipe, the water will flow slower, with less volume passing any particular point per unit time.

"Ohms" is a measure of electrical resistance. You don't think of it, but substances conduct or resist electricity on a scale. Rubber isn't very conducive, copper is. This is why some premium cable are made out of silver or gold and such - these substances conduct electricity better than copper, but are more expensive.

When the electricity passes into the speaker, it is "resisted." to a certain extent. The ohms rating of the speaker is how much is resisted, and an indication of how much energy it takes to drive it - the higher the ohms rating, the more difficult it is to drive. The relationship between volts, amperage and resistance is explained by the equation V=IR, where V=volts, I=amperage, and R=resistance in ohms.

Decibels

The term dB has meaning in all kinds of scientific measurement -- from sound, to electrical or mechanical power, to voltage, and so on. The decibel scale is an example of a logarithmic scale. Other examples of logarithmic scales used in scientific measurement are the Richter scale (used to denote the energy of earthquakes) and the pH scale (used to indicate the concentration of hydrogen ions in a solution).

Now suppose, if you will, that you’ve found that the smallest sound intensity that most people can hear is .000000000001 W/m2. You also discovered that the intensity that makes people start to wince in pain is 1 W/m2. Of course, you’ve taken a bunch of measurements in between as well, like .000792710162 and .000006288415. Just try conveniently comparing those numbers! Quick -- what’s the difference between .000792710162 and .000006288415 ? Try figuring that one out in your head!
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Enclosures

The moving coil drive unit (speaker driver) is as open to the air at the rear as it is to the front, hence it emits sound forwards and backwards. The backward-radiated sound (backwave) causes a problem. Sound diffracts readily, particularly at low frequencies, and much of the energy will 'bend' around to the front. Since the movement of the diaphragm to the rear is in the opposite direction to the movement to the front, this leaked sound is inverted (or we can say 180 degrees out of phase) and the combination of the two will tend to cancel each other out. This occurs at frequencies where the wavelength is larger than the diameter of the drive unit. For a 200 mm drive unit the frequency at which cancellation would start to become significant is 1700 Hz, the cancellation getting worse at lower frequencies. The simple solution to this is to mount the drive unit on a baffle. A baffle is simply a flat sheet of wood with a hole cut out for the drive unit. That way, the backwave has to travel much farther before it cancels the front wave. Amazingly, it works. But to work well down to sufficiently low frequencies it has to be extremely large. The wavelength at 50 Hz, for example, is almost 7 meters. The baffle can be folded around the drive unit to create an open back cabinet, which you will still find in use for electric guitar loudspeakers. The drawback is that the partially enclosed space creates a resonance that colors the sound.

Basically, there are two types of speaker enclosures. Sealed, and bass reflex. The sealed enclosure simply seals off the speaker backwave, in order to prevent the backwave from interfering (or canceling) the front wave. This type of enclosure requires a driver with lower "spring rate" around the speaker cone. Since the enclosure is sealed, the air inside the enclosure dampens the driver cone movement. This dampening effect is usually compensated by using a weaker spring surround around the speaker cone.

The bass reflex enclosure makes use of the backwave. By installing a tube into the enclosure, the backwave can actually be routed out through the tube, and if the tube is made a particularly length (tuned), then the backwave (at certain frequencies) will be more "in phase" with the front wave, and the backwave then reinforces the front wave.

Speaker sensitivity
Speaker sensitivity is a specification provided by all manufacturers of high-quality speakers. The sensitivity rating has no relation to sound quality, as some of the very best speakers have low ratings. Sensitivity ratings simply tell you how much sound a speaker will produce for a given power input.

Sensitivity ratings are given in decibels per watt at one meter, or db/Wm. So, with an input of one watt (usually white noise), a speaker with a sensitivity of 90 db/Wm will produce 90 decibels of sound at a distance of one meter. A sensitivity of 90 is considered average, with ratings of 87 and below considered low sensitivity and above 93 considered high sensitivity. To increase the volume by 3 db, you must double the power. So, using the example above, to make 93 db you would need two watts, and to make 96 decibels, four watts.

"Sensitivity," which is expressed in dB, should not be confused with "efficiency" that is expressed as a percentage of power out relative to power in. Efficiency data for loudspeakers suffers from many problems such as failure to consider variations in frequency response.

Speaker efficiency is the ability of the speaker to do work or use power. The more efficient the speaker; the less power is required for the speaker to produce sound. The original Bose 901 speakers had 9 4" drivers (speakers) in each cabinet. They required a minimum of 50 watts per channel to drive those speakers. On the other hand the Klipsch Klipschhorn speakers with the horn and 15" woofers were very efficient and could be driven at 10 watts per channel. Voice coil design, type and size of the magnets, speaker cone design and material, speaker size, etc. all play a critical role in determining speaker efficiency. However, speaker size is a good general method for guessing efficiency.

Typical speaker efficiency (for physicists) is about 5%. Meaning that for 100% power input, you get about 5% acoustical work back.