An Audio speaker (or Loudspeaker) is an electronic device which produces sound waves. This is accomplished by generating vibrations which cause the air pressure to fluctuate slightly in waves. The more rapid these fluctuations are (the higher the frequency of fluctuation) the higher the perceived pitch is. The stronger each fluctuation is (the greater the amplitude), the greater the volume of the sound.
Each modern dynamic speaker contains a number of parts working together. Every speaker contains one or more driver, which is the device to actually produce sound. Each driver contains a diaphragm (a flexible cone or dome, made of paper, plastic, or metal), which is attached to a flexible rim, called the suspension (or surround). The suspension is attached to the driver's frame, which is called the basket. The inner part of the diaphragm (the back of the concave surface) is attached to the voice coil. This voice coil is then attached to the basket by a spider (a ring of flexible material) which allows vibration but keeps the coil in place.
Several of these drivers are usually installed into an enclosure (except in rare cases, when only one is needed). Many speakers are designed generically and sold for off-the-shelf use. However, speakers can also be custom designed, which generally results in better sound quality, since they can be made specifically for the kind (or genre) of sound they will be reproducing. Some speakers are even installed into enclosures which also function as furniture.
In a traditional dynamic speaker, the voice coil is nothing more than a simple electromagnet. It is supplied with power to magnetize it, but the power flow is cycled from one direction to the other thus also toggling the magnetic field polarization (north to south and back again) many times per second during operation. Opposite to the electromagnet is a permanent magnet (which requires no power, and generates a static magnetic field). Opposite magnetism attracts, while similar magnetism repels, so based on the charge put on the electromagnet, the two magnets either pull towards one another, or push away. Since the voice coil is attached to the back of the diaphragm, this movement causes the diaphragm itself to move. The diaphragm in turn moves the air molecules, causing high and low pressure waves to move out from it. Now by adjusting the rate and strength of the electromagnet cycles, the sound waves' amplitude and frequency can be altered rapidly. Once designed and configured properly, this device now produces whatever sound is desired, within the speaker's range of ability.
However, each driver does have limitations. The human ear can perceive a much greater range of tones than a driver can produce. For this reason, most speakers include multiple drivers. Although drivers can be produced to best output any variety of tones, they are generally given the classification of as Woofers, Tweeters, or Midrange.
The largest kind of drivers are classified as woofers. These produce low-pitch sounds, such and thunder and dogs barking (as the same suggests). They usually produce sounds between 20 and 2,000Hz. There are other low-pitch classifications which are similar to this one. These drivers are also the largest in size.
Midwoofers have a slightly higher range than regular woofers. They are able to produce sounds between 200 and 5,000Hz, although they are best between 500 and 2,000Hz. Outside of this optimal range, the quality degrades. These consist of a single speaker driver in an enclosure.
Subwoofers can produce sounds between 20 and 200Hz. Although they have a small auditory range, multiple drivers are usually installed in each enclosure. Unlike midwoofers, subwoofers can produce equal quality throughout their limited auditory range.
A less common kind of subwoofer is the rotary subwoofer, which uses fans instead of the diaphragm and magnet design. Rather than moving a diaphragm to create sound waves, the fans are used to make pressure waves. These can produce sounds at 20Hz and below, and make them audible when they usually would not be.
Midrange speakers produce, as the name suggests, sounds in the approximate middle of the auditory range. For many purposes, these are the speakers which are used most, since many sounds are neither very high nor very low. However, these alone are not enough to represent sound properly without losing quality. These speakers are much smaller than woofers, but still larger than tweeters.
Tweeter drivers produce high-pitch sounds, such as bird calls like the name suggests. These are the smallest kind of speaker driver, and for this reason are often fit into speaker cases last, since they can fill the physical gaps.
For speakers with multiple drivers to function properly, the audio stream must be divided up among the different drivers, so that each one produces what it is best at. The speaker crossover is responsible for this task. Most such devices are passive crossovers, which use some simple circuitry including inductors and capacitors, and need no additional power source. Using the power from the audio signal itself, the crossover can perform its task. However, some use active crossovers, which need an additional power supply. These offer several benefits, including the option to easily adjust frequency ranges. Most musical performers use active crossovers, and control them using sound boards.
Although much less common, kinds of speakers other than dynamic also exist. These are a few other kinds:
Electrostatic speakers also use electromagnetism, but in an entirely different way. A pair of conductive plates are placed on either side of a flat diaphragm, with a narrow gap in between them. The plates are then charged, so that the diaphragm has a positive charge on one side, and a negative charge on the other. Now, the audio signal is run through the suspended diaphragm. As the audio signal flows, the diaphragm is rapidly cycled between a positive and negative charge. Opposite charges attract, so while charged negative, it pulls towards the positive plate, and while charged positive, it pulls towards the negative plate. This process takes place so rapidly that high-frequency sound is produced. In fact, this system tends to respond more quickly than the traditional dynamic speaker, which can be useful in some situations.
Planar magnetic speakers are very similar to electrostatic speakers, since they use a similar design with a metal ribbon suspended between two plates. However, the surrounding plates are permanently magnetized rather than electrically charged. This kind of speaker also produces high-frequency sound.
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