A speaker is an electroacoustic transducer, which produces sound, in response to an electrical audio signal input. The most common speakers use a paper cone supporting a voice coil electromagnet acting on a permanent magnet.
Common speakers use a lightweight diaphragm that is connected to a frame using a flexible suspension wire that is wrapped around a fixed magnet. When an electrical signal is applied to the voice coil, a magnetic field is created by the electric current flowing through the voice coil. The coil and the fixed magnet interact generating a mechanical force that caused the coil and the attached diaphragm to move back and forth, thereby reproducing sound.
The speakers diaphragm is usually manufactured with a cone or dome shaped profile, and commonly constructed from paper, plastic, or metal. The cone needs to be rigid enough to hold its shape while being light weight so that it takes little force to generate movement. It also needs to be well damped to reduce unwanted resonance after the signal has stopped. For the best of all of these attributes, many speakers are now made of a composite material. Carbon fiber, Kevlar, glass, hemp, or bamboo fibers can be added for strength, or a coating might be applied for additional strength.
The speakers frame is designed to be rigid, avoiding deformation that could change critical alignments. The frames are usually constructed of cast aluminum alloy, or stamped from sheet metal. Molded plastic compound speaker frames are becoming more common, especially for low-mass speakers.
The suspension system keeps the coil centered and provides the restoring force that returns the diaphragm to the neutral position. There are two parts to the suspension system, the spider, which connects the diaphragm and voice coil to the frame. Then there is the surround, which helps center the diaphragm cone and the coil.
The speaker cone surround can be rubber or foam or a resin coated fabric. It is then attached to the diaphragm and the frame. Each one of these materials have there are pros and cons as they can dramatically affect the output of the speaker.
The voice coil is made usually from copper, though aluminum or silver could be used. Voice coil wire sections can be circular, rectangular, or hexagonal adding to the wire volume coverage in the magnetic gap. The coil moves back and forth within a small circular groove in the fixed magnet.
Modern day magnets are usually always a permanent magnet that is made of ceramic, ferrite, alnico, or more rare materials like neodymium or samarium cobalt. When high field-strength permanent magnets became available, Alnico, an alloy of aluminum, nickel, and cobalt became popular. Alnico magnets can be partially demagnetized by accidental ‘pops’ or ‘clicks’ caused by loose connections but the damage can be reversed by “recharging” the magnet. In the 1980s most manufacturers switched to ferrite magnets, which are made from a mix of ceramic clay and fine particles of barium or strontium ferrite. These magnets are less expensive, which lets designers use larger magnets to achieve better performance.
Since speakers provide optimal performance within a limited pitch range, multiple speakers are generally combined into a complete sound system. These speakers include subwoofers, mid-range, and tweeters.
A subwoofer is a speaker used only for the lowest part of the audio spectrum, typically below 200 Hz for consumer systems. Because the subwoofer is intended for a limited frequency range, the speaker itself is usually designed in a simpler way, often consisting of a single driver enclosed in a box or enclosure. To reproduce very low bass notes without unwanted resonance, subwoofers are solidly constructed and properly braced. A good subwoofer is usually quite heavy. Subwoofers are either “active” or “passive.” An active or “powered” subwoofer includes a power amplifier and electronic sub-filters with additional controls relevant to the production of making low-frequency sounds. “Passive” subwoofers will require external amplification.
A mid-range speaker is a driver that reproduces middle frequencies. Mid-range speakers can be direct radiation drivers (like a small subwoofer) or they can be compression drivers. If the mid-range is a direct radiator, it can be mounted on the front baffle of an enclosure, or, if a compression driver, mounted at the throat of a horn for added output level and control.
A tweeter is a high-frequency driver that reproduces the highest frequencies in a speaker system. Tweeter designs range to allow different abilities with regard to frequency, output, power handling, or maximum output level. Most widely found in stereo systems is called a soft-dome tweeter.
A coaxial driver is a speaker with two or more combined concentric drivers. These are typically found in automobiles because the design allows a more compact speaker housing. In automobiles 2 or 3 way speakers mount the tweeter, or the tweeter and a mid-range, in front of the woofer, partially obscuring it.