An introduction to a vital part of any home theater
by David Gibbons
Revised April 2007
Loudspeakers are perhaps the most common example of the device known as the linear electric motor.
All electric motors work because of the electro-motive principle. Really, it's a very simple one. If you run electricity through a wire which is in a magnetic field , that is to say near a magnet of some type, the wire tries to move. If you run electricity back and forth through the wire, the wire will try to move back and forth through the magnetic field. Seems like magic, but it obviously works.
The usual electric motor uses this principle to produce circular motion or rotation. In a speaker, the arrangement of wires and magnet is set up so that motion occurs in back and forth in a straight line. (thus the term linear)
In almost all speakers, the moving wire is wrapped up into a coil for better effect, and so you'll hear the term "speaker coil " used.
Pushing a lot of electricity through any wire causes the wire to heat up. Think of a toaster. Due to the necessity of keeping the speaker coil easy to move, speaker coils are light weight and made with fine wire. Therefore common speaker coils cannot take too much heat before they burn up and fall apart. Typically , the speaker coil is buried deep down inside a closed magnetic structure. This makes it even harder to cool. This is one of the factors that limits how much power a speaker can take.
The stronger the magnetic field, the more "push" you can get out of a current-carrying wire sitting in that field. Big heavy magnets are often used in speakers to get strong fields, but the size of the gap between the metal pieces on either side of the of the speaker coil also really affects the strength of the the magnetic field. The smaller the gap, the stronger the field, and the greater the electro-motive effect. The problem is that a really narrow gap allows for little cooling air flow, and increases the chances that the moving coil will scrape or rub against the magnet structure. When a speaker coil touches other parts of the speaker, it will at least produce bad sound, if it does not destroy itself due to the wear. A part of the speaker called a "spider" hopefully keeps the voice coil centered in the gap so that it does not touch anything else.
A speaker voice coil moving through a magnetic field actually resists the flow of electricity trying to get through the coil. This resistance, plus the plain resistance of the material of the wire itself, is called "impedance." Impedance is measured with a unit called Ohms. (A guy named Ohm was an early pioneer in electrical science, and thus the name.)
Let's compare this to plumbing to make it easier to understand. A 12 inch water pipe is easy to push water through, so we would say it has a low resistance to the flow of water. We could assign it a flow resistance number of, say, one. A 1/8th inch pipe might be 100 times harder to push water through. We could then assign it a flow resistance value of 100, as it is 100 times harder to push water through than the 12 inch pipe. This works the same way for Ohms--the biggest wire has the lowest resistance value.
You don't need a big water pump to push water through that 1/8 inch pipe we are talking about. However, to maintain the same pressure while pumping water through that 12 inch pipe, you are going to need a BIG pump. This works the same way for amplifiers and speakers - a "4 Ohm" speaker will need an amplifier that can push twice as much electrical current compared to an amplifier connected to an "8 Ohm" speaker.
Most amplifiers will cheerfully work with 8 and even 6 Ohm speakers, but 4 Ohm speakers should be mated with amplifiers especially rated to work with them. Please note: the Ohm value of a speaker depends on internal speaker construction and design, and not on speaker size, power, or cost.
For a regular electric motor or a car motor, some means of getting the power from the motor to what needs to be moved has to be arranged. In normal speakers, the speaker cone does the job. The cone connects the relatively tiny speaker coil to the air. The cone works like the transmission, axles , and wheels work in a car, getting the power of the motor to the road. In the speaker cone's case, it gets the power to the air.
Just like with the car, where the transmission, axles, and wheels must be matched to the size of the engine so they don't wear out too soon or break, the speaker cone must be able to connect the speaker coil to the air without breaking down.
Paper cones were used for many years, but today various metals and plastics are also used in an effort to improve speaker performance, or are at least are used in an effort to wow buyers with fancy looks or with high-tech-sounding terminology. No matter the material, the cone should move back and forth as a unit with the speaker coil, without bending or distorting its shape, which distorts the sound it is supposed to generate.
The speaker cone has to be supported at its outside edge. This is done by an item called the "surround." The surround helps keep the cone stable and centered, while not reducing its ability to move back and forth freely. This part also helps the spider keep the speaker coil centered in the magnet gap. These days surrounds are often made of very flexible synthetic rubber materials.
Speaker coils and cones come in different sizes, and the physics of sound require that this be so. Consider: A bulldozer motor and transmission must be suited to move large loads at low speeds and a race car motor and transmission must be suited to move light loads at high speeds. Same kind of thing for speakers.
To generate low frequency sound, a speaker cone must move a lot of air, and move long distances. Therefore, low frequency speakers (known as bass speakers, and now sub-woofers) are large speakers, with cones that typically measure 8, 10, 12 or more inches across. Their construction must allow for lots of back and forth movement. Sometimes manufacturers can fool with specially shaped speaker boxes and openings in those boxes to make a smaller speaker act like a bigger speaker, but nothing really replaces a large speaker cone to produce low frequencies.
A large speaker cone also typically requires a large magnet and a speaker coil which can move back and forth a long distance to work properly. However, as you try to produce higher and higher frequencies with a large speaker, the amount of sound produced will diminish rapidly because the large heavy cone cannot go fast enough to reproduce those high frequencies. The size and weight just won't let it work well.
To generate medium frequency sounds (such as the male human voice range,) speaker cones are usually between 3 to 6 in. in diameter. Speakers of this size are often called midrange speakers. As with the big speaker, the cone's size and weight will still prevent it from producing the highest frequencies. Also, the midrange speaker isn't big enough to do the low frequencies, so it gets weak as you go down to the low frequencies.
From the top of the human voice range up to the highest audible frequencies, very small cones can be useful, except that at these frequencies the cone shape begins to have trouble particularly when higher power is demanded. Often a dome shape is used in place of the cone. These small speakers are usually called tweeters.
Usual speakers require a frame which keeps everything together and in alignment. Sturdy stamped metal frames are OK, and fancy speakers will sometimes have heavy cast metal frames. Heavy cast metal frames are best suited for the 12 inch and larger speakers.
There are various high-end speakers which do not have cones or even speaker coils, but these special designs are not necessary for good sound. Good quality conventional speakers can do an excellent job for you at a reasonable price. Do your research, and listen before you buy. There are several sites on the web which provide good information on buying speakers: the Consumer Reports website is a great place to start for common sense advice, now that you know a little more about how speakers work.
Good Listening,
David Gibbons
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Contents copyright 2002-2007 by David C. Gibbons