Thứ Bảy, 18 tháng 6, 2011

Speakers – why magnets, why cones, why different costs, why different sizes

Speakers – why magnets, why cones, why different costs, why different sizes

Making Sound: Diaphragm

A driver produces sound waves by rapidly vibrating a flexible cone, or diaphragm.
  • The cone, usually made of paper, plastic or metal, is attached on the wide end to the suspension.
  • The suspension, or surround, is a rim of flexible material that allows the cone to move, and is attached to the driver's metal frame, called the basket.
  • The narrow end of the cone is connected to the voice coil.
  • The coil is attached to the basket by the spider, a ring of flexible material. The spider holds the coil in position, but allows it to move freely back and forth.
Some drivers have a dome instead of a cone. A dome is just a diaphragm that extends out instead of tapering in.

 A typical speaker driver, with a metal basket, heavy permanent magnet and paper diaphragm

Making Sound: Voice Coil

The voice coil is a basic electromagnet. An electromagnet is a coil of wire, usually wrapped around a piece of magnetic metal, such as iron. Running electrical current through the wire creates a magnetic field around the coil, magnetizing the metal it is wrapped around. The field acts just like the magnetic field around a permanent magnet: It has a polar orientation -- a "north" end and and a "south" end -- and it is attracted to iron objects. But unlike a permanent magnet, in an electromagnet you can alter the orientation of the poles. If you reverse the flow of the current, the north and south ends of the electromagnet switch.
This is exactly what a stereo signal does -- it constantly reverses the flow of electricity. If you've ever hooked up a stereo system, then you know that there are two output wires for each speaker -- typically a black one and a red one.

The wire that runs through the speaker system connects to two hook-up jacks on the driver.
Essentially, the amplifier is constantly switching the electrical signal, fluctuating between a positive charge and a negative charge on the red wire. Since electrons always flow in the same direction between positively charged particles and negatively charged particles, the current going through the speaker moves one way and then reverses and flows the other way. This alternating current causes the polar orientation of the electromagnet to reverse itself many times a second.

Making Sound: Magnets

So how does the fluctuation make the speaker coil move back and forth? The electromagnet is positioned in a constant magnetic field created by a permanent magnet. These two magnets -- the electromagnet and the permanent magnet -- interact with each other as any two magnets do. The positive end of the electromagnet is attracted to the negative pole of the permanent magnetic field, and the negative pole of the electromagnet is repelled by the permanent magnet's negative pole. When the electromagnet's polar orientation switches, so does the direction of repulsion and attraction. In this way, the alternating current constantly reverses the magnetic forces between the voice coil and the permanent magnet. This pushes the coil back and forth rapidly, like a piston.
When the coil moves, it pushes and pulls on the speaker cone. This vibrates the air in front of the speaker, creating sound waves. The electrical audio signal can also be interpreted as a wave. The frequency and amplitude of this wave, which represents the original sound wave, dictates the rate and distance that the voice coil moves. This, in turn, determines the frequency and amplitude of the sound waves produced by the diaphragm.
Different driver sizes are better suited for certain frequency ranges. For this reason, loudspeaker units typically divide a wide frequency range among multiple drivers. In the next section, we'll find out how speakers divide up the frequency range, and we'll look at the main driver types used in loudspeakers.

Driver Types

In the last section, we saw that traditional speakers produce sound by pushing and pulling an electromagnet attached to a flexible cone. Although drivers are all based on the same concept, there is a wide range in driver size and power. The basic driver types are:
  • Woofers
  • Tweeters
  • Midrange



Woofers are the biggest drivers, and are designed to produce low frequency sounds. Tweeters are much smaller units, designed to produce the highest frequencies. Midrange speakers produce a range of frequencies in the middle of the sound spectrum.
And if you think about it, this makes perfect sense. To create higher frequency waves -- waves in which the points of high pressure and low pressure are closer together -- the driver diaphragm must vibrate more quickly. This is harder to do with a large cone because of the mass of the cone. Conversely, it's harder to get a small driver to vibrate slowly enough to produce very low frequency sounds. It's more suited to rapid movement.
Things that make a difference in speaker qualityThe operating principles that make a typical speaker work today are identical to those used one hundred years ago for the same purpose. The most obvious difference that might be noticed between those first speakers and common units today, is the replacement of the electrical field coil used to create the magnetic field in those early units, by a powerful, permanent (non-electrical) magnet made from highly refined metallurgical materials. Gauss density, which is a measure of the strength of magnets, is many times higher in modern alloys. The higher the gauss density, the greater the field strength. This is the force within the speaker motor that principally determines the amount of power from the amplifier that the speaker will be able to handle.
Another important factor used to assess power handling, is the weight of the magnet itself, which usually has a bearing on the amount of magnetic energy it can produce.
Important tip!
Among important things to remember when evaluating speakers, is to be sure to compare the right sorts of claims. Some manufacturers specify weight for the magnet only, while others weigh the entire magnet structure, which may be up to twice the weight of the magnet proper.
Another important parameter is the material of which the magnet is made. Newer speakers have magnets made of neodymium, strontium, and barium, among other technologically advanced metal composites. Older and cheaper units use alnico 5 and other less dense alloys that may have only one third or less of the magnetic potential of the newer composites.