by Norris Chambers
Before learning how speakers work, it is well to consider what sound is. Sound is a vibration of the air in the area that is impressed upon the human ear. The ear processes the variations in air pressure as speaking, music or whatever noises may be present. These variations have to be pretty fast to be detected by the ear...about the lowest most people will hear is around 30 movements per second, and the highest is around 15 to twenty thousand. This is referred to as "frequency response". Movements of air below and above these frequencies will not be heard by the human ear, Some animals hear much higher or lower, accounting for silent dog whistles, electronic pest removers, etc. On the whistle, the air is moved back and forth many times faster than we can hear, but the dog's ear gets it loud and clear, The pest removers work the same way...the little device emits sound waves at a high frequency that we can't hear, but it drives the pests batty because they hear it as an irritating noise. Sound is a combination of air vibrations and a speaker only restores sound from the vibrations that were turned into electrical representations of the original sound.
When we talked about magnetism we learned that current in a wire produces a magnetic field around it. If the wire is formed into a coil, the field is much stronger because the fields of the wire are concentrated and strengthened by the closeness of the wires. If the coil is wound around a permanent magnet and a current is run through the coil, the strength of the permanent magnet will increase or decrease, depending on the polarity of the applied voltage. If a thin sheet of iron or steel is placed in front of the magnet, it will bend toward the magnet. When current is applied, it will bend closer because the magnet is stronger. If the voltage is removed, it will return to its regular position.
This is the principal most speakers and earphones work on. As the diaphragm moves back and forth in accordance with the electrical impulses sent from an amplifier, radio or other sound producing device, it pushes air in an out, much like a hand moved fan does. This motion of air is picked up by the ear, and is interpreted as a sound, since sound received by the ear is only movement of air pressure variations that are determined by the source of the sound, whether it be talking, thunder or variations caused by the diaphragm of a speaker. The old speakers of the twenties were nothing more than a coil of wire around a magnet with a diaphragm mechanically fed into a horn. The radio output was run through the coil around the magnet and the changing magnetic field caused the diaphragm to move in and out. The horn directed the vibrations in one general direction and made it louder than if it had scattered in all directions. In the early thirties a radio speaker was introduced with a paper cone, similar to the modern speakers. This cone was attached to a metal reed that was placed between the poles of a horseshoe magnet with a coil wrapped around it. This speaker was called a "magnetic speaker". It worked very well, but there was a limit of the amount of volume that could be produced because of the narrow gap between the poles of the magnet, preventing large movements of the cone. The cone must travel considerable distance when reproducing music or voice at high volume, especially at lower frequencies.
Next came what is know as the dynamic speaker. This type is still in use. It is constructed by using a round steel bar that is highly magnetized. On the bottom of the cone a round form is attached on which is wound a small coil. The coil and cone is mounted so that the coil can move back and forth over the magnet and travel considerable distance. When an electrical signal is applied to the coil, it will move the cone back and forth at the same rate as the electrical frequency and voice or music can be heard at high volume. Normally the magnetic rod is a permanent magnet and the speakers are called PM speakers. Early models of the speaker sometimes had a larger coil on the opposite end of the coil with a direct current applied. This current magnetized the rod in operation. This coil was called the "field" coil. Modern speakers use very strong permanent magnets and are capable of hundreds of watts of power projection. Remember, the power is equal to the voltage applied times the current through the conductor.
Another type of speaker uses crystals that vibrate when current is flowing. The crystal is physically attached to the cone and it vibrates and moves the air. This type of speaker is usually used only in head sets or alarm signals.
Headsets, or ear phones, usually use the same arrangement as the old telephone receivers. The magnet vibrates a thin diaphragm of iron or steel and the vibrations are fed directly into your ear.
Now that you know how speakers work, you can sit back in your easy chair and listen to your favorite music. You can visualize the speaker cone moving in and out at unbelievable speed and be glad that you are listening and HAVING FUN.
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Lesson Five