We can represent an AC electrical signal with a sinusoidal waveform. The waveform below has a frequency in Hz and an amplitude, just like with sound waves. The frequency is just the number of full wave cycles one peak and one valley per second. Most of us know that when two magnets come close together, they will want to move.
Opposite poles attract and the same poles repel. A speaker creates movement by manipulating the interactions between an electromagnet the voice coil and a permanent magnet. The voice coil is suspended in the magnetic field of the doughnut-shaped permanent magnet. When we have no electricity flowing through the voice coil, we have no electromagnetic field being generated. Therefore, no interaction or movement will place. When we do have electricity flowing, we generate an electromagnetic field.
Then, the poles of the electromagnet and permanent magnet interact with each other and cause the suspended voice coil to move. We can control the strength of the electromagnet and therefore the amount the voice coil moves by controlling the electrical power flowing through the voice coil. The greater the electrical power, the greater the displacement of the voice coil from its de-energized equilibrium position will be.
The power is directly related to the signal amplitude. A higher amplitude means higher power and more sound! When using an AC electrical signal to energize the voice coil, the poles of the electromagnetic field will flip positions. Electricity flows through the wire coils, and the magnetic center pole remember the right-hand rules? So how does the human ear hear the sound of the speaker?
Sound enters the ear primarily through the eardrum; pressure waves are amplified within the ear via area reduction. When the pressure waves reach the inner ear, they cause the large number of extremely sensitive nerve endings in the inner ear to vibrate. These nerve endings detect their own vibration and convert it to electrical signals that the brain can recognize.
This electromagnet will attract or repel other magnets as if it were an ordinary magnet. One difference, however, is that we can easily reverse the polarity of the electromagnet by changing the direction of current flow. An oscillating current in the coil will result in an oscillating force if the coil is near a permanent magnet.
This is the basis of a dynamic speaker. A coil surrounded by a permanent magnet is attached to a cone made of cardboard, plastic or other flexible material. When an oscillating electrical signal flows through the coil there is an oscillating force on the coil and cone. This makes the cone vibrate back and forth, producing sound waves in the air next to the cone.
Dynamic speakers have two basic designs depending on the range of sound frequencies being reproduced. Bass speakers or woofers have large, flat cones attached to the coil and are designed to produce low frequencies.
A sketch and cutaway picture are shown below. The voice coil is attached to the cone or diaphragm. Electrical signals enter from the leads and go through thin wires which are glued to the spider, a lightweight flexible support system for the coil. Oscillating current in the voice coil causes an alternating magnetic force between the coil and the permanent magnet. This alternating force on the coil is transmitted to the cone which causes air to vibrate, creating sound.
Tweeters are designed similarly to bass speakers but have a small 2 cm to 5 cm , dome shaped vibrating part in place of the cone. Because they are smaller and lighter they are better able to produce higher frequencies.
In both types of speakers the permanent magnet does not move. This is because magnets are heavy; if the permanent magnet was attached to the cone instead of the coil, the extra inertia would keep the cone from responding to fast variations in the signal. It is also possible to make an electrostatic speaker that uses electrical forces, rather than magnetic forces. For this type of speaker, a thin sheet of flexible plastic a diaphragm with a metallic coating is suspended between two thin metal screens or grids.
There is a small air gap between the grids and the plastic sheet. The sheet is charged by an external source and the oscillating signal is sent to the grids. The oscillating voltage between the grids causes the diaphragm to vibrate, producing sound.
0コメント