by Norris Chambers
Before understanding how microphones work, we have to learn just a little more about physics - sound waves, to be more specific. What makes a sound? A sound is nothing more than a vibration in the atmosphere that causes a response from your ear or other receiving device. Sounds are composed of different frequencies of vibration. Think of a vibration as you did AC current - a change from negative to positive at some rate - the rate is called the frequency and is expressed as so many cycles per second. In the matter of sound, it would be so many vibrations per second. A low rate of vibration produces a low tone, and a high rate of vibration produces a high pitched tone. The human ear normally can hear frequencies from around 30 cycles per second to well over 15,000. Some people can hear higher frequencies and some lower.
To simplify sound waves, as we did AC current with the battery that we kept changing the polarity to illustrate, we will take a piece of plywood about two feet in diameter and holding it in front of us, will push it forward the length of our forearms. Then we will draw it to our chest. We have performed one cycle, and in doing so, we pushed the air that was in front of the plywood forward, and when we brought it toward us, we pulled some back. This disturbance went forward through the air. That was a 1 cycle sound wave. Now imagine that you could do this about 50 times per second. If you did, you would be generating a sound wave that a person could hear as a very low roar. Now pretend that you could do this 2000 times per second. You would be>generating a shrill whistle that could be heard for blocks. That is the way sound waves are made - a sound vibrates the air and the vibrations are picked up by the ear.
The ear detects sound waves by means of a thin portion that vibrates, or moves in and out with the air pressure of the sound it is receiving. This vibration is transmitted to the brain and interpreted as a sound. Although the ear cannot hear extremely high frequencies because it cannot vibrate and transmit that fast, that does not mean that they are not there. For instance, a dog can hear much higher frequencies and that is why the silent dog whistle is heard by the dog, but not by people.When you speak, you force air past your vocal cords, causing them to vibrate in accordance with the sounds you wish to transmit. Speech is composed of several different frequencies emitted at the same time, but the ear and brain translate them into intelligent sounds.
Now, back to the microphone.
We will examine four basic types of microphones: the carbon, the crystal, the dynamic and the condenser. These four types all convert sound waves into electronic components that vary in accordance with the sounds that they are expected to pick up.
The carbon microphone is the first practical mic and was the one used in early telephones and radio transmissions. As the name implies, it uses carbon granules, or small bits of carbon. Carbon is a conductor of electrons, but it offers considerable resistance to its flow. When the granules are packed together tighter, the resistance is less. When they are looser, it is more. When they are compressed, more current will flow through the carbon, and when they are not compressed, less current will flow. In the microphone, a diaphragm, or thin metal disk, is positioned against one side of the carbon grains, and one side of the battery is connected to it. The other pole is connected to the other side of the carbon. When a sound wave hits the thin diaphragm, the resistance of the carbon goes up and down in accordance with the vibrations of the sound wave, and of course the current goes up and down with the resistance. You have converted the sound waves into variations of electrical current. The battery was connected through the low resistance winding of a transformer, and as you remember, variations in the primary cause corresponding variations in the secondary. Because there are a few turns in the primary and many in the secondary, an AC voltage is produced that is equivalent to the sound wave.
The crystal microphone works on an entirely different principle. It uses a crystal like material called "Rochelle salts." This material is in a thin strip, and has the peculiar property of causing electrons to gather on one side when it is bent. The more it is bent, the more electrons. In a crystal microphone, the thin disk or diaphragm is mechanically connected to the crystal, causing it to bend slightly when the diaphragm vibrates. As the sound waves strike it, it in turn vibrates the crystal, causing it to produce a voltage that varies in proportion to the frequency and amplitude of the sound that is picked up. The same type crystal has been widely used in phonograph pickups. The needle is connected to the crystal, and as it moves from side to side in the groove of the record, in accordance with the recording, a voltage is produced that is proportional to the frequency and amplitude of the recorded material.
A dynamic microphone uses a diaphragm also, but it is connected to a round coil of a very few turns that surrounds one pole of a permanent magnet. It is wound on a round, insulated form that fits closely over the magnet. When the diaphragm vibrates with the sound waves that strike it, it moves back and forth through the magnetic field. As you remember from the magnetic theory, when a coil is moved through a magnetic field, a voltage is generated in it. So as the sound moves it through the permanent magnetic field, it generates a voltage that varies with the vibration of the sound waves striking the diaphragm.
The condenser mic is what the name suggests....a condenser. As you recall, a condenser (or capacitor, as it is sometimes called) consists of two plates separated by an insulator, In a condenser microphone, one plate is fixed and the other is a diaphragm of very thin conducting material. A DC voltage is applied to the condenser through a high value resistor. When the voltage is applied, the electrons form on one plate and retreat from the other. But when sound waves hit the thin diaphragm plate, it vibrates back and forth. When the plates are closer together, the capacity increases. When farther apart, the capacity decreases. The sound wave that causes the diaphragm to vibrate in and out causes the electrons to advance and retreat in accordance with the sound. Therefore, you have a varying voltage across the resistor that corresponds with your sound.
These four microphones have all done the same thing - presented you with a varying electrical voltage that is a representation of the sound waves. You could listen to this voltage with a headset, or amplify it and cause it to come over a loud speaker, as in a public address system. You could also use it to broadcast on radio or TV.
Next time we will find out how a speaker works. Aren't we having FUN?
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Copyright © 2007 Norris Chambers