Electronic Devices: How Speakers Work

Electronic speakers may have complicated features and configurations, but they all operate on a simple combination of electromagnetics and soundwave production.

Before examining how electronic speakers actually work, it might be helpful to understand the scientific principles behind sound generation. Many of us have heard the old riddle "If a tree falls in the woods and no one is there to hear it, does it still make a sound?" For our purposes, the answer is no. The tree may very well push a lot of air out of the way as it falls, but 'sound' is a phenomenon which occurs in the listener's mind. Humans have sensitive eardrums which receive waves of air pressure from a vibrating or moving object. In and of themselves these waves of air pressure are meaningless, but our ears feed this information to our brain, which in turn translate these waves into recognizable sounds. The tree in our riddle can be large or small, far or distant. What it generates is disturbances in the air, which we TRANSLATE as sound.

If we apply that same principle to the world of electronic sound reproduction, we see that speakers duplicate the work of the tree. Speakers also disturb the air in front of them, causing soundwaves to be projected through space. Eventually our ears pick up enough of these waves to send to our brains. In turn, our brains convert the vibrations into music or speech or even unintelligible noise. Our ears are usually passive receivers- they will feed any and all external noises to the brain without interpretation or prejudice. Our brains must decide which sounds are worthwhile and which are ignorable. This is why you can listen to a stereo and not realize the air conditioner is running- your brain can filter out extraneous soundwaves.

So how does a speaker actually work? The answer lies in a combination of electromagnetism and human comprehension. A speaker is made of several important components: a permanent magnet, an electromagnet called a voice coil, a suspension device called a spider, and a conical membrane called a cone. All of these parts are held in place by a metallic structure called a basket.

A vibrating object such as a guitar string generates soundwaves much like rings on a pond. These waves are actually disturbances in the air which radiate in a steady pattern or frequency. A sensitive microphone has a thin piece of material which begins to duplicate the wavelength of the guitar string because air particles are striking it. This vibration is turned into electronic impulses and recorded on an electronic storage medium (CD, cassette tape, vinyl album, etc.). When the medium is placed inside an appropriate player, the electronic impulses are fed through an amplification system which can increase the volume (amplitude) of the signals. These signals are sent out to the speakers.

A typical speaker contains a coil of wire wrapped around a material which is easily magnetized, such as steel. This 'voice coil' is wrapped with wires leading from the amplifier and back. As electricity passes through the wire coil, an electromagnetic field is generated. Electromagnets naturally have positive and negative poles, but the current flowing through the wire coils of a speaker is constantly reversed by the amplifier. Voice coils change polarity many times per second, depending on the amplitude and frequency of the tone being generated.

The main principle of a speaker depends on this constant change in the voice coil's polarity. The voice coil is suspended over a permanent magnet by a flexible holder called a spider. The spider must be sturdy enough to hold the voice coil in place, but flexible enough to allow the voice coil to vibrate freely. The permanent magnet has a constant polarity. When the voice coil's polarity is negative, the entire assembly is pulled towards the positive end of the permanent magnet. When the polarity becomes positive, the two magnets repel each other and the voice coil is slightly raised. This occurs hundreds or even thousands of times per second, which causes the voice coil to vibrate in the same pattern as the original guitar string. The relationship between the vibrating voice coil and the stationary permanent magnet is responsible for recreating the disturbance in air pressure.

Because this initial vibration is not very strong, the soundwaves must be amplified further. The voice coil is attached to the bottom center of a cone made from paper or other flexible material. As the coil vibrates, the cone is also forced to reproduce the changes in air pressure. Because it is shaped like a cone, almost all of the soundwaves are forced out of the front of a speaker. These amplified waves go out in all directions, but many reach human ears. The eardrum picks up the air pressure changes and vibrates in a pattern the brain recognizes and interprets as sound.

Just about every speaker works by the same principle of vibrations from a voice coil, but not every speaker is ideal for sound reproduction.

Higher frequency soundwaves work best with smaller speakers, while low frequency soundwaves are best reproduced on larger ones. Stereo systems often feature two or three different speakers stacked together in a tower. The smallest speaker is called a tweeter, the medium speaker is called a midrange and the largest speaker is called a woofer. Each speaker receives a specific set of electrical signals from a device called a crossover.

Crossovers work on the principal of controlling electrical flow through special filters. A controller called a capacitor will only allow the higher frequencies of sound to flow to the tweeter's voice coil. Therefore, tweeters only reproduce sounds above a specific frequency. Woofers are controlled by inductors, which limit the high range but allow low frequency electrical impulses to flow to the voice coil. Midranges have both an inductor and a capacitor, which effectively dampens the lows and highs and leaves only the middle frequency impulses to reach the midrange voice coil.

Some consumers will also purchase specialized speakers such as subwoofers, which deepen the bass sounds of recordings significantly. Some home theater systems also feature double-side speakers which work best from behind the listener. Sound is reflected to the rear, left and right of the listener. Modern speakers may contain better quality membranes or more sensitive coils for a higher quality reproduction.

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