Experiments

How do speakers recreate sound?

posted: 04/11/12
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Good speakers can virtually put you in the front row of Carnegie Hall by recreating the complex, sophisticated sounds of a full orchestra in your living room. To understand how these surprisingly simple devices can do that, we first need to delve into how our sense of hearing works.

Good Vibrations

Sound is really just a disturbance of air molecules that travel in waves. A vibrating body, like a guitar string, strikes against the air molecules around it, which disturb the molecules around them and so on, until the sound wave has traveled across an entire room. The disruption of these molecules causes them to fluctuate, alternately pushing into each other (compression) and pulling apart from each other (rarefaction). This means that the air pressure in those areas fluctuates above and below the normal atmospheric pressure, which our ears pick up on.

Our eardrums are remarkably sensitive to the vibrations of changes in pressure, and the brain interprets these vibrations as sound. The different patterns of compression and rarefaction determine the pitch and volume of that sound. Microphones, like our ears, are sensitive to the signals. These devices pick up acoustical patterns and allow us to record them by translating them into electrical signals. Then, we store these signals digitally on CDs. The speaker, as you might have guessed, must be able to do the reverse: translate the electrical signals back into vibrations for our ears to interpret what seems like the original sound.

Making Waves

If you've ever looked through a speaker grille cover, you've probably noticed a large cone inside. This cone moves back and forth, vibrating the surrounding air molecules in the appropriate pattern to recreate sound. But it's the stuff behind the cone that causes it to vibrate and form that pattern. Two important parts are the voice coil that the cone's connected to, and the permanent magnet, which is attached to the voice coil.

The stereo amplifier transmits the recorded electrical signals to the voice coil. When the electrical current flows through it, the coil turns into an electromagnet. Unlike a permanent magnet, an electromagnet's polar orientations (its negative and positive sides) can switch places. The direction of the current determines which side of the coil is negative and which is positive. In a speaker, the direction of the current changes back and forth rapidly. This causes the coil to interact with the nearby permanent magnet, alternately being repelled by and attracted to it.

As the coil is alternately repelled by and attracted to the permanent magnet, it moves the cone in a precise pattern. This setup allows the speaker to translate electrical signals back into the vibrations necessary to recreate the original sound.

But just one speaker can't produce the full array of frequencies audible to the human ear. Different classes of speakers pick up on various ranges of sound, such as woofers (for low frequencies), tweeters (for high frequencies) and midrange speakers.

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