Three-Minute Tech: Speakers
[In our Three-Minute Tech series, we tell you everything you really need to know about a technology in three minutes or less.]
Speakers—sometimes referred to as loudspeakers—are everywhere. Just looking around my office, I find them inside my iPhone, IP telephone, Apple Cinema Display, headphones, and laptop. Even my alarm clock and my microwave have speakers, not to mention the speakers connected to my stereo. But how do they work?
Sound—whether it’s the music blasting from your speakers or the noise of your fingers on your keyboard as you type—is essentially moving air. Sound waves move air, and when that air reaches your ears, your brain interprets it as sound. Speakers, no matter the size, perform the simple task of creating sound waves. More specifically, they take an amplifier's electrical representation of an audio signal, and they vibrate to create the corresponding sound waves that recreate the original sound. (When recording music, sound is converted to digital or analog representations of sound waves. When playing music, you need equipment to convert that data back to the appropriate electrical signals that can be fed to your speakers. Your stereo components or a device’s digital and analog circuitry serve this purpose.)
How speakers work
The term “speaker” is often used ambiguously. It can refer to an individual speaker—otherwise known as a driver—or an enclosure that holds multiple drivers. I’ll use driver when I'm talking about a single driver within an enclosure that contains several drivers, and speaker to refer to an enclosure with one or more drivers.
Most drivers use a very simple design. At the back of the driver, a permanent magnet (generally circular) is held firmly in place in a rigid frame. As electricity is fed into the driver, changes in the electrical field cause a copper coil inside the magnet to vibrate. Attached to the coil is a membrane, usually made of paper or plastic, that vibrates back and forth to move the air in front of the driver—thus creating sound waves. When these waves reach your ears, you hear sounds.
When the electric current flows in one direction, the membrane pushes away from the magnet. When it flows the other way, the membrane pulls back. The flow of current is changed back and forth to match the frequency of sound waves the speaker is meant to create. For low frequencies (bass), this may be a few dozen times per second. For high frequencies (treble), this happens up to 20,000 times—or more—per second.
The size of a driver affects the audio frequencies it can best reproduce. A larger driver can move more air, but it can't move as quickly, making it better for producing bass. A smaller driver doesn't move as much air, but it can move much more quickly, making it better for producing treble. Because of this, most hi-fi speakers use multiple drivers.
(Some speakers have a single driver—a telephone, for example. However, though there are single-driver speakers that do a good job of producing nearly full-range sound, speakers designed for listening to music generally use two or more drivers to allow them to produce the full spectrum of audible frequencies.)
A basic hi-fi speaker has two drivers: a woofer (for lower frequencies—bass) and a tweeter (for higher frequencies—treble). Midrange frequencies—which cover the range that includes most vocals—are generally reproduced by the tweeter, although some speakers with smaller woofers (such as bookshelf speakers) let the woofer handle some midrange duties.
How does the speaker determine which driver handles which frequencies? A speaker with multiple drivers includes a circuit called a crossover that routes electrical signals to the appropriate driver according to the frequency it covers. For example, in a small bookshelf speaker, the crossover might send all frequencies at or above 3kHz to the tweeter, and anything below 3kHz to the woofer.
Some hi-fi speakers have one or more additional drivers specifically to handle midrange frequencies—thus requiring one or more crossover circuits—and you may even have a separate subwoofer in your speaker setup (which requires a subwoofer crossover somewhere in the signal chain). Each of these different drivers or speakers is optimized to produce different frequencies. From low frequencies to high, and thus larger driver size to smaller, the order is: subwoofer, woofer, midrange, tweeter.
The length of a particular sound wave depends on its frequency. You may not think much about wavelengths, but they can be important when you decide how you are going to position your speakers.
Take your subwoofer, which produces low-pitch sounds (bass), which have long wavelengths. There’s a reason you (usually) only have a single subwoofer: A 40Hz sound wave—a very low frequency that generally requires a subwoofer—is more than 28 feet long. In a normal-sized room, you simply can’t hear stereo with waves of that length, so you can get by with a single subwoofer. In addition, because of the wavelength, your ears can’t even tell precisely where the wave is coming from, which is why you can position your subwoofer almost anywhere in your room (though it’s best to have it on the same side of the room as the rest of your speakers).
[For more on determining the right number of speakers for you, see “How many speakers is enough?]
As for tweeters, a 4000Hz (4kHz) sound wave—that’s around the highest notes a guitar or violin can play (not taking into account harmonics)—is only about 3.4 inches long. Since these sound waves are so short, and vibrate so quickly, you need to be directly in their path to hear them clearly. This is why tweeters are always at the top of loudspeakers—so there’s a better chance they’ll be located at or near ear level. This means you should position your left/right speakers so the tweeters are around the same height as your ears when you’re listening to music or watching TV. If you use smaller speakers that don't sit on the floor, they should be on stands or shelves so their tweeters are roughly at ear level. Speakers on your desk, connected to your computer, should also be positioned (on a shelf or stand, or tilted up) so their tweeters point toward your ears.
Woofers and midrange drivers produce frequencies that are in between treble and low bass, so their positioning is less important than that of tweeters. (This is why floor-standing speakers’ woofers are always at the bottom.) However, the higher the frequency, the more you’ll notice if your ears aren’t “on axis”—a phrase that means in the direct line of the speaker’s sound-wave transmission—with the driver.
Speakers are all about sound waves and air. Their technology is simple, but understanding wavelengths can help you position speakers so your sound is optimal.