Speakers: Understanding the Technology

How (Most) Speakers Work
The last couple of decades have seen many dramatic changes in the electronics we use in our audio systems. We have moved from an analog world ruled by LPs and cassettes to systems that encompass both audio and video carried on digital media such as CD and DVD. Yet through it all, basic speaker design has changed very little. An amplified electrical audio signal travels through wires to the speaker, where it forces the diaphragms (usually cones or domes) of one or more drivers to vibrate back and forth like pistons. These vibrations push and pull on the air to create pressure waves that mimic the sound of the original event. The explanation is easy, but performing the task accurately and economically is anything but.

Because we can hear sound from approximately 20 Hz, or 20 vibrations per second, all the way up to 20 kHz (20,000 Hz) — or less if you’re past your teens — most speakers use two or more different-size drivers to do the job. That’s the most straightforward way of handling the very different physical requirements for producing very low and very high frequencies. The low-frequency sound waves of the deepest bass are many feet long, whereas the highest treble wavelengths are mere fractions of an inch. Moving enough air to generate strong bass therefore requires a relatively large driver, called a woofer, while radiating high frequencies evenly over a broad area usually requires a very small one, called a tweeter.

Some designs take it a step or two further, dividing the audio spectrum into three or even four bands, with the frequencies in the middle handled by intermediate-size midrange drivers. A passive electrical circuit called a crossover splits the incoming signal between the various drivers. Crossover design is one of the trickiest and most critical elements of speaker engineering. Consequently, it’s not too surprising that some of the finest speakers ever created have been simple two-way designs. Some speakers of questionable quality, on the other hand, are built with scads of drivers to appeal to unsophisticated shoppers. This is not to say that three-, four-, and more-way speakers are bad — there are many superb examples — only that you can’t assume they’re necessarily better, either.

A popular multidriver design technique that doesn’t involve a crossover is using two or more woofers in tandem to cover the same frequency range, instead of a single larger woofer. This is a way of getting strong bass while avoiding the problems that can occur in trying to blend a tweeter’s output smoothly with that of a big woofer — and without resorting to a three-way design with a midrange driver. It allows the cabinet to be slimmer as well, pleasing the eye as well as the ear.

Sensitivity
A speaker’s sensitivity rating lets you know how loud it will play for a given power input, which is standardized for the purposes of the specification at 2.83 volts (equivalent to 1 watt into 8 ohms), with the speaker’s output (sound-pressure level in decibels) measured at a distance of 1 meter. Most speakers have sensitivities somewhere in a range from the mid 80s to the mid 90s. It’s important to realize, however, that sensitivity is measured on a logarithmic scale. A speaker with a sensitivity rating of 89 dB needs only half as much power to play at a given volume as one rated at 86 dB, and a speaker with 96-dB sensitivity needs only one-tenth the power. (A 3-dB sensitivity difference is equivalent to a 2:1 power difference, while a 10-dB sensitivity difference is equivalent to a 10:1 power difference.) With that in mind, it’s easy to understand why some horn speakers with sensitivities of more than 100 dB can blast you out of the room with just a couple of watts. (A horn enables a driver to transfer energy to the air more efficiently, which is why horn speakers tend to have unusually high sensitivities.)

Impedance
Impedance is another specification to take note of, because many amplifiers will deliver more power into 4 ohms than they can into 8 ohms. This will work only up to a point, however. The increased power results from a larger flow of current, which generates more heat in an amplifier’s output transistors. Some amps deal with this better than others. Whereas a high-end power amplifier may be able to work comfortably into 2-ohm loads and lower, a budget receiver may shut itself down or even fail if asked to work hard into an impedance lower than 6 ohms or so.

Sealed vs. Ported Enclosures
It’s easy to overlook the role of the speaker box, or enclosure, but without one you’ll generally get very little bass. Enclosures come in a pretty bewildering array of sizes, shapes, and finishes, but all of them perform the same basic task of containing and sometimes rerouting the sound that radiates from the backs of the drivers, especially the woofers.

Most enclosures fall into one of two basic categories: sealed (acoustic-suspension) and ported (bass-reflex). Acoustic-suspension woofers depend on the pressure of the air in the sealed enclosure to provide a very linear (and thus low-distortion) restoring force. Ideally, all the sound radiation from the woofer’s back is contained and dissipated within the box. Bass-reflex speakers, on the other hand, have a port, or hole, in the box that’s tuned to a frequency that will allow some of the sound coming off the back of the woofer to reinforce the bass output from the front.

There are many variations on the ported-enclosure theme, including passive-radiator, bandpass, and transmission-line designs. A passive radiator is a diaphragm (usually a cone) that looks like a woofer but is not driven by the electrical signal from an amplifier. Instead, it responds to the back wave from the woofer. Like an open port, it is tuned to a frequency that will reinforce the deep-bass output from the front of the woofer. Bandpass enclosures have multiple ports, one of which is often inside the box, communicating between internal chambers. A transmission-line enclosure, on the other hand, has a defined path (often a folding tunnel) from the back of the woofer to the port that is filled along its length with sound-absorbing material.

Although each type of enclosure has adherents among engineers and enthusiasts, any of them can yield excellent results. It’s a matter of what works best to achieve a particular set of design objectives. For a shopper, however, one thing to keep in mind about any ported speaker is that it will require at least a little “breathing room” around the port. If a speaker has a port on its back panel, for example, don’t jam it back against a wall, inadvertently sealing off the opening.

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