ULTRA AUDIO -- Archived Article

August 1, 2004

The Ideal Speaker

Hang on to your hats, gentle readers, for I have designed the ideal speaker. It has a perfectly flat frequency response, instantaneous driver acceleration and deceleration, ideal dispersion, accurate bass, high efficiency, and -- oh yes, pleasing appearance. Want to know the secret? I’ll tell you, but I must warn you: This speaker will probably not only never be built, it probably never should be built, for reasons that will soon become obvious.

To get an idea of the nature of the ideal speaker, it’s helpful to have a solid grasp of the non-ideal speaker; e.g., the ones you have in your living room right now. Though your speakers are undoubtedly the best available for the price and look as if sculpted by Michelangelo, typical common-man speakers are egregious heaps of crap that look and feel like packing crates -- or, more recently and worse, like the Jetsons’ household robot. They have rickety plastic drivers, shoddy internal wiring, crossovers dug out of cereal boxes, and their ill-fitting particleboard cabinets are coated in dusty spray paint.

But the real failings of real speakers are in their sound. What you want, of course, is a full-spectrum frequency response that covers the enormous 20Hz-20kHz bandwidth of human hearing. Vibrating at 20,000 times a second may be no problem for a tiny mass, but it’s not feasible for a large woofer. On the other hand, a tiny tweeter moving at 20Hz would be like poking a needle into peanut butter -- it would have no appreciable effect on the surrounding air. You need a large woofer to transfer enough energy through your room’s air for loud (not necessarily good) bass. Thus, most speakers have multiple drivers.

But multiple drivers require crossovers, which split the frequencies among the drivers using electronic filters that roll off gradually. Now we have problems. No crossover is perfect; ergo, every crossover makes mistakes. Getting a tiny tweeter to sound as if cut from the same sonic cloth as a 12" woofer is tough. Ditto with the crossover electronics, and the potential nasty interference effects at the crossover points when both a midrange driver, say, and a woofer are trying to make the same sound. When in operation, large drivers have elastic properties that cause them to spring back, producing a phenomenon called back-EMF. This occurs when a driver’s piston-and-voice-coil assembly acts as a generator. The back-EMF then propagates to the other drivers, where it wreaks havoc.

The speaker cabinet is another major source of sonic problems. The cabinet’s function is to support the drivers and to provide an internal volume of air that helps couple the sound from the drivers to the air in the room. What’s supposed to happen is that the drivers vibrate the cabinet air, and the cabinet air vibrates the room air. What actually happens is that the drivers vibrate the cabinet air and the room air at the same time. Tweeters tend to primarily vibrate the room air, in a directional beam, while woofers tend to mainly vibrate the cabinet air, which then oozes into the room as a flatulent, nondirectional rumble.

There’s more. Interactions between internal and external vibrations, diffraction effects as sound bends around the front baffles of the speaker, and the problem of dispersing all the frequencies through the room so that the right balance reaches the listening position -- all rear their ugly heads. In addition, the cabinet may rattle at a variety of audible frequencies unrelated to your music. Even with a super-rigid, super-damped cabinet, physics dictates that a volume of air in an enclosure will itself resonate, with more undesirable sonic consequences.

But let’s leave the poor old conventional speaker alone and get to the good part. When I began to muse on the various options, I realized that I wanted to get away from multiple drivers and crossovers altogether. A single driver should be capable of reproducing the entire audio spectrum, and it should disperse all of these frequencies evenly in all directions. Also, there should be no cabinet resonances, which means no cabinet. By now, you should be imagining a sphere hanging in space.

No, I’m not about to foist some alien technology on you. Such a speaker must be feasible using practical present-day technology. Among the plethora of nonstandard speaker technologies available, only the ion driver meets these specs.

An ion driver applies a very-high-frequency AC carrier current to a pointed electrode tip to produce a ball of plasma. The size of the ball can be varied almost instantaneously in accordance with a musical signal because the ball has almost zero mass. This design has been used for 50 years in the ion tweeter, which has been commercialized and works beautifully. Unfortunately, the plasma balls in these tweeters are tiny -- less than 1mm in diameter -- and therefore can’t reproduce low frequencies. Scaling up the size of the plasma ball has been considered impractical because of the huge voltages required. For some reason, a speaker that can launch a bolt of lightning into your chest from the next room has never caught on.

But there’s another way. Instead of enlarging the plasma ball by upping the voltage, it’s possible to decrease the ionization energy properties of the air using -- you guessed it -- ionizing radiation.

Before you run for cover: Some processes of radioactive decay involve particles known as alpha particles, which travel only a few millimeters in air. They efficiently ionize the air in their immediate vicinity, but pose no danger to living things. A spherical electrode coated with a pure alpha emitter such as polonium, for example, would permit the formation of a large ball of plasma -- say, over the surface of a 2" sphere -- that could reproduce satisfying bass and blissfully clean treble at the same time from a point source.

So there you are. Go down to your local Chernobyl Surplus store, pick up some polonium, and start building.

Or not.

...Ross Mantle

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