by Chris Woodford. Last updated: June 28, 2018.
You're about to walk past the Mona Lisa in the Louvre art gallery without even noticing but, just as you pass by, the woman in the picture calls out and starts telling you her story—who she is, how she came to be painted, and how she's lived her cramped life inside this simple wooden frame for almost 500 years. Suitably informed, you walk on through the gallery and the same thing happens with the next painting... and the next... and the next!
Sounds like a joke? It's likely to happen before much longer as museums, galleries, and exhibitions start taking advantage of a clever new type of loudspeaker. Instead of pumping air out randomly over a wide area, directional speakers can target sound like a stage spotlight to a precise place where only certain people can hear it. Directional loudspeakers have all kinds of uses, from high-tech megaphones on naval warships to billboards that catch your ear as well as your eye. Let's take a closer look at how they work!
Photo: Thanks to directional speakers, museum exhibits could soon be talking to you—and only you!.
What is a directional loudspeaker?
Photo: U.S. Army soldiers send a warning with a directional speaker system called the Long Range Acoustic Device® (LRAD®). Photo by Bobby L. Allen Jr. courtesy of U.S. Army.
A conventional loudspeaker is designed to spread sound over a fairly wide area: it has a paper or plastic cone that moves back and forth, pumping sound in a wide arc in front of it. The more energy you feed into a speaker (in the form of electric current), the more energy it can pump out as sound, the further the sound waves can travel, and the louder they seem to be. Giant speakers used at rock festivals produce so much energy that they can be heard over a huge area, whether you want to hear them or not.
Most of the time this is exactly how we want speakers to behave, but there are times when it would be helpful if they could work more selectively. Suppose you're the captain of a giant, fast-moving warship and you see a tiny fishing boat moored just up ahead and locked firmly in your path. If it doesn't respond to radio contact, you have a problem. You could use a megaphone to try to call out, but that's just a basic loudspeaker, really, and the sound it makes will probably not reach far enough. Wouldn't it be neat if you could send out a very focused "shout," in a tight beam of sound, that would travel all the way to the fishing boat to catch its attention, even from a huge distance away? This is essentially what a directional loudspeaker does: it's a kind of "sound flashlight" that can "shine" sound energy into a precise spot, even from some distance away.
How directional speakers work—in simple terms
You've seen ripples spreading out when you prod the surface of a still pond with your finger? That happens because the waves of energy you're putting into the water travel outward in all directions: the spreading, diverging pattern of waves is called diffraction. The further the waves travel, the bigger the area over which their energy spreads. Eventually, the waves have so little energy that they disappear completely. Exactly the same diffraction process happens with sound waves. Whether you shout with your voice or pump sound through a loudspeaker, the sound waves spread outward from the source and their energy is gradually dissipated.
Photo: A conventional (electromagnetic) speaker has a single, large, sound-producing cone.
Directional speakers work in an entirely different way from conventional loudspeakers. The biggest difference is that they don't produce ordinary, audible sound waves with a single, moving electromagnetic coil and cone. Instead, they generate ultrasound (high-frequency sound) waves that are too high pitched for our ears to hear using an array of electrical devices called piezoelectric transducers. These are simply crystals, such as quartz, that vibrate back and forth tens of thousands of times a second when you feed electric currents through them, producing very high frequencies of sound. Ultrasound is used because its higher-frequency waves have a correspondingly shorter wavelength and diffract (spread out) less as they travel, which means they stay together in a beam for longer than ordinary sound would. Also, having an array of many, small transducers makes sound diffract less than it would do from a single, large transducer (for reasons briefly explained in this article on directional sound).
Photo: A parametric, directional speaker has an array of many ultrasonic transducers. This is a closeup of a Sennheiser Audiobeam, which has 152 small transducers.
Effectively, then, the ultrasound travels out from a directional speaker in a narrowly focused column, like a flashlight beam. But when it hits something, it turns back into ordinary sound you can hear. So, in the case of our talking Mona Lisa, there's a concealed directional loudspeaker next to the picture. It fires out ultrasound that travels out from the front of the picture and gradually dissipates into the room. If (and only if) someone walks into the beam, the ultrasound waves collide, turn back into normal sound, and Mona Lisa's voice is magically heard.
One of the biggest advantages of directional loudspeakers is how far they can make sound travel. In theory, sound pumping out from a conventional speaker follows what's called the inverse-square law, so doubling the distance from the speaker reduces the intensity by much more than half. But a directional speaker sends its sound in a much more tightly focused column, with far less energy dissipation. In practice, that means it can travel something 20 times further than sound from a conventional speaker.
How directional speakers work—a more complex explanation
If that's as much as you want to know, fine. But the more technically minded among you will recognize this explanation as a bit of a gloss, to say the least. You might be wondering how the ultrasound turns back into audible sound: if you can't hear Mona Lisa when you stand to one side, why can you suddenly hear her if you walk through the sound beam?
The speaker array actually produces a modulated wave made of two separate ultrasound waves. One of them is a steady, reference tone of a constant 200,000 hertz (Hz) frequency (the carrier wave) and the other is the signal that fluctuates between 200,200 Hz and 220,000Hz (the modulating wave). Although they're combined, it's easiest to think of them as two separate waves traveling out in parallel straight lines through a column of air without overlapping. If they meet an obstruction (such as your curious head), they suddenly slow down and mix together so they interfere constructively (by adding together) and destructively (by subtracting from one another). By the principle of wave superposition, two ultrasound waves of those frequencies can subtract from one another to produce a third wave with a much lower frequency in the range 200–20,000 Hz—and that's in the frequency range that your ears hear. An electronic circuit attached to the piezoelectric transducers constantly alters the frequency of the two waves so they produce the correct lower, audible frequency when they collide and "demodulate." (It also encodes the signal in a unique way that ensures there's less noise and distortion when it separates out in the listener's ear.) The process by which the two ultrasound waves mix together is technically called parametric interaction, which is why speakers that work this way are sometimes called parametric loudspeakers.