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!.
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.
Photo: Shining a sound spotlight. Army personnel practice riot control
techniques with an LRAD® speaker mounted on the back of a truck. Photo by David Overson
courtesy of US Army and
DVIDS.
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How directional speakers work—in simple terms
Photo: Energy ripples out in waves across the surface of water. The further the waves travel,
the more their energy dissipates, because it's spread further, over a widening area.
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 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 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 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 disperse less than it would do
from a single, large transducer (for reasons briefly
explained in this article on directional sound).
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.
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.
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.
Summary of how directional speakers work
The piezoelectric transducers (gray circles) in the directional speaker produce two ultrasonic waves (red and blue), both of which are at frequencies way too high to hear). The transducers pump out the waves in a focused column (like the light in a flashlight beam). The waves are actually modulated (like radio waves) and travel as one wave, but it's simplest to imagine them as two quite separate waves.
When the two waves hit something (or someone), they slow down and demodulate, producing a new wave (green) whose frequency is much lower—equal to the difference in frequencies between the two original waves. This is a wave you can hear.
When there's no-one standing in the beam, the waves keep on traveling without producing an audible sound wave—so if there's no-one standing in front of the speaker, there's nothing you can hear.
People standing outside the beam can't hear anything because (unlike with a conventional loudspeaker) the sound waves are not diverging from the source of sound to reach their ears.
What are directional speakers used for?
Photo: A Long Range Acoustic Device (LRAD®) is being used here to send a warning from the USS Blue Ridge to a small, incoming craft during an attack drill. Note how you have to swivel the LRAD® and point it in the direction you want the sound to go, just like a flashlight. Photo by Tucker M. Yates courtesy of US Navy and
Wikimedia Commons.
The possibilities are truly limitless. Imagine advertisements or
in-store displays that talk only to you as you walk past. Or hospital
televisions that beam their sound only to the patients in certain
beds, leaving the others undisturbed. What about megaphones that
police officers could use to address only one or two troublemakers in a
rioting crowd? Or speakers on a concert stage that performers could use to
send private messages to certain people in the audience! How about
hands-free speaker phones that only a few people, sitting nearby, could hear? Great
for noisy offices!
The U.S. military has been using directional speakers since 2004.
The system they use is called LRAD® (long-range acoustic device)
and consists of giant flat loudspeakers mounted on the side of ships
so they can send loud audio warnings to vessels at a potential range
of over 500m (a third of a mile). It's particularly useful on loud and noisy
aircraft carriers where any conventional loudspeaker
would be drowned out by the background noise from jet planes and
helicopters. Some police departments have
been using LRAD as a means of crowd control for over a decade, though the practice is
controversial. Following one protest in New York City in 2014, lawyers
complained that LRAD devices "are designed to perform crowd control and other functions—to modify behavior, and force compliance, by hurting people," according to
The New York Times. Protesters reached a settlement with the NYPD to limit use of LRADs in 2021.
Photo: Front and back views of two different LRAD® models. Left: You can get a sense of how big and bulky this LRAD is: it's being carried here by two men. Photo by Jordan Kirkjohnson courtesy of
US Navy.
Right: Here's the back of a lightweight LRAD 500X (Extreme) with the control panel and microphone unit just underneath.
This model is 64cm (24in) square and can beam sounds up to 2km (1.2 miles) away.
Photo by Amanda Dunford courtesy of
US Navy.
Similar principles have also been used in some types of hearing aids. Ultrasonic sound is used to ferry inaudible, high-fidelty sound directly into someone's ear canal—and, in this case, the demodulation happens inside their inner ear,
producing sound only they can hear.
Photo: The control panel and microphone around the back of an LRAD®1000Xi speaker, which can throw 153dB of sound at a distance of up to 3km (~1.9 miles).
Photo by Riley McDowell courtesy of US Navy and
DVIDS.
Who invented directional speakers?
The kind of directional-speaker technology we're talking about in this article was originally developed by
naval scientists who were using parametric arrays with sonar (underwater navigation) systems.
One of the first people to perfect the technology for use with audible sound was US inventor
Woody Norris;
his system, called HyperSonic Sound (HSS)®, is marketed by Genasys
(formerly called LRAD Corporation and American Technology Corporation, ATC), which makes the LRAD® and a number of related products. The Holosonics®
Audio Spotlight® uses broadly similar ultrasonic technology developed by former MIT student Dr Joseph Pompei.
Introduction to diffraction: This good little background introduction explains how sound waves travel through a medium and what happens when they encounter objects in their path. [Requires Flash.]
Sound Cannon by Emily Buiso. New York Times, December 13, 2009. Explores some of the interesting uses for LRAD, including scaring birds away from wind turbines.
Sound ideas by Jon Excell. The Engineer, June 2007. A good background introduction to directional sound technology, including a comparison of HyperSonic Sound (HSS)® and the Audio Spotlight®.
Cruise lines turn to sonic weapon by Adam Blenford. BBC News, November 8, 2005. Describes how cruise liners are using LRAD devices as a deterrent against pirates.
In the audio spotlight by David Schneider, Scientific American, October 1998, pp40–41. An early introduction to directional sound and the work of Woody Norris.
Books
Extremely Loud: Sound as a Weapon by Juliette Volcler, The New Press, 2015/2013. There's quite a bit of coverage of LRAD-type devices in this book.
Woody Norris: Inventing the next amazing thing: In this 15-minute TED talk from 2009, one of the pioneers of directional sound explains (sort of) how the invention works and gives a demonstration to a studio audience. He also describes his life as a self-taught inventor and some of the other inventions he's develoiped.
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