Why isn't there an audio equivalent of the laser?

Cecil's Columns/Staff Reports
1

http://www.straightdope.com/classics/a2_114.html

Before answering this question, we must first state what a laser is. A laser is an amplified beam of coherent light. Ok, now what’s coherent light? It’s light of a uniform frequency and polarity. Polarity is the orientation of the light wave. Think of a physical (sine) wave moving through space, or draw a wave on a piece of cardboard and move it through space. Notice that we can orient the wave (or cardboard) at any angle along its direction of motion (spin it like a spiralling football): vertically, horizontally, etc. This is the polarity. Coherent light is when all the light waves, which have the same frequency, are oriented at the same angle.

So, how does this apply to sound waves? With electronic technology, we can already amplify sound waves of uniform frequencies (tuning fork, voice, synthesizer, etc.). What about the polarity? Unlike light waves (which are called transverse waves) sound waves don’t have a polarity. Light waves can be thought of as disturbances in the electro-magnetic field and the oscillations of the constituent parts are perpendicular (transverse) to the direction of the wave. Sound waves are the disturbances of the densities of air molecules and thus the oscillations of the constituent parts are longitudinal to the direction of the wave (hence longitudinal waves).

So in effect, it’s even easier to create an audio “laser” because we need not worry about the non-existent polarity. As a matter of fact, a singer shattering a wine glass is quite similar to a laser cooking a piece of food.

2

Note that while some lasers produce polarized light, polarization is not otherwise an intrinisic quality of lasers. The phrase that you really needed to use instead is “uniform frequency and phase.” If you want to learn the basics about lasers, try this Wikipedia article.

3

Oops, silly me, forgot to explicitly mention phase. :smack:
Thanks for the peer-review.

And you’re right, lasers don’t have to be polarized, but it helps. :wink:

4

While there is not any true example of a “sound laser,” I think there is something close. This device uses ultrasound, which does not spread out near as much as regular sound as it travels through the air. By manipulating the ultrasound, interference patterns can be made that create audible sound to anyone in the “beam” of the ultrasound.
Here’s a link to learn more http://www.holosonics.com/technology.html

While this may use the same principals to produce the beam of sound as lasers use to produce a beam of light, I think the final product is close enough to a audio equivalent to the laser to be mentioned.

5

It’s got nothing in common with Cecil’s musings on the subject back in '87, but some physicists have discussed a phonon laser or SASER. I’ve no idea how far any of this has actually got or even whether it’s taken seriously.

6

I pointed this out around last year. I think that for practical purposes, “Audio Spotlight”-type technologies are close enough to what people would imagine an “audio laser” to function.

The thread, though, expectedly sank like lead.

7

Cecil’s original article was a bit muddled. Let’s see if we can clarify the issue here.

Lasers have many attributes. Let’s compare these to sound.

(1) Directionality-- perhaps the most notable feature of laser beams is how directional they are. You can aim your laser pointer at a stop sign a block away and the spot is still mighty tiny. It’s this way for two main reasons: (1) The atoms in the laser are operating coherently, cooperating, emitting light in lock-step with each other, and (2) Light waves are transverse waves, which can travel without any dispersion away from their direction of travel. So we can both generate and propagate laser light in tight beams.

(1s) Now for sound we can easily do #1, emit sound that is coherent-- any loudspeaker gets the air molecules to move in unison. The glitch is #2-- sound waves are waves of compression-- the movement of the particles is in the direction of motion… and due to the geometry of the situation, these kinds of waves tend to disperse. So sound waves are not going to stay focused in a small beam.

(2) Monochromaticity-- a fancy way of saying “one color”. All the atoms are in lock-step, so they all have to be vibrating at the same rate, that ensures they all stick to the same color.

(2s) For sound this is no big deal, it’s easy to generate a pure tone, use a piano key, or tuning fork, or anything that moves in a sine wave fashion.

(3) Polarization-- Most lasers are designed yo emit light of a particular polarization. But this isnt a requirement for lasing, it is possible to get a laser tube to emit randomly polarized light.

(3s) Sound waves, being pressure waves, due to the basic geometry, don’t even have the possibility of being polarized, the concept doesnt apply.

Now that holosound business, that’s got NOTHING to do with lasers. It’s just an ingeneous way of getting air to act in a non-linear fashion in a certain spot. It’s also possibly hazardous due to the extremely high unltrasonic sound levels required.