Here’s a shower thought: if light and sound both travel as waves, can you make something that works similarly to a laser but uses sound instead of light?
It turns out that the answer is “yes”! If you want to see one in action, the YouTube channel Electron Impressions has you covered. The channel’s latest video demonstrates a gun that shoots a narrow beam of soundwaves, along with its most striking property: you can only hear the sound if you’re within the beam’s width. The YouTuber demonstrates this by pointing the device at his camera; when it’s pointing directly at the lens, the music it’s transmitting is clearly audible, but the moment it’s moved away, the music stops.
So how does the device work? The defining characteristic of laser light is its coherency; lasers produce light of a single wavelength, and the crests of those waves are all aligned. This device works in a similar fashion, but instead of a narrow beam of single-colored light, it creates a narrow beam of sound waves. If these were audible soundwaves, you’d hear them as a single, very loud tone.
Thankfully, though, the device doesn’t use audible soundwaves. Instead, it operates at a frequency of kHz, which puts it well into the realms of ultrasound, frequencies that are way too high for humans to hear. This allows it to blast out waves at amplitudes that would be extremely loud if they fell into the audible range.
This is all well and good, but if what we have is a device that’s blasting out a single frequency that’s too high for us to hear anyway, where’s the music coming from?
The condensed answer is that the ultrasonic frequency acts as what’s called a carrier wave. By superimposing the waveform of the actual music onto the carrier wave, you get a combined signal for transmission; the waveform of the music can be extracted at the other end by subtracting the carrier wave’s waveform from that combined signal. (This is how radio transmissions work.)
But this raises another question: we can hear radio broadcasts because our radios take the carrier wave and convert it back into audible sound. But there’s no radio in use here, so… what’s decoding the signal? This is the really clever part, and while a detailed explanation of the answer is beyond the scope of this post, the short version is that the sound basically decodes itself.
Sound travels at different speeds through low- and high-pressure air, and the ultrasonic beam is powerful enough to cause small changes in the pressure of the air through which it’s traveling. This causes the wave to distort as it travels. This seems like it should be a problem, but instead, the effect is kind of magical: the ultrasonic frequency falls away. Or, as Electron Impressions puts it, “The air itself demodulates the signal, creating audible sound in mid-air.”
The result, he continues, “is like having an invisible speaker positioned right in front of your target, making it seem almost as if the sound is being generated inside their head.” It’s a neat party trick, and the video’s comment section is already rife with “creative” ideas for how such an effect could be used. A note of caution, though: the accompanying write-up contains the caveat that while “high power ultrasound (over 110dB) has been approved for use in directional sound applications by many health institutions … there remains some uncertainty as to whether this safety standard should be revisited.”
Read the full article here
