A speaker is typically used to broadcast audio far and wide. Whether it is amplifying the voice of someone giving a lecture, or the guitars of a rock band, speakers are meant to be heard. But what if you want to selectively deliver audio to one individual in a large group? Or what if you want to play loud music in the middle of the night without waking your neighbors?
Well, headphones would work for most cases. But what if you need to deliver the audio without the receiver having any sort of equipment? It’s a niche application, to be sure, but those who find themselves in this situation have few options. A particularly interesting option was recently demonstrated by Electron Impressions. He created what he calls a sound laser. Not only is this DIY device cool, but he also dives into the physics behind how it works, which is quite interesting.
The circuit (📷: Electron Impressions)
Under normal conditions, the face of a speaker is far smaller than the wavelength of the sound it produces. This causes sound waves to diffract around the edges of the speaker, leading it to spread out in every direction. To avoid this, most sound in the audible range would require a speaker face the size of a billboard.
So to transmit sound in a single direction, Electron Impressions worked with ultrasonic waves. The wavelengths are short enough that an 8 mm transducer is as large as a full wavelength. That prevents diffraction at the edges, but there is a pretty obvious problem with this approach: humans can’t hear ultrasonic sound.
The fix for this issue relied on a property of air called acoustic nonlinearity, which states that sound travels faster when air pressure is higher. This means that if the ultrasonic waves are transmitted with a sufficiently high amplitude, the waves distort their own shape. If the amplitude of these waves is modulated, this effect will cause the sound waves to demodulate themselves mid-air, producing audible sound.
A test of the device (📷: Electron Impressions)
With the approach worked out, Electron Impressions got to work on the device. This handheld sound laser uses a 555 timer to generate a 40 kHz carrier wave. A Bluetooth audio amplifier is connected to the timer’s trigger pin to modulate the signal. The output is fed into an h-bridge for amplification, before finally being sent to an array of 73 piezoelectric transducers that are wired up in parallel.
The spacing of these transducers has to be very precise to create a unified wavefront that generates a strong sound wave that can travel a large distance. This spacing also has the effect of creating destructive interference that cancels out any sound that is emitted outside of the target region.
The sound quality of the device is not great, but it most definitely serves its purpose of transmitting directional sound at a distance. Check out the video below to see it in action.
