A Very Short History of Ultrasonic Directional Speakers

A project log for Ultrasonic Directional Speaker v1

Producing highly directional sound from 40kHz ultrasonic transducers

Alan GreenAlan Green 01/06/2016 at 09:140 Comments

This log is a summary of what I've found about directional speakers.

Joe Pompei and The Audio Spotlight®

The modern directional speaker began with Joe Pompei at the MIT Media Lab. Large parts of his 2002 PhD thesis go straight over my head, however there is a lot of good information in there, including a summary of previous research into directional speakers (spoiler: the military did a lot of work with sonar), and a human-readable summary of the math.

Pompei combines two ideas to produce a directional speaker.

First, sound waves are directional if the width of the wavefront is much larger than the wavelength. Middle-C (262-ish Hz) has a wavelength of 1.32m, so if a speaker were, say 10m wide, you might be able to get a directional wavefront from it. Ultrasonics however have much smaller wavelength - about 8mm at 40kHz - so reasonably sized speakers will produce a directional wave front.

Second, ultrasonic sound waves will demodulate in air. The resulting audible sound is much quieter than the ultrasound that generates it.

Joe Pompei called his speaker the Audio Spotlight® and founded Holosonics to produce it. There are a number of YouTube videos showing off the speaker: this one with Joe Pompei causing trouble in the library is my favorite, although this other one has some more technical details.

Kazunori Miura's Ultrasonic Directive Speaker

In March 2011, Elektor Magazine published an article by Kazunori Miura describing a directional speaker built from ultrasonic transducers. It was the inspiration for this project.

Briefly, Miura discovered that using pulse width modulation over a 40kHz square wave carrier produced a better result than several other schemes. He also seems to have pioneered using those 16mm transducers from ultrasonic range finders. The article has lots of useful hints about construction too, including the need to determine the polarity of the transducers (more on this later).

Miura's resulting device looks a lot like a class D amplifier (explanatory video here). The incoming signal modulates a 40kHz wave, which is then pumped through a MOSFET H-bridge to drive an array of ultrasonic transducers. If one were to replace the array of transducers with a low-pass filter and an audio speaker, it would look a lot like the the circuit used in the class D amplifier Wikipedia article.

As far as I can tell the differences between Pompei's and Miura's devices are:

  1. Pompei uses amplitude modulation (AM) while Miura uses pulse width modulation (PWM). AM can use power more efficiently since the power required to produce the ultrasonic wave is (approximately) proportional to the power of the audible signal, while the PWM scheme uses full power all the time. AM also requires more sophisticated processing.
  2. Pompei uses an array of circles of piezo-electric film, about 40mm across (as far as I can tell) while Miura uses 16mm piezo electric transducers. The 16mm transducers are relatively common and simple to mount on a PCB interface with (ie. solder to the terminals). I suspect that using the film is fiddly.
  3. Pompei's device runs at 65kHz, while Miura's runs at 40kHz. 40kHz is audible to dogs, while 65kHz is not. Cats, however, can hear both. Neither frequency seems to have any effect on humans, even at quite high levels, but there hasn't been conclusive research either.

Kazunori Miura is selling kits from his website. They're quite good value considering that they include the transducers, which are a little difficult to get hold of.

A Sidebar about Elektor

If you're into hobbyist electronics you should probably subscribe to Elektor. Every issue has two or three neat ideas that get me thinking. Look in the Jaycar Catalog, page 63 for a discount code.

And Others

SoundLazer is likely a device that Hackaday readers have heard of. The first SoundLazer device was a successfully funded Kickstarter project, with all backers receiving access to schematics and parts lists, however these are not available to non-backers and the company is now focused on building and selling complete products.

The LRAD is a directional loud hailer device/crowd-control device/weapon sold to governments for use by the military and police. Here's inventor Woody Norris giving a demonstration of his newer devices in a TED talk, though it is extremely light on technical detail.

In Summary

If what you want is a working directional speaker, there are many good options for kits and pre-built devices. However, I'm going to build my own. It's more expensive, but I'm enjoying the process.