Making a rickroll laser: A parametric speaker

What's really interesting about these piezoelectric ultrasound emitters (Not far off from what you see in novelty fog fountains with the little waterfall and maybe some elves and mushrooms), in different arrangements, are actually capable of something called acoustic levitation of small objects - and what's more this is actually something you can DIY as well

https://www.youtube.com/watch?v=yVDWrWpaBho

https://www.instructables.com/Acoustic-Levitator/

https://pubs.aip.org/aip/rsi/article/88/8/085105/962938/Tiny...

https://www.youtube.com/results?search_query=acoustic+levita...

There's some interesting applications combining it with projection here (Acoustic holography):

https://www.youtube.com/watch?v=Q9GybXczNAc

Pretty cool! I’ve seen similar implementation in the Elektor 2011 issue magazine. The US navy uses the commercial brand of these speakers; LRAD (long range acoustic device) to ward off pirates at seas. LRAD is the company that makes them for defense purposes and they patented the name and acronym. But parametric speakers are used everywhere even museums and apparently for trolling Rick roll too! :)

I worked on this long ago, the main problem with the poor sound quality is that the self-mixing process is non-linear, a good approximation is squaring the output signal.

To improve the quality you can pre-distort the output signal. Taking the square root works quite well, but expands the bandwidth significantly (infinitely, in theory). There is a lot of literature on pre-distortion with bandwidth constraints for telecom power amplifier linearisation. You will also need a linear amplifier to power the array.

The ultrasonic transducers used in this post are very narrowband, having a resonance peak of merely a few 100Hz. You can reduce the Q factor with resistive loading but the output power significantly drops. It seemed these transducers quickly start making an audible whining noise when used for continuous transmission at higher powers. I don't know what caused that, apart from this effect they seemed to hold up for essentially infinite duration.

Using a larger wideband ultrasonic transducer instead of an array of small narrowband transducers again increases the sound quality a lot. We did not find a commercial supplier of such transducer for a reasonable cost, but made some improvised custom electrostatic ones with conductive foil. There is a lot of literature on how to construct ultrasonic transducers but this is not my field.

You will not be able to play bass notes due to physics, the power required would be insane.

Directionality is a normally an almost completely unwanted quality in speakers. You want speakers to be heard equally well from all directions (that are frontward of the speaker's baffle). That's one reason why speaker boxes use multiple speakers, and these get smaller for the higher frequency ranges. "Dome tweeters" are shaped as such in order to radiate all around.

A "full range" speaker will send the lows in all directions but the highs mainly in the direction of its axis. A listener caught in the beam will hear a shrill sound, whereas someone off axis hears it muffled. Guitar speakers are like this; particularly the 12" ones and particularly in the 4x12 cabinet arrangement. Sometimes musicians use dispersing devices mounted on the speakers, like "beam blockers". Or the speaker is picked up by a microphone close to it, so that the audience hears it fro the PA system (which solves the sound dispersion problems in its own way).

There are situations in which it is desirable for a speaker box to "beam", like when it is mounted far away or high above a target listening area that is relatively small, calling for the speaker to be a kind of spotlight.

Is this dangerous in terms of hearing damage due to the perceived low sound level alongside inaudible louder ultrasound?

"The beam also bounces off objects, making it seam like the sound is coming from somewhere else. Strangely, the sound is actually louder when bouncing off a hard object like a wall then when listening to it directly. I’m guessing that the surface creates areas of higher ultrasound intensity, creating more sound then would be created otherwise."

Would this just be due to the fact that the reflecting surface isn't perfectly smooth so the reflections do not reflect back 180° and pretty much scatter up reflection?

Quite a few years ago as a teenager, who knew even less about the physics of sound than I do now, assumed that it wouldn't be long before technology was developed that lets a single device (say a consumer TV) output two different audio tracks aimed at two different people, and even track the people moving around the room to keep their audio being directed at their head.

Visually the technology for two people to see different video on the same TV has existed for a while, there's just no demand or market for it (either glasses syncing with TV to block certain frames, or there was a technique that depended on the angle you're viewing from).

And for tracking people walking around the room, to then know where to point audio or video at, there's released & integrate-able technology available like head and eye tracking from Tobii.

Is there also some very expensive option for having audio split between people in the same room (without using any devices like earphones) and just equally no general demand from consumers wanting to use or pay for it, or do the laws of physics prevent sound waves from working well this way?

The Country Music Hall of Fame in Nashville has these. Bad sound but the directionality works well. Want one of these in the bathroom so I can take a long shower and listen to podcasts without waking up my lovely wife.

So how does this work? Is it the same as the kind of phase cancellation that you often see with two speakers playing the same tone, but just with lots of elements (and an ultrasonic source)?

And just like a phased array antenna, by properly spreading the emission of sounds you can change where the speaker is emitting.

With proper mixing you could emit different sounds in different directions, at the same time.

Should put these random places in the streets and have them play at random times

so, basically, beamforming but for sound ?

Awesome - very cool project!

I would like to make the unsubstantiated assertion that unscrupulous police and occasionally private entities use this existing technology (available for organizations such as libraries, venues, corporate) off-label on occasion as an “investigative tool”, which is to say maliciously.

Lasers? Ah, one of the early lasers was He-Ne gas in a narrow tube, a radio transmitter antenna wrapped around the tube (or some such), and mirrors at both ends of the tube. So, photons would go back and forth between the mirrors, a tiny fraction would leak out of a mirror as the laser light, and the rest would stay in tube and help generate more photons from the radio and He-Ne interactions (laser, light amplification via stimulated emission of radiation).

So, right, just thinking from the OP, between mirrors there was some highly favorable line of amplification, and that line meant that the beam out of the laser would be an extension of that line and form a "narrow" beam!!!

Right, if use some voltage on some piezoelectric crystal to make tiny adjustments in the distance between the mirrors, then will make small changes in the frequency of the light, i.e., there is a highly favorable wavelength that fits a whole number of times between the mirrors or some such.

The changes in frequency of the light still have to correspond to the thermally moving gas atoms generating the light. Right, if have the favorable frequency in the middle of the feasable range, will get slightly less power in the beam, a dip, called the Lamb dip. Could that dip be used as a length standard? First job, worked on that, physicist, NIST, then the NBS, US National Bureau of Standards.

That is, at the end of the laser we have a tiny light source that puts out a very narrow beam. How? As above and not from antenna theory.