Making a rickroll laser: A parametric speaker | Hacker News Reader

Making a rickroll laser: A parametric speaker

10maurycy10.github.io

190 pointspabs361 comments

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

The novelty fog fountains are typically a piezo micro mesh with tiny holes, which is similar but distinct from ultrasonic atomization

I think you can get them on amazon etc as 'ultrasonic foggers', they're different somehow?

Yeah there are two ways I know of to achieve atomization using ultrasonic transducers. 1. Blast water with ultrasonic waves such that the top of the waves break off into droplets. This is well studied in the literature, but you need a decent amount of power to make it work. 2. Metal mesh piezo atomization. This uses the planar vibrational modes of a piezo disc to rapidly shove liquid through tiny holes.

Shit so they're basically just mis-representing what they're doing in there? I know about these things because I was looking into people who were experimenting with trying to grow food with roots dangling in nutritious fog for a while

Aeroponics is exactly what got me interested in the technology as well. I haven’t actually looked at the link so no idea about what they are potentially misrepresenting, but I did find it quite difficult to find good info on the metal mesh atomizers.

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! :)

Fwiw lrad is regular speakers. Not ultrasound. Just tweeters.

Those things are so goddamn loud, I've seen them tested before a bunch. I used to see them for sale on ebay and amazon which was pretty funny. Not anymore. Still on aliexpress, as well as these sound laser directional speakers albeit the LRAD will run you at least 5 grand and you can probably make the directional speaker cheaper as in this DIY project

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.

Speaker directionality gets interesting once you get to arrangements for festivals and concerts. Ideally the audience should all be able to hear well, which is difficult to achieve over such a large area and with multiple sources.

But you also don't want the neighbours to complain about the thumping bass. A number of subwoofers pointing backwards, with the right phase delay, destructively interferes with the most offensive frequencies and reduces the sound intensity behind the stage. It's called a 'cardiod' arrangement, because of the shape of the resulting sound intensity distribution.

Yes. And then you have the vexing problem of the stage sound where you want the musicians to hear themselves and each other. Without feeding back into their microphones, and not too loud please.

Modern in-ear monitoring has solved some of this, but it’s still tricky (and expensive).

Larger venus also have a problem with delay, since longer runs are needed. I can’t remember how they deal with that though.

> Sometimes musicians use dispersing devices mounted on the speakers, like "beam blockers".

Beam blockers seem to be the most common thing employed, though to my knowledge a much more effective measure is a "Mitchell donut". Basically, a soft foam disc with a soup can diameter hole cut in the center. The disc is sized to completely obstruct the speaker when mounted in front of it on the grill(cloth). With properly chosen foam of the right thickness, frequencies above about 1khz will be attenuated except when traveling through the center hole, meaning they'll behave more like they're coming from a point source and the constructive interference that causes the beam will be much reduced. Equalization may need adjusting since some highs are lost, and because if the player adjusted initially for on-axis sound they are now hearing the off-axis sound.

From a stroll through the audio cyclopedia, I think "mitchell donuts" are more properly called "acoustic lenses". A key difference between them and most beam blockers I see is that the blockers assume high frequencies come from the center of the speaker (cause that's where the beam is, and when close micing that's usually where you get the most treble), whereas donuts/lenses assume the entire speaker emits the highs, and that it is their constructive interference that causes both the beam and the trebliness of close micing a speaker on axis. I have not fully verified all the physics yet, but so far my understanding is that this later explanation is correct, at least in the context of guitar frequency ranges.

The consequence of that is that beam blockers usually/likely add a comb filtering effect from what I've read, and will still develop some beaminess in the far field despite removing it in the near field. The donuts/lenses should be effective in both the near and far fields. Anecdotally, my experience confirms the donut behavior. A previously painful-on-axis 4x12 was almost completely evened out by the addition of donuts.

If anyone has good pointers for honest to goodness physics books on speaker drivers and speaker cabinets, it would be much appreciated.

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

This reminds me of when I played around with laser diodes as a kid (of course to burn random things).

I ordered a powerful green laser diode from eBay, wired it up and pointed it at some black paper, hit record on the camera, excitedly connected the battery and... nothing. I checked the wiring, all was good. I looked in the end of the diode, I could see a faint red glow inside, but nothing else. I must've got a dud unit.

Later I looked at the recording and my heart sank. When past me connected the battery, the room immediately lit up with a bright white glow. The diode emmitted an intense beam of infrared light... and I pointed that thing directly at my eye.

There wasn't a wider point to that, this just reminded me and I wanted to share. I suppose be careful of what you can't see.

I got lucky. That sort of thing can cause big problems. Especially those that stay unnoticed until old age. Hearing is also one of those.

I guess you should be fine. My face gets blasted with point based IR day in and out by FaceID and Windows Hello and I can still see fine lol.

Can FaceID or Windows Hello burn things? No? Maybe they're not high-intensity lasers.

IR lasers are dangerous. (Well, lasers in general.) Please don't downplay their risks.

Can the laser in OP’s laser burn things? No? Point of my comment is to console OP’s concern with long term damage to his eyes. Some Windows Hello’s IR emitter is powerful enough I can see bright red blinking.

I have a friend who built one of these as their thesis. IIRC they were telling me that the air through which the sound beam propagates acts as a low pass filter, so if you're at the correct distance from the device the high frequency energy should have dissipated.

Interesting stuff, I wish I had more time to learn about what they where doing.

I was thinking about how the air itself must be contributing to the construction of the (standing?) sound wave as a resonator.

Not sure if there are standing waves involved, or resonance. I presume it is very similar to a phased array [1] for beamforming in antennas, except that then anisotropic properties of medium may not be negligible to construct the wavefront (temperature gradients & wind), which is probably also why these devices do not sound great. To produce a high quality waveform at the receiving end the physics probably becomes quite involved rather quickly.

[1]: https://www.analog.com/en/resources/analog-dialogue/articles...

I'm not an expert on this, but there don't seem to be any reported cases of hearing loss from sounds above 30 kHz, but there are documented cases of unpleasant effects. In any case, I'd keep some distance, just to be safe.

No, we don’t hear ultrasonic frequencies because our ears do not resonate at those frequencies.

We hear sounds when the cilia in the cochlea resonate with the incoming sound. We don’t have cilia of the length required to resonate with ultrasonic sounds, so there’s no danger of hearing loss.

Animals may get their hearing damaged, if they are in the path of the sound, are close enough that it’s still ultrasonic at their location, and are sensitive to the frequency used, I believe. Maybe someone who knows for sure can say for sure.

"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?

That sounds like the thing where you turn your bass speaker towards the wall (ideally a corner) and the bass gets louder/better. A friend showed me this in high school; never figured out how this could possibly work.

some bass bins are in a folded configuration so that the speakers are not firing directly out of the front of the cabinet but from an angled position that hits the back before exiting the front of the cabinet.

With low frequencies, directionality is negligible, so it's not about turning it "toward" the wall, but about placing it against the wall. I suppose it's possible that the sound radiates more from one side than the other (possibly the side with the cone, or maybe the side with the port if one exists!) so turning it can result in effectively moving it closer. Or maybe you were listening to a more directional frequency range than I'm thinking of. Anyway, why does closer matter?

If you suspend a subwoofer up in the air so it has no boundaries, its sound radiates in all directions (full space). If you put it on the floor (let's assume all such boundaries are infinitely dense and thick, for simplicity) its sound now radiates only upwards/outwards (half space). Now push it up against a wall: quarter space. And finally, put it in the corner of the room: eighth space. Of course for a few millimeters it goes toward the boundary but then is reflected back, and so long as the distance isn't so significant relative to the wavelength that destructive interference (cancellation) occurs within the audible range, all interference is constructive (additive). The SPL in the listening area increases by 3 dB for each of these boundaries/halvings, although in practice it's slightly less since typical boundary material is a little bit acoustically absorptive (sound converts to heat) and acoustically transparent (sound is transmitted through to the other side), but even complete absorption isn't any worse than complete transmission to a place with no listener (i.e. the absence of a boundary): the sound goes unused either way.

Fun fact: typically half-space is used when citing the efficiency of a speaker in terms of dBSPL/Watt, since the vast majority of the power is for bass, and it's considered rare to emit bass against fewer than one boundary (the floor).

The next time you are deciding where to position your bluetooth speaker, if it's lacking in bass, boundary-load it.

But this is all about turning non-directional sound into directional sound (through boundary loading, which is a very close cousin to horn loading, as a sibling comment mentions with folded horn cabinet designs). TFA isn't about that though, it's about sound that's already directional! So I doubt any of what I've said is relevant to this mysterious effect.

> The next time you are deciding where to position your bluetooth speaker, if it's lacking in bass, boundary-load it.

There's only so much bass that a silly bluetooth speaker can provide. Even the silly Bose speaker where the sound travels through a maze before exiting the unit just can't make bass. Small speakers just can't move enough air nor have enough surface area to generate the waves for bass. Anything less than 12" is a joke. 15" is ideal for me as 18" tends to not be able to cycle fast enough for the music I listen. They do fine for longer sustained lows like in hiphop/rap. A running 16ths bassline at 135bpm sounds better in a cabinet of 15"s than 18"s. To me. No bluetooth unit has ever impressed me.

Couldn't agree more. But even as a bass head myself (living room flat to about 27 Hz) I find myself occasionally somewhere with a portable speaker, like when traveling... Someone puts it on a table and starts playing it, and is blown away when I move it to be against the wall. The -10dB point moves from, say, 200Hz down to 150Hz.

As for cone size tradeoffs, an interesting peculiarity is in live music: 10" is most common in bass rigs (assuming 4+ of them in a cab) while 12" is most common in guitar rigs. Go figure. Ultimately, the combination of driver specs and cabinet volume can be tuned for any desired response, within reason.

Bass speakers are not directional. Near a wall, air cannot displace, only the pressure can vary. When standing waves form in a room, the places near walls are nodes with respect to displacement. That makes them antinodes for pressure. A speaker next to a wall is in a pressure antinode for standing waves of multiple frequencies, an ideal position for driving those waves.

Corners maximize this effect. A speaker in a corner where two walls and the floor meet is in an ideal position to drive standing waves along three axes.

I suspect, turning the driver toward the wall helps it get closer to the wall, and to create pressure in that space.

I suspect it's because the surface itself acts as a speaker.

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.

Surely waterproof headphones would be far easier and sound far better

You correct. But I’m clumsy and interrupt-driven. I would somehow forget about them, drop them on the floor, then trip on them and break a hip.

OK I bought some water resistant bone conduction earphones

They work perfectly and hook to my ears so they can’t fall off!

Update: you are correct, the water-resistant bone conduction set works like a charm. And they won’t fall off. I’d say your solution is conservatively .5% what I would have paid for the less satisfying parametric array.

They make a "speaker" for loud environments like trade shows or museums that are parabolic dishes. There's a speaker at the node and the dish reflects all of the sound back down into a collimated "beam". The ones I've specifically used were meant to be hung overhead so you only hear it when you step underneath. They were clear acrylic bowls that were 24"-30" diameter. It was mainly meant for delivering dialog/narration to a video playing without annoying the people working the booth. It was effective for the purpose

"Bad sound but the directionality works well."

When this tech was new, there was hope that they could be used to produce good bass sounds. If you could produce good bass from such small speakers, we could probably handle the engineering behind de-directionalizing the sound, e.g., "bounce it off things".

Unfortunately, if it is possible, nobody seems to have figured out how to do it. The frequency response curve on these things are bad. Very bad. This has relegated them to small niches as a result.

I wouldn't guarantee that they would work with your podcasts; if any of them come from someone with a deep voice you could well lose the primaries and even some of the harmonics of the voice almost entirely. Trying to make out the resulting words over a shower is probably awful because the only frequencies these can play are going to cross awfully badly with the splattering of the water.

You’re killing me here

> Unfortunately, if it is possible, nobody seems to have figured out how to do it.

I'm not sure if you're referring to directional bass or to small speakers making decent bass. The former is fortunately a solved problem today - cardiod speakers (and their variants) do so, albeit DSP may be required/help.

For the latter, the issue is partly because you need to move double the air volume for every halvening of an ocatve. So going from 120hz to 30hz requires 4 times more moved air for same dBspl... but because of human ear (insensitivity), we need way more loudness at lower frequencies to sound "as loud" (Fletcher Munson curve or dbA weighting). Consequently it's kinda hard to get loud enough bass from small drivers (I can explain in more detail if you'd like). There's some interesting tech like Mayht speakers and Resonado but in its current form there are limitations.

I was referring specifically to this ultrasound technology producing good bass, not the general problem.

It's still kind of the same issue. At the end of the day you need to move a lot of air for the bass. In theory some kind of air multiplier might allow these small-excursion ultrasonic speakers to do that, but unless there's a very good reason (or DARPA $$$) it's just more practical to do it conventionally.

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)?

As far as I understand yes, I would also guess that it is analogous to a phased array antenna, but with sound waves

It's a sound laser, or maybe I'm remembering the video in this link wrong on the terminology and the SASER is different? Maybe it's just "sound from ultrasound"

https://en.wikipedia.org/wiki/Phonon

https://en.wikipedia.org/wiki/Sound_amplification_by_stimula...

https://www.youtube.com/watch?v=aBdVfUnS-pM

https://en.wikipedia.org/wiki/Sound_from_ultrasound

This is not a sound laser; the beam has some similar properties to a laser (e.g. collimation), but it works more like a directional antenna.

Yeah I think you're correct. Really interesting distinction

Its quite interesting. The directional ultrasonic array broadcasts a carrier frequency, not unlike am radio. You can read about the function of this sort of device at hackaday.

https://hackaday.com/2019/02/14/creating-coherent-sound-beam...

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

The security state decrees that this is not a good idea https://en.wikipedia.org/wiki/2007_Boston_Mooninite_panic

I want a compact version I can point out a car window at the glass of another car and turn it into a voice speaker.

You can surely do this and more! The device presents as a panel, however.

You need to use an array of transducers to form a coherent sound beam, but it will possibly sound through the window and gain the attention of an occupant, or enter the vehicle and seem to come from inside the car its self.

You can also just point them at someone.

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.

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