Kuro5hin.org: technology and culture, from the trenches
create account | help/FAQ | contact | links | search | IRC | site news
[ Everything | Diaries | Technology | Science | Culture | Politics | Media | News | Internet | Op-Ed | Fiction | Meta | MLP ]
We need your support: buy an ad | premium membership

[P]
Smoke and Laser Microphone

By imon2nd in Technology
Sat Sep 19, 2009 at 07:01:41 AM EST
Tags: technology, audio, microphone, laser, smoke, fog, diaphragm (all tags)

About 150 years ago, the first audio recorder, a phonautograph built by Scott de Martinville incorporated a stretched membrane with an attached pen to intercept sound waves and write them on paper.


The phonautograph was the first, but hardly the last audio device to use a diaphragm to capture sound pressure waves. You can see drawings of de Martinville's invention and listen to it here:

http://www.firstsounds.org/press/032708/images.php

Even today, a diaphragm or plate is integral to every kind of microphone. Because diaphragms interfere with sound, they are a necessary evil. At least they were until now.

In 2005, I designed a diaphragm-free audio transducer based on a laser, smoke and photocell. The idea occurred to me shortly after having dinner in a fancy restaurant with my wife. On the table between us was a little oil lamp. A thin column of white smoke rose from it, passing between us. Every time either of us spoke, the smoke wavered. Of course, most of the movement was due to exhaled breath. But I figured that somewhere in that stream, sound waves had made microscopic disturbances.

As a result of much research, bench-top experimenting, and legal business, U.S. Patent 7580533 issued a few weeks ago. It can be downloaded for free from www.uspto.gov.  Here's the link:

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&am p;u=/netahtml/PTO/search-bool.html&r=1&f=G&l=50&co1=AND&d=PT XT&s1=7580533.PN.&OS=PN/7580533&RS=PN/7580533

Note that the patent contains further links to prior art, which show many attempts to directly detect sound in air.

Both the first not-completely-ugly proof of concept and the first prototype of the smoke and laser mic are demonstrated in videos on YouTube. Here's a link to one of them:

http://www.youtube.com/watch?v=V2K5LJHTouQ

I know, it sounds like crap. The next prototype will be better.

This is the first somewhat public writing about it. I'm hoping some of the people who read about the mic here will be inspired to study the patent, look at the videos, and build their own version to study the concept. There is much I don't know about how to optimize this device. Any insights will be appreciated.

As most of you know, modern microphones are very good. About the only improvements anyone spends time on are to make them smaller and cheaper. However, no matter how good they are, they contain a distortion producing mechanism: the diaphragm or plate. Because that element has mass, it has intertia, which inhibits instant response. The diaphragm also must be supported, which restricts its motion. It resonates, adding its own signal to the sound. Ideally, sound would be detected directly from the air.

Over the past 30 years, more than a dozen engineers have tried to directly detect sound pressure waves in air. The most notable attempts are shown in patents issued to Bell Labs and Brookhaven National Labs. They are referenced in my patent, above. Neither of those designs were ever publicly demonstrated or commercialized.

The key to "seeing" sound pressure waves in air is to introduce small particles or droplets into the air. That smoke or fog moves with the air. A laser passing through the mix will be disturbed by the variations caused by sound. Those variations can be detected by a fast photocell and converted to an electrical signal.

Because a small stream of smoke or fog has little mass and can be constantly replenished, it is easily modulated by sound and does not have any appreciable reaction force. In theory, the dynamic range of this kind of mic could exceed 130 dB and its response to rapidly changing sounds should be nearly instantaneous. Of course, it may be another year before we get close to those goals.

Sponsors

Voxel dot net
o Managed Hosting
o VoxCAST Content Delivery
o Raw Infrastructure

Login

Related Links
o http://www .firstsounds.org/press/032708/images.php
o http://pat ft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&am p;u=/netahtml/PTO/search-bool.html&r=1&f=G&l=50&co1=AND&d=PT XT&s1=7580533.PN.&OS=PN/7580533&RS=PN/7580533
o http://www .youtube.com/watch?v=V2K5LJHTouQ
o Also by imon2nd


Display: Sort:
Smoke and Laser Microphone | 43 comments (34 topical, 9 editorial, 0 hidden)
DIE NULLO FAGGIT (1.14 / 21) (#2)
by I Did It All For The Horse Cock on Thu Sep 17, 2009 at 06:18:12 PM EST

DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT
DIE NULLO FAGGIT



\\\
  \ \        ^.^._______  This comment brought to you by the penis-nosed fox!
    \\______/_________|_)
    / /    \ \
    \_\_    \ \

How about animation? (none / 1) (#4)
by imon2nd on Thu Sep 17, 2009 at 06:39:13 PM EST

Hey, I-did-it;

I'd like to see your fox in the hen house.

Have a nice day!

Dave


[ Parent ]

Smoke has mass. (2.50 / 2) (#7)
by Pentashagon on Thu Sep 17, 2009 at 07:40:53 PM EST

So does air.  So does water in the air.  It all distorts the sound to some extent.

If you can't reduce the S/N ratio below what existing microphones can achieve, at least you'll be able to eavesdrop on smokers miles away.

Blah (none / 1) (#9)
by Vampire Zombie Abu Musab al Zarqawi on Thu Sep 17, 2009 at 08:12:07 PM EST

Air doesn't distort sound any more than a brick distorts its own shape. Without air, there would be no sound.

[ Parent ]
You've obviously never been in the wind (none / 1) (#10)
by Pentashagon on Thu Sep 17, 2009 at 08:21:15 PM EST

or listened to someone through a column of heat from a camp fire.  Small particulates, unless they seriously affect the density/mass of the air, aren't going to be much of an issue.  Density changes are going to bend sound waves and mess with the pitch, and turbulence will just drown them in white noise.

[ Parent ]
Yes yes (none / 1) (#11)
by Vampire Zombie Abu Musab al Zarqawi on Thu Sep 17, 2009 at 08:25:59 PM EST

But that's still the actual sound, not a reproduction.

[ Parent ]
Much as thermoclines in water screw up sonar. $ (none / 1) (#19)
by Nimey on Thu Sep 17, 2009 at 10:51:32 PM EST


--
Never mind, it was just the dog cumming -- jandev
You Sir, are an Ignorant Motherfucker. -- Crawford
I am arguably too manic to do that. -- Crawford
I already fuck my mother -- trane
Nimey is right -- Blastard
i am in complete agreement with Nimey -- i am a pretty big deal

[ Parent ]
One can make a sound lens from a large balloon (none / 0) (#21)
by MichaelCrawford on Fri Sep 18, 2009 at 02:16:41 AM EST

The Exploratorium in San Francisco had one of these - it was quite cool. Nothing more than a big balloon filled with I think carbon dioxide, it had the effect of focussing sound so that you could clearly hear someone whispering from quite some distance away.


--

Live your fucking life. Sue someone on the Internet. Write a fucking music player. Like the great man Michael David Crawford has shown us all: Hard work, a strong will to stalk, and a few fries short of a happy meal goes a long way. -- bride of spidy


[ Parent ]

Or explode a stick of dynamite (none / 0) (#26)
by Pentashagon on Fri Sep 18, 2009 at 01:52:34 PM EST

and turn them into a sack of jelly, I hope.

[ Parent ]
Distortion Chain (3.00 / 2) (#29)
by localman on Fri Sep 18, 2009 at 05:50:38 PM EST

Yes, all the things you mention have mass.  But they all affect a normal microphone as well.  So if the smoke has less mass and resistance than the diaphram, all other things being equal, it is superior in theory.

That said, though this is cool on a geek level, there aren't that many practical applications.  For nearly all purposes, current microphones are more than good enough, far outperforming human perception and doing so at a good price.

The only things I could think of that would significantly benefit from more sensitivity are military/intelligence or scientific applications.

In scientific cases, you're almost always going to be measuring some thing other than air, so you can just laser that directly (and they do).  But maybe there are some cases where it would give new data?

For the military/intelligence community, eavesdropping that requires smoke placement limits the use, but there are possibly some cases where there is enough ambient smoke to be used?

Anyway, interesting.

[ Parent ]

Also: audiophiles (3.00 / 3) (#32)
by some nerd on Fri Sep 18, 2009 at 07:52:28 PM EST

they'll buy anything that supposedly improves fidelity, even if it doesn't. I'm sure there's some way this could be marketed to them.

--
Home Sweet Home

[ Parent ]
Expensive speaker -> smoke microphone -> (3.00 / 4) (#36)
by Pentashagon on Mon Sep 21, 2009 at 07:50:52 PM EST

another expensive speaker.

"Adds ambiance."

$13942930678348906138902534905903248906.99

[ Parent ]

reversible transducers (none / 1) (#43)
by iggymanz on Mon Dec 28, 2009 at 11:12:48 PM EST

speakers can be used as microphones, and sound can emerge from microphones.  I propose a smoke speaker to complement the smoke microphone for the discerning audiophile.

[ Parent ]
-1 on general principle (2.50 / 4) (#8)
by N0574 on Thu Sep 17, 2009 at 07:52:40 PM EST

null0 queue abuse must end.

- NCCTG N0574 CANCER PROTOCOL
So there's more room for (none / 1) (#14)
by rhdntd on Thu Sep 17, 2009 at 09:22:27 PM EST

Vampire Zombie Abu Musab al Zarqawi?

I -1'd the other submission, and this one would have been still better for a trip through the edit queue, but WTF, why not a technology article?

-- 
"book chicks really seem to like anal"
  — Lady 3Jane
[ Parent ]

too many (3.00 / 2) (#16)
by N0574 on Thu Sep 17, 2009 at 09:40:15 PM EST

unembedded links, didn't go through editing, etc.

- NCCTG N0574 CANCER PROTOCOL
[ Parent ]
+1 FP on general principle (none / 1) (#30)
by localman on Fri Sep 18, 2009 at 05:53:33 PM EST

k5 is dying

[ Parent ]
So you have almsot massless sound detection (none / 1) (#12)
by localroger on Thu Sep 17, 2009 at 08:51:43 PM EST

...and it still sounds like crap? What?

And that is what is so great about the internet. It enables pompous blowhards to connect with other pompous blowhards in a vast circle jerk of pomposity. -- Bill Maher
Talking dog! (none / 1) (#20)
by imon2nd on Fri Sep 18, 2009 at 12:13:40 AM EST

Hello, Roger.

Given the amount of development that has gone into the device so far, I'm happy that it works at all. The old saying about the talking dog applies.

Cheers,

Dave

[ Parent ]

He is critical but has a point re measurements (none / 1) (#24)
by Blarney on Fri Sep 18, 2009 at 10:41:02 AM EST

Have you tried playing sinewave tones into the thing? Taken a look at the THD and individual harmonics (odd, even) versus frequency & intensity? If so, I think those go in the article. We're not as specialized as AES but we sure can be nerds around here.

Don't feel bad if localroger is slamming your work. Last week people were making hellacious fun of his. K5 is like that.

[ Parent ]

Criticism is a good thing (none / 1) (#25)
by imon2nd on Fri Sep 18, 2009 at 10:55:55 AM EST

Hello, Blarney.

Roger's point was well taken.

Yes, I do own a spectrum analyzer, a 'scope and a waveform generator. The numbers for Prototype One suck, as expected. Until we get a handle on the 26 variables we're evaluating, we won't know how to make improvements efficiently, so that's the priority.

There's a small probability that Proto Two will get us in the ballpark, where talking about the numbers will be meaningful. I doubt it, but we'll see.

Best Regards,

Dave

[ Parent ]

fuck it front page (2.60 / 5) (#18)
by bride of spidy on Thu Sep 17, 2009 at 10:20:55 PM EST

vote +1 fp to everything. who cares man. it's like the titanic it's all coming down anyway!


regarding diaphragm microphones (none / 0) (#27)
by JackStraw on Fri Sep 18, 2009 at 03:59:24 PM EST

Can't the frequency response (i.e., the damped response at high frequencies) just be adjusted for by dividing by using a well-tuned equalizer?

And--if the diaphragm's resonances are well above the frequencies that we can hear, do we care about them?

Cool idea, though! Are you using doppler on the smoke particles, or measuring the diffraction they cause?

Is it possible to use a stable liquid emulsion, instead of smoke in air? I imagine the liquid would introduce more distortion than a good diaphragm, even if you tried to compensate for it...
-The bus came by, I got on... that's when it all began.

Detection mechanism (none / 0) (#28)
by imon2nd on Fri Sep 18, 2009 at 04:20:52 PM EST

Hello, Jack.

I'm reasonably certain the principal modulation mechanism is forward scattering, though variable opacity may also be at work. I'm designing some experiments to figure that out.

The frequency response should be extreme, given the speed and bandwidth of the photocell. Low-pass filtering in either analog or digital domains will be required since the high frequencies alias into the audio. An equalizer is a set of filters, so you're correct about the need for it.

Liquid won't work well since it has too much mass. One advantage of smoke or fog is that we can pump it past the detector very rapidly, creating a "fresh" slate to write on, so to speak.

Cheers,

Dave


[ Parent ]

Not sure diaphragms are an inherent problem, (none / 0) (#31)
by levesque on Fri Sep 18, 2009 at 06:02:14 PM EST

as our ears depend on them, and they shape sound in a way our brains expect.

One diaphragm is better than two (none / 0) (#33)
by imon2nd on Sat Sep 19, 2009 at 02:18:14 PM EST

Hello, Levesque.

One diaphragm in the sound to brain path is enough. Adding any barrier is a problem. Playback of recorded music does suffer from the artifacts of loudspeakers or headphones, too.

Best Regards,

Dave

[ Parent ]

The ear is a barrier to sound ? (none / 0) (#34)
by levesque on Sun Sep 20, 2009 at 06:44:45 PM EST



[ Parent ]
We don't want an extra barrier (none / 0) (#35)
by imon2nd on Sun Sep 20, 2009 at 09:13:05 PM EST

Levesque;

Our eardrums are enough. We don't need another device in the chain with eardrums hearing for us.

When we listen to live music, the sound goes from instrument to eardrum. When we listen to recorded music, the sound goes from intrument to microphone (diaphragm), to recording device, to disc to player to amplifier to speakers to our eardrums. That chain needs to be as linear as possible. Elminating sources of distortion and non-linearity is the name of the audio engineering game.

Cheers,

Dave

[ Parent ]

Addendum (none / 0) (#37)
by levesque on Tue Sep 22, 2009 at 07:13:28 PM EST

Ears are transducers that convert traveling waves into neural firing, what we experience mentally and what we call traveling waves are two different things.

[ Parent ]
Update on prototypes (none / 1) (#38)
by imon2nd on Mon Sep 28, 2009 at 12:32:51 PM EST

Hello, all.

Proto 2 was better than Proto 1. We expect Proto 3, presently under construction will be a bigger step forward. That's because we got the non-occluded-beam operating mode working a few days ago.

Non-occluded means the laser beam is not impeded by smoke except when sound pressure waves make the smoke move into the beam path. The result is a very low "idle" noise floor.

The problem with implementing this method is the precision necessary to maintain a clean boundary between the laser beam and the moving smoke ribbon. With any luck, we should be able to post a YouTube video of Proto 3 within a week.

Cheers,

Dave

won't the signal be rectified? (none / 0) (#39)
by Blarney on Tue Sep 29, 2009 at 08:52:25 PM EST

Serious THD here.

[ Parent ]
More like upside down (none / 0) (#40)
by imon2nd on Thu Oct 01, 2009 at 05:14:07 PM EST

Hello, Blarney.

You're correct. Even in occluded operating mode the output of the photo detector's amplifier is DC offset and inverted. Nonetheless it is analogous to the incoming signal. The DC offset is due to the fact that the photocell outputs a voltage even when occluded by a constant ribbon of smoke. Then, when the sound waves create peaks and troughs in the smoke the output of the photocell goes down with the peaks and up with the troughs. AC couple the output to get rid of the DC offset and you're good to go, though inverted.

In non-occluded operating mode, the photocell is always putting out its maximum value. Then, when the smoke ribbon impinges into the laser beam, the louder the signal, the lower the photocell's output voltage because more of the beam is occluded. This also creates an upside down signal in terms of peaks and troughs. However, the mic preamp on the recorder side of the PC has auto gain and soft-clipping. The recording software has signal inversion. The net effect of all that is a usable recording.

The proper thing to do is, use digital signal processing on the photo detector's output to both remove DC offset and invert the signal. We'll get there, eventually.

Best Regards,

Dave

[ Parent ]

are you joking? (none / 1) (#41)
by Blarney on Thu Oct 01, 2009 at 11:43:39 PM EST

I take it you're a digital guy. Have you messed around much with analog? Phase inversion. Who cares? Seriously, why would it matter to anybody? Do you think all microphones produce a positive voltage for a high-pressure input or vice versa? I'd be surprised if anybody cared enough to check this. I don't even know if the inputs in my mixing board are inverting or noninverting. Ask if my guitar's pickups give a positive signal when the string comes closer to them, or if it's negative - I wouldn't know and I bet if you called up Fender or Gibson they wouldn't know either. Nobody cares about phase inversion. It's inaudible. Yes, some golden-ear nuts claim they can hear 'absolute phase' whatever that is. When one of them can do this in a A/B double blind test, it'll be believable.

Anyway, from an analog point of view, you have much worse things to worry about than absolute phase.

Your description of 'occluded' operating mode suggests that you'd basically be full-wave rectifying the signal. Say the ribbon of smoke moves to the left by a bit and now the photodiode detects a tiny bit more signal, so let's just say that your output voltage moves positive. Ok, now let's say the ribbon of smoke moves to the right the same amount. This will brighten the photocell exactly the same, and again, your output voltage moves positive. You are reading the absolute value of the signal, and not the signal itself. Here's an experiment - play a 1000 Hz wave through a speaker at the setup. Record it. Do a FFT. If you see 2000, 4000, 6000 Hz etc but no fundamental, then this is exactly what you're doing. It's even worse because if you are recording anything besides pure sine waves, you'll get intermodulation and utterly distort things. Honestly that's what I suspected just from listening to your recording. I like to square or full-wave rectify my guitar signal, and have a variety of commerial and homemade effects which do this with various filtering. I enjoy the intermodulation products produced in this particular application. And I know that sound. So . . . any chance you're actually doing that?

Your non-occluded mode sounds more like a half-wave rectifier. What I mean is - the wave goes one way, the smoke goes in front of the photocell, your signal goes down, you get let's just call it a negative output signal. Say the wave goes the other way - do you get a positive signal? - no you don't because it went away from your light beam and never got picked up. So you only get the negative half of the signal and the positive is just a flatline. How will it sound? Like a mixture of the real sound and the full-wave rectified one mentioned previously.

[ Parent ]

You're right. (none / 1) (#42)
by imon2nd on Tue Oct 13, 2009 at 05:07:53 PM EST

Blarney;

The more we worked with the non-occluded mode, the more we realized it sucked for a number of reasons. We're going forward with the smoke or vapor occlusion method.

As for the importance of phase inversion; many of the studio recording engineers we met at the AES convention last week are totally obsessed with that level of perfection whether we can hear it or not. And we got a lot of negative feedback to our proposed use of a CMOS detector with digital output. There are a lot of "analog forever" types out there.

Best Regards,

David

[ Parent ]

Smoke and Laser Microphone | 43 comments (34 topical, 9 editorial, 0 hidden)
Display: Sort:

kuro5hin.org

[XML]
All trademarks and copyrights on this page are owned by their respective companies. The Rest 2000 - Present Kuro5hin.org Inc.
See our legalese page for copyright policies. Please also read our Privacy Policy.
Kuro5hin.org is powered by Free Software, including Apache, Perl, and Linux, The Scoop Engine that runs this site is freely available, under the terms of the GPL.
Need some help? Email help@kuro5hin.org.
My heart's the long stairs.

Powered by Scoop create account | help/FAQ | mission | links | search | IRC | YOU choose the stories!