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Natural Science Forum / Physics / Acoustics / October 2004The sound of a laser.
Hi. Physics newbie question here: what does a laser sound like? Some sources I've read claim that a sufficiently powerful laser would ionise the air along its path and produce a "crackle" or "pop" noise, while others claim that regardless of power, a laser has to be of a particular colour to ionise the air and thus produce a sound - but fail to mention what that colour is. Is any of this information accurate? As a follow-up question, if a laser actually can make noise, is there an easy way to derive an approximate relationship between how powerful the laser is and how loud it is? Thanks, - David Prokopetz. > Hi. > [quoted text clipped - 12 lines] > > - David Prokopetz. At high instantaneous laser powers the ossilating electric field in the beam can ionize gasses, including air. An easy way to experimentally investigate this is to bring a laser beam to a focus in gasses of varying pressures. Vary both the gas pressure and focal length of the lens. Longer lens focal lengths result in lower electric fields at the focus. It will be necessary to use a pulsed laser with short pulse lengths as this maximizes the instantaneous electric field. The sound of the electric breakdown is a loud crack. i suspect the sound reported from any high energy laser is more likely to be the crack due to the relaxation of the dielectric in the capacitors. Itd have to be some energy/(not power) level to heat the air up enough to produce a shock wavefront. Still... if you dress it up enough and give it a swishy name, theres bound to be some hi-fi nut with a turntable and valve amp that would think it was the best tweeter he'd ever heard ie inertialess. > > Hi. > > [quoted text clipped - 23 lines] > > The sound of the electric breakdown is a loud crack. A few mJ is enough for air breakdown from a low f/ number focused Q-switched laser. Yes, there is the sound of the caps discharging. That's not what is being discussed here. --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header is ignored. To contact me, please use the feedback form on the S.E.R FAQ Web sites. > i suspect the sound reported from any high energy laser is more likely to be > the crack due to the relaxation of the dielectric in the capacitors. Itd [quoted text clipped - 36 lines] > > > > The sound of the electric breakdown is a loud crack. > Hi. > > Physics newbie question here: what does a laser sound like? Some sources > I've read claim that a sufficiently powerful laser would ionise the air > along its path and produce a "crackle" or "pop" noise, Biggest laser I ever worked with was 2 W, so I have no first hand knowledge. What you're talking about is usually associated with high voltages, such as a lightning bolt, an electric discharge, or the corona around a high-voltage line. That wouldn't be a mechanism for a laser to ionize air. However, if it made the air hot, that *would* be a mechanism for this sort of thing. So the question is, under what circumstances would laser heat the air along its path? You can see a powerful laser beam from the side because of scattering off dust in the air. I could imagine a powerful enough laser heating the dust enough to boil it. Hence, a crackling sound. > while others claim > that regardless of power, a laser has to be of a particular colour to ionise > the air and thus produce a sound Not true. However, dust and air molecules, like everything else, absorb different wavelengths differently. Black dust absorbs most visible wavelengths, so it would get hotter. Air is transparent in the visible, but not 100% transparent. so the air along the path is going to get microscopically hotter. However, obviously you'll put more energy into the air, making it hotter, if you choose a wavelength where it absorbs. There are probably some good choices in the infrared. I don't have anything like an absorption spectrum for nitrogen handy, but that would tell you which wavelengths are preferentially absorbed. - Randy One aspect of the physics of laser-induced gas breakdown that I've not seen mentioned yet in this thread (maybe I missed some of the messages) is that it's very much a "runaway" process. A small amount of initial heating and/or ionization of the atoms in a tiny volume of gas can result from even rather weak direct absorption by the atoms, or from laser-induced multiphoton ionization of the gas atoms, or the presence of a small amount of random ionization from other causes, laser absorption by dust particles or impurity gases, possibly other causes. Once heating starts or some initial ionization occurs, the initial volume can rapidly become much more absorbing to the laser light as a result of excited state absorption in heated neutral atoms, additional absorption lines in the ions that have been produced as well as in the heated atoms, and free-electron and plasma absorption processes, with relative strengths depending in complex fashion on wavelength, optical intensity, and presence of impurities. Basic point is that more heating produces more absorption produces more heating produces . . . Rates of further heating and ionization can then increase rapidly in a runaway fashion, leading to intense heating and ionization, formation of a visible spark, expanding shock wave, and generation of an audible "snap". Most of the same things also happen in the pulsed laser-induced damage spots produced when a laser beam is focused inside a "transparent" dielectric medium to do subsurface laser marking. > One aspect of the physics of laser-induced gas breakdown that I've not > seen mentioned yet in this thread (maybe I missed some of the messages) [quoted text clipped - 24 lines] > spots produced when a laser beam is focused inside a "transparent" > dielectric medium to do subsurface laser marking. I have read that there are laboratories $pending government money$ trying to use this process to establish a transient line-of-sight conductor to be used for a high-current and high-voltage discharge. Can you say "death ray" my friends? I *knew* you could. Tom Davidson Richmond, VA Dear tadchem: ... > I have read that there are laboratories $pending government money$ trying to > use this process to establish a transient line-of-sight conductor to be used > for a high-current and high-voltage discharge. > > Can you say "death ray" my friends? I *knew* you could. Not a very efficient way to transmit the message "you are dead"! Doesn't smell as bad as a bullet proof vest wrapped around fish, I guess... David A. Smith > Dear tadchem: > [quoted text clipped - 9 lines] > Not a very efficient way to transmit the message "you are dead"! Doesn't > smell as bad as a bullet proof vest wrapped around fish, I guess... My guess is that it is intended to kill mostly microchips/guidance systems/avionics/ECM hardware et cetera. Tom Davidson Richmond, VA >> One aspect of the physics of laser-induced gas breakdown that I've not >> seen mentioned yet in this thread (maybe I missed some of the messages) [quoted text clipped - 30 lines] > >Can you say "death ray" my friends? I *knew* you could. I thought this was the plan for a German taser that would shock a person by sending a current over ionized air paths rather than through a pair of wires coupled to the target by spikes sticking in him. But last I heard, the power supply was the size of a refrigerator.  Signature"And don't skimp on the mayonnaise!" <snip repost> > I thought this was the plan for a German taser that would shock a person > by sending a current over ionized air paths rather than through a pair of > wires coupled to the target by spikes sticking in him. But last I heard, > the power supply was the size of a refrigerator. That would make it a lot easier to see it coming. It would also be difficult to use in confined spaces. Protest rallies held indoors would become more frequent. Tom Davidson Richmond, VA ><snip repost> > [quoted text clipped - 6 lines] >difficult to use in confined spaces. Protest rallies held indoors would >become more frequent. Or on hills... Signature"The result of this experiment was inconclusive, so we had to use statistics." (Overheard at international physics conference) >><snip repost> >> [quoted text clipped - 8 lines] > >Or on hills... The sound of money leaving the wallet as the laser is purchased. The bigger the laser, the louder the sound and the longer it lasts. :-) > Hi. > [quoted text clipped - 3 lines] > that regardless of power, a laser has to be of a particular colour to ionise > the air and thus produce a sound - but fail to mention what that colour is. I don't know if high intensity laser {beams] produce any sound, but to ionize air you need something powerful and up in the ultraviolet range; perhaps from a nitrogen laser. Ron |
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