Naked Science Forum
Non Life Sciences => Technology => Topic started by: Paul Anderson on 03/03/2009 11:30:02
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Paul Anderson asked the Naked Scientists:
Hi Chris and team,
Helicopters use heat sensing gear to track down folk and bodies. Could helicopters use radiation cancelling gear similar to noise cancelling headphone technology, to combat forest fires?
Regards
Paul
NZ
What do you think?
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Sound and light are both waves. Sound is a pressure wave moving through a medium (in this case air) and infra-red light is an electromagnetic wave moving through space. THere are a number of key differences in the scenario you describe though. Sound cancellation relies on receiving the sound then reproducing an inverted wave so as to reach the receiver's ears at the same time. The incoming wave can be a multiplicity of waves at different frequencies and phases but provided the complex waveform is faithfully inverted and the design allows the incident wave and the inverted wave to arrive at the ear without any significant phase or amplitude errors, it will cancel very well. This is easy to achieve with sound because the methods to receive and transmit sound with precision are relatively easy. This is not at all the case with light.
Light could be detected and the frequency components analysed, but detecting the relative phases is not easy and certainly not practical in a short time. Detecting the complex wave shape and reproducing it in inverted form in any way analogous to sound cancellation is beyond technology at present.
The nearest equivalent is to produce a transparent material with a half wavelength coating on the surface which would be transparent to optical light but designed to reflect the average wavelength of infra-red. If you did not want to see out, a completely reflective material would be better.
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You are suggesting an 'energy cancelling' system. This is not a meaningful concept. You can't even cancel sound waves, really. You just re-direct them. Where would you re-direct the heat? Upwards, possibly. But the cancellation only can work with coherent waves. The photons from a fire are all randomly phased and timed. You couldn't deal with each one individually. You couldn't even deal with one photon of IR because the frequency is too high and no device can observe individual photon amplitude and phase - just the energy, possibly.
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"You can't even cancel sound waves, really. You just re-direct them."
Sound wave cancelling is actually a fairly good description of what happens. By creating a wave in anti-phase it is as close to the description that would be given by the word cancelling that I can think of. But it is true that you are adding energy to the system and at other points, away from the point at which you want to null the sound, you may be (and at some points will be) increasing it - where the phases add. In that sense you could say you are redirecting the sound.
Re-reading the question though, I think that it may have been some idea of beaming antiphase IR radiation from an independent body (like a helicopter) to prevent heating of (say) a person near to a radiation source. I agree that this is not possible with today's technology and may not be with any technology. You would have to know the amplitude and phase of the incident radiation close to the body to create an antiphase signal. You can't do this remotely.
As far as coherance length, you could think of this as how good cancellation (if it were possible) would be if the wavelength range was large. You would only cancel an average band which I thought may still be significant, though thinking about it, as it would be blackbody radiation it would be quite wideband. But I was suggesting a means of protecting an item from in the region of the item itself, which is not as intended.
I wonder is their a market for a large, IR reflecting tent made from non-flammable material, that could self inflate over an object to be protected and have its own air-supply. That might work as long as a burning tree didn't fall on it.
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But, when you 'cancel' a sound wave, you don't absorb the energy - you produce an antiphase version of it, locally. That means that the sound energy travelling in that particular direction (or entering your ear canal) must be reflected away. It can't just disappear unless you absorb it.
In the case of a forest fire, you would need to redirect a lot of energy - what would you absorb it with? It can't just cease to exist, can it?
Any standing wave / null that could be produced by introducing an extra signal would require both sources to be cohrent. You can only produce destructive interference with an appropriate antiphase signal. If the two sources are not very near (within a very few wavelengths) the 'null' regions in the interference pattern you could produce would be very small - the size of interference fringes - and to protect an object from 'heat' this way would be impossible because the source is not monochromatic and the different wavelengths would all produce different interference patterns and the nulls would all be in different places.
But the radiation from a fire is neither coherent nor monochromatic, anyway, so it's a non starter. I think there's a fundamental misunderstanding of what's required here and it's not just a matter of technology.
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"But, when you 'cancel' a sound wave, you don't absorb the energy - you produce an antiphase version of it, locally. That means that the sound energy travelling in that particular direction (or entering your ear canal) must be reflected away. It can't just disappear unless you absorb it."
I think you will find I said that :-)
In the case of noise cancelling earphones the description "noise cancelling" is very good. This is because the waves from the transducer are feeding a small cavity much smaller than the wavelengths and are best analysed as a pressure wave. The re-inforced wave will be from the back of the transducer and will be absorbed mostly by foam etc. Noise cancelling in cars has to be carefully designed to cancel at the driver's ears. There will be other points where the sound will be made louder.
What you say about IR (or light in general) is true, which is why I said it would not be possible. My suggestion of a glass surface with a half wavelength (average wavelength) coating works though, in the same way a quarter wavelength coating works to minimise reflection on a camera lens. If you don't need to see out then a 100% relector would be even better.
Have you seen the auditorium sound systems that have been developed by (I think) the Fraunhofer Institute? They use a vast number of small speakers and a good deal of DSP processing to give every member of the audience a perfect sterio (more like balanced surround sound) image. It uses "beam forming" technology originally developed for aerials but works well with sound too.
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I agree that the function of sound cancelling is more or less as you describe - but that it doesn't 'eliminate' the energy.
Both that and the travelling phase sound system in the auditorium are dealing with coherent waves and you know what wave vector you are dealing with.
The thermal example has no relation to those models - and can never have. If you are to produce a suitable wave to cause destructive interference then you have to be able to phase lock onto the wave train in some way and then produce your antiphase version. With a coherence length of a few microns, perhaps, you would need to have your 'transmitter' placed within the fire itself.
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"but that it doesn't 'eliminate' the energy." - I said that too :-)
SC, I just thought you would be interested in the Fraunhofer auditorium beam forming system. I wasn't suggesting anything to do with using it for optics.
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Yes, it is very interesting. I once saw an excellent demo of an analogue VHF radio service, serving a length of road in France. There was a string of transmitters (using the same frequency) with careful phasing of the signal which avoided the multipath problem and which gave perfect reception all the way, allowing a driver to move from service area to service area with no 'mush' in between. The system produced a travelling wave which swept along the road.
But, again, it used a coherent signal to achieve the required effect.
I suppose, with the advent of digital transmissions, which are designed to eliminate multipath / multi-signal problems, the system was a bit of a dead end. But impressive, none the less.