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OT: The sound of light

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  • OT: The sound of light

    Last year I built an aeolian harp. It is a modern version of an old instrument and uses a solar powered FM transmitter to relay the sounds of the harp to the house. This has proven to be problematic in the winter even though the batteries can run the transmitter for 4 days without recharge and even though the solar array can do a full recharge in only 5 hours. There simply isn't enough daylight or strong enough solar radiation in the middle of winter. I needed to make a drastic revision to the energy budget. I settled on using an amplitude modulated diode laser to beam the sound to the house. So, I built the receiver which will be attached permanently under the eaves above in the photo,

    And added laser capability to the transmitter. The laser system uses one fourth the power that the transmitter requires. This may still not be good enough but makes it easier to run from batteries.

    Of course, I could just run an audio and power cable but where's the fun in that?

    However, I digress as the above is only a background to explain why I was wandering about with a long black tube with a wire to a box that has a cord plugged in to my ear.

    The receiver is dead simple. There is a monocrystalline silicon solar cell in the end with a cheap plastic lens that is intentionally somewhat defocused and which forms an out of focus image on the solar cell. The long tube prevents the detector from being swamped with ambient light which would saturate it so that it could not respond to an amplitude modulated light source.

    The output of the solar cell is amplified by an LM 386 audio amplifier IC set for a gain of 200 with a pair of earbud phones for output.

    It should be noted that the frequency response of single crystal silicon solar cells can range into the megahertz depending on the size of the cell.

    After building the receiver and amplfier I took them outside to test. I first used the idea of having an entirely passive retroreflector on the harp and simply bouncing a laser beam, in this case from the level, from the harp.

    The LM386 IC is not a particularly quiet amplifier as it generates a noticable amount of 1/f hiss when the gain is set to maximum. I had already verified the operation inside by pointing the receiver at a lamp and noting the 60hz hum it detected.

    I had expected not to hear anything unusual as sunlight is effectively DC. On stepping outside I immediately noticed an increase in noise level. This noise varies depending on what the receiver is pointed toward. It seems to be acting as an optical microphone of sorts. The visually richer the objects in the field of view the greater the level of noise generated. Trees in particular are fairly quiet near the bottom but quite noisy near the top.

    I soon realized that the most likely explanation for this is the accumulated tiny random movements of the millions of small features such as branches, twigs and needles all reflecting a bit of light and all randomly contributing to the optical equivalent of white noise.

    I find this facinating in itself but there was something even more interesting to come. The noise level made the retroreflector unworkable but I still wanted to explore the background optical noise effect so I set the unit on a tripod and began aiming it a various parts of the surroundings. At one point I had left it aimed at the trees that line the edge of the driveway. At that time my wife arrived home from work.

    To my astonishment I could clearly hear the sound of the car including the crunching of gravel beneath the tires as she pulled in to park, out of my sight, around the corner of the garage. I heard this via the amplifier, not via my own ears directly through sound in the air.

    The obvious explanation is that the sound pressure waves are disturbing all of the tiny movable features on the trees and grass etc in a coordinated manner corresponding to the passage of the sound pressure wave over these features. This is entirely reasonable as trees will produce very clear echos in the right circumstances such as when lining the edge of a lake.

    However, in this case the echo is of a third order as it is composed of reflected light that is modulated by the movement of the features under the influence of reflected sound. This was heard again when a neighbour arrived home as well, out of direct sight but revealed by the modulated light reflecting from the nearby trees.

    This is a most interesting development. I can not find any direct reference to such an environmental opto/acoustic effect although there are many and varied related techniques that make use of integral light sources to make what are called "optical microphones". My example though uses only the existing natural light to illuminate the optoacoustic envrionment.

    This has many possibilities to explore, such as the optical acoustic environment during a thunder storm, especially the sound of lightning on windblown trees and even more the optical equivalent of thunder. Another exciting possibility is to try and listen to the aurora borealis.

    I am surprised at not being able to find any mention of this online anywhere. Surely I cannot be the first person to notice it? Has anyone here heard of this?
    Last edited by Evan; 04-06-2008, 04:12 AM.
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  • #2
    I'm sorry you discovered this.

    The black helicopters have been dispatched, national security.

    Evan, Who?
    Today I will gladly share my experience and advice, for there no sweeter words than "I told you so."


    • #3
      Turn the radio/stero on in your car with all the windows closed and aim your reciever at the one of the car's side windows.


      • #4
        Ah it's well seen your retired have fun Alistair
        Please excuse my typing as I have a form of parkinsons disease


        • #5

          It's been used for clandestine surveillance for several years.
          Here is an expensive one:
          Licensed security experts specializing in GPS tracking, video surveillance, and covert solutions to meet your security needs.

          I cut it off twice and it's still too short!


          • #6
            Just Curious

            Hi Evan
            I'm curious does aiming it at a strong magnetic field have any noticeable change in sound? Again I'm thinking about lines of flux in the field.


            • #7
              Guys like Evan is why I roam the streets in a tinfoil hat.


              • #8

                If you point it right at the aeolian harp, can you pick up the sound directly?


                • #9
                  Well, that has certainly piqued my inquisitive nature today. It stands to reason that it would work that way. I'm sure it will be interesting to hear thunder with that pickup method.

                  By the way, as you are probably aware, the noise produced by the 386 can be reduced using a suitable feedback circuit. It likely won't be of much value to have the full gain of 200 going out to 20k, so why not increase the feedback for higher frequencies to reduce the gain- for what you're doing you could start dropping the gain at 5k and the only thing you'd lose is noise.

                  I'm wondering also about the frequency response and noise performance of todays amorphous cells as compared to the monocrystalline cells.

                  It will be interesting to hear of your future findings with that apparatus. I had another thought- maybe you can arrange to detect that mystery plane that flies over your area with some variation of that. But I suppose you wouldn't be wanting to aim a laser at a plane these days-
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                  • #10
                    If your sensor is able to pick up IR it would be endlessly fascinating to know what happens when you aim your pickup at a stand of trees at night and then illuminate them with IR. Or even visible light.

                    I wonder what a raccoon sounds like in IR

                    Many years ago I discovered it was possible to use a laser aimed at a window a very long ways off across Seattle and pick up conversations in the room. It was not what I expected - we were trying to use passive reflectors to extend the range of a laser data communications link.

                    It is also possible to illuminate the white board inside such a room and get even better audio as multi-pane glass affects the quality. It's a bit creepy, to be honest, and I didn't explore it further.


                    • #11
                      I wonder if the monocrystalline silicon solar cell, in addition to a photoelectric response to light, has a piezoelectric response to sonic frequencies.
                      Allan Ostling

                      Phoenix, Arizona


                      • #12

                        That isn't the same. It supplies it's own coherent light source which is then modulated when it is reflected. I am listening to the sound from the modulation of incoherent environmental light.

                        Today I was out with the device listening to various things and I noticed that the sky, which was quite dark with clouds, was very noisy. This was the opposite of yesterday when the dark areas were quiet. About 15 minutes later it began to snow/frozen rain. I think I was picking up the precipitation on it's way down.

                        Allan, the solar cell isn't the least bit microphonic. You can tap, rap, scratch and bang on it with no result.


                        I have some very low noise FET op amps. I'll just build a front end.

                        But I suppose you wouldn't be wanting to aim a laser at a plane these days-
                        I just noticed this sentence. I'm not using the laser to pick up the sounds, not at all. The sound is coming from the modulation of daylight by moving objects.
                        Last edited by Evan; 04-06-2008, 10:09 PM.
                        Free software for calculating bolt circles and similar: Click Here


                        • #13
                          Originally posted by Evan
                          Allan, the solar cell isn't the least bit microphonic. You can tap, rap, scratch and bang on it with no result.
                          Okay, the sun is soon to set. I trust you will try aiming the tube in the dark, and report what effect the absence of daylight has.
                          Allan Ostling

                          Phoenix, Arizona


                          • #14
                            I have already tried it. All I hear is the hiss of warm electrons in the circuit and the 60 hz hum of my yard lights.

                            I am going to build another detector using an amorphous solar cell. They have a much enhanced red sensitivity compared to crystalline silicon.
                            Free software for calculating bolt circles and similar: Click Here


                            • #15
                              I like it.. Sounds like you are perceiving the super sub-harmonics from possibly light waves. What I dont get is the energy level. They would be so small, are you picking up the variations in light level, not the modulated frequency.

                              Say the detector is set at a determined "noise" level and it is picking up variations in the light received but not the modulated frequency of the light wave.

                              Both would give similar events. The receiver is picking up a varying frequency. Be it a varying amplitude of signal received (which I think the case is) or a varying noise level attributed to the changing wave length through modulation...

                              Either way I like it!!! Fun project.. JR