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Author Topic: Can we make a sonic hologram?  (Read 1174 times)

DWoodsTEC

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Can we make a sonic hologram?
« on: May 15, 2014, 11:41:54 AM »
Using a laser it is possible to expose a film with an interference pattern which when viewed with the same frequency light reproduces the image of the object used to create the interference pattern.


Is it possible to digitally accomplish the same thing?


The nut to crack is to create a hologram of an underwater environment in a single shot.  In theory this could be done with lasers but the impurities in the water scatter the laser rendering it useless at a distance over maybe 20 feet.  Also the wave length is short enough that particulates in the water can reflect.


If the "laser" was at a much lower frequency say about 3 to 4Khz the resolution would be approximately 1 inch so particulates would be invisible.  At a distance of hundreds of feet the inch resolution would be an insignificant error.


It is possible to create emitters to send the signal but is is possible to create a reciever (the film) to record the interference pattern?


Can this receiver be something similar to a CCD in digital cameras tuned to the much lower frequency?


Can we then digitally view the image of the structure?


If this could be done we could then see better than dolphins in murky water.  An immediate application is underwater archaeology.  It could save days and weeks of measurements.


It appears current technology for acoustic imaging is similar to that used with laser imagers where a beam is sent from a stationary source and the echo time is measured and tied to the angle.  It suffers from a very beam width which hurts the resolution hugely.   It seems the hologram would eliminate the beam width problem.  It would be necessary to have very sensitive receptors.  The other problem is to filter out the background noise.  Under water is very noisy.
 



Dave Woods

Real inventors turn ideas into products in the market.

Matt

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Re: Can we make a sonic hologram?
« Reply #1 on: May 15, 2014, 04:19:16 PM »
I'm a bit confused. Are you talking about making a hologram where the interference patterns are generated by sound waves in water vs coherent electromagnetic radiation? Or are you talking about simply using a much lower EM frequency? Keep in mind that EM radiation at the frequency you're talking about has a HUGE wavelength (~100 KILOMETERS for 3 KHz) This is computed as Wavelength = c / frequency where c is the speed of light in meters per second and frequency is in hertz. A wavelength that size would be nearly useless for anything other than mountains/continents. Not to mention the problems of creating a small array of detectors to pick up this frequency. Submarines use frequencies like this to communicate but the antennas used to pick up the signal are huge.. see the article on VLF here: http://en.wikipedia.org/wiki/Very_low_frequency

If you're talking about using an audio frequency at 3 KHz then a whole new set of problems present themselves. First, audio waves are entirely different beasts. Producing a coherent audio source is not terribly difficult (much easier than lasing light due to the way audio is generated vs the random quantum process that causes the emission of photons). However, a coherent source of waves is not the only requirement for good holography. You also need a coherent medium for the wave to propagate through. The speed of sound varies greatly with the depth/pressure of the water and temperature/salinity gradients. This is why thermals show up on sonar plots. In optical holography the interference pattern is useful for reconstructing the image because the multipath returns of the light waves are all equivalent (at least close enough) that all of the variations in the pattern can be chalked up to the shape of the item you are imaging, not noise introduced by the speed of light being different for one path vs another (they're generally all the same). This would be far from true for audio. Then you get to the sensor problem. It's quite easy to build filters for narrow band pass EM filters, audio, not so much.  This is due to the fact that EM radiation has a real particle (photon) behind it while sound travels as a virtual wave through whatever medium it passes. The frequency of of light is unique per photon.. So, for example the laser scanner product my company produces emits UV laser light at a very stable frequency. It is then able to receive a reflection of that light at up to 330 meters and if only two or three photons make it back to our sensor we're still able to detect them and know that they're "our" photons rather than random background noise. This is not so easy with sound.

I think a better implementation would be to stick with standard echo location technology but implement a multi source/sensor array with different frequencies that you can use to model the environment more clearly and remove multipath ambiguities by carefully choosing your phases per source/sensor and using the different geometric arrangements of the sources/sensors to cancel out background noise caused by thermals and depth changes.

Disclaimer: I'm not an expert and all of this may be total BS :)
-Matt

P47

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Re: Can we make a sonic hologram?
« Reply #2 on: May 15, 2014, 06:07:00 PM »
Dave,

This is a cool idea. In my youth I had interest in Holography and spent many days at "The Museum of Holography" at 11 Mercer Street in NYC. It has since closed it doors, but it left a lasting impression. They had lots of gear and awesome holograms on display. The museum sparked my desire to make my own holograms, and I had reasonably good results. I built a sandbox rig to isolate vibrations, and built a ruby laser driven by a strobe light. I also built a helium laser (this didn't last long due to impurities).  Because the film is recording the interference pattern between the object beam and the reference beam, vibrations in the environment was the greatest obstacle to overcome (hence the sandbox). To stress the importance of damping, reference material I read stated that if a car or a truck drove past my house the vibrations caused would render the hologram useless. Most of the holograms I've seen are in a controlled environment. My suggestion would be to make a fish tank and emulate the motion of the flipper in the water, then try your hand at making a hologram. Let me know if you need any help with this as it sounds pretty cool.

Regards,

Pat
« Last Edit: May 15, 2014, 06:13:34 PM by P47 »

DWoodsTEC

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Re: Can we make a sonic hologram?
« Reply #3 on: May 15, 2014, 07:04:29 PM »
This is the sort of conversation I wanted to get started.  I really know nothing of holograms.  The application I was envisioning was for under water archaeology.  It would need a range of a few hundred feet and a resolution of about an inch.  I was thinking sound rather than EM.  No doubt there are numerous mechanisms which will cause distortion.  It seems the hardest part is creating a pressure receiver which is sensitive enough to pick up the subtle reflections from the objects.

My initial inquiry into this indicates the speed of sound is about 4 times faster in water than in the air so that would actually make the frequency required to get 1 inch resolution be about 60KHZ. (rather than 3 to 4 KHZ as I stated earlier,  oops.)  At that frequency most of the background noise would be way out of the spectrum.  Changes in salinity, in most cases, are small enough to be ignored.  Temperature differentials and current could have a measurable effect.  That is a problem to be dealt with once an actual hologram is created.

The first issue is the receptor.  Can we make an inexpensive narrow frequency pressure receptor in an array dense enough to create data which could represent an audio hologram?  After that it is just data processing - lots of it.
Dave Woods

Real inventors turn ideas into products in the market.