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Invisible hollowgraphy
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Acoustic holography is the process in which sound waves are recorded and arranged into a visual pattern using a computer. The sound field can be modeled to reconstruct its structure using three-dimensional (3D) images.

Parametric speakers are sort of like a laser beam with the sound focused at high intensity into a relatively small area. The result is that two people can be standing only a few feet apart from each other yet only one of them will hear the directional audio waves emanating from the parametric audio source.

I wonder if these might not be combined with a bit of geometry to create a basic hologram made of sound.

Initial sound waves are sent through an acoustic splitter so that parts of each sound wave are redirected 90 degrees from the target object, and then both the redirected and the original wave are then reflected from slight parabolas towards an array of microphones.

By playing the interference patterns on a continuous loop in reverse, and reflecting them off of the same surfaces, the re-convergence pattern of sound waves should create standing wave nodes in the shape of the original object.

I've done a sketch of how I think it might work. [link]

The effect would be an invisible three dimensional shape until mist or very fine particles were released, which would then remain suspended within the standing wave nodes to act as a screen for projection.
If this projection came from underneath the audiologram the result would be a Real three dimensional hologram rather than just the illusion of one.

Hollowgram http://s68.photobuc...jpg.html?sort=3&o=0
[2 fries shy of a happy meal, Aug 12 2014, last modified Aug 14 2014]


       Very original, the main problem I see is to keep the phase at the transducers, at mics while recording and speakers while playing.
piluso, Aug 12 2014

       //array of microphones// ah, well... hmm...   

       Visual holography works because tunnels in the film reflect light in different directions: it's not actually a "2D" surface.
FlyingToaster, Aug 12 2014

       //a basic hologram made of sound//
//What's one of those ?//

       A three dimensional representation of a shape using sonar.   

       //phase at the transducers//
Ooh, new words to look up.

       //tunnels in the film reflect light in different directions: it's not actually a "2D" surface.//
Ok... but do you think it would be possible to record the shape of an object in reflected sound and then re-broadcast a reverse recording in such a way as to control the convergence of the sound waves to create an invisible standing wave node copy of that shape in three dimensions?

       If so, would fine particles hang suspended in these wave nodes to make cymatic shapes which are not bound by symmetrical geometry?   

       If so, then would future adaptations at higher frequencies allow for virtual objects which could feel solid to the touch?
...because if so, then we could eventually build the holodeck and force fields.

       This idea reminds me of an idea that's been on my list to post for a few months. My list says it came to me in a dream, though I don't remember that. Anyway, mine does a similar thing, for the purpose of digitally imaging the target in 2.5D by interferometric sonar/radar/lidar/*dar, but doesn't use a traditional holography/interferometry configuration. It might still be a synthetic holography, though; I don't think I'll be sure until I've written it up, but for now I can say it could be described as "swept-frequency interferometric imaging *dar".   

       // tunnels in the film //   

       Huh? Are you saying film is structured similarly to a microchannel plate?   

       // Visual holography works because tunnels in the film reflect light in different directions //   

       How does that work?
notexactly, Sep 28 2019

       Hitting a recorded interference pattern surface resulting in a reflection is going to be different to having standing waves generate a misty form. It seems to me they are two different entities. High number cunching computing with acoustic ray tracing may be able to convert between the two.
wjt, Sep 29 2019


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