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Audio is low enough frequency that you can process the signal directly. The time delay/phase information between each mic allows you to know which direction the

Audio is low enough frequency that you can process the signal directly. The time delay/phase information between each mic allows you to know which direction the sound is coming from. This is essentially the opposite of beam forming. Theoretically you could do it with visible light and not need a lens if you had a computer and sensors that could operate fast enough. But optical sensors typically only tell you the amplitude of light and not phase for example. Edit: To clarify, the "opposite" of beam forming means using processing you can choose which direction you want to listen at any one time, like a beam. Then you can scan the beam across x,y and make an image.

6gvONxR4sf7o 5y ago
Thanks!
- _Microft 5y ago
  Fun fact: we manage to record amplitude and phase of radiowaves, though. That allows us to record them at different points on Earth, ship the recorded data to a datacenter and computationally merge them to get a planet-wide virtual telescope dish with a much better angular resolution that a single telescope dish ever could have.
  - ddalex 5y ago
    we don't record phase, there is no way to recover the phase from a single signal what we do we make sure that all receivers are synchronized, i.e take samples at the exact same time then you can correlate the signal received between dishes (which will arrive at different times due to delays in propagation), and find out the time difference of the signal which then points out to signal origin (beam forming) - this is how phased radar works once you align the signals you can use the minute differences in the signals to compute a synthetic aperture, i.e improving the angular resolution
marginalia_nu 5y ago
Wouldn't a light version of this basically just be a fancy camera obscura?
- photonic37 5y ago
  The light version of this would be closer to a light field camera [https://en.wikipedia.org/wiki/Light_field_camera https://en.wikipedia.org/wiki/Light_field_camera] The major difference between a microphone array and an imaging sensor is the availability of phase information for the received wave. A microphone oscillates with the sound pressure wave, and that oscillation is translated directly to a voltage. Your software can see the full time series of that wave, so the information about it is 'complete'. An optical image sensor, essentially, turns photons into electrons. The optical wave is too fast to turn into a voltage time series, so you only see the wave's amplitude at a given sample in time. Therefore, in order to turn it into an image, you need to recover some fraction of the phase information in some way. A pinhole is one way to do that. One way to think of a pinhole is that it maps every source point to a distinct imaging plane point, so the phase of the wave doesn't matter as much to the final image. It acts as a filter that cuts out ambiguous information that phase would have disambiguated. A lens performs a similar operation by interacting with the light wave's phase to bend wavefronts in a way that maps points on the object to an imaging plane. Those approaches don't recover 100% of the phase information, but they recover or filter enough to form the image you care about. Light field cameras attempt to recover more complete phase information through various ways better explored in the wikipedia link. Could you create a sound blocking plane with a pinhole that makes an acoustic camera that follows similar principles to an optical camera obscura? I bet at some level you could, but I also bet it would not be very advantageous. You still need a microphone array to act as the imagine plane. The size of the pinhole is probably very constrained by sound wave diffraction (it's a pretty long wave after all, compared to light). Using the directly available acoustic phase information is more compact and efficient.
  - marginalia_nu 5y ago
    I figured if you were to create an optical camera on the same principles of an acoustic camera you would get into trouble with the very short coherence length of sunlight. It's easy enough to build something that can deal with a laser, but sunlight has a coherence length of just a couple of dozen micrometers. If you are working on a larger scale than that, the phase information effectively becomes useless.

6gvONxR4sf7o 5y ago

Thanks!

_Microft 5y ago
Fun fact: we manage to record amplitude and phase of radiowaves, though. That allows us to record them at different points on Earth, ship the recorded data to a datacenter and computationally merge them to get a planet-wide virtual telescope dish with a much better angular resolution that a single telescope dish ever could have.
- ddalex 5y ago
  we don't record phase, there is no way to recover the phase from a single signal what we do we make sure that all receivers are synchronized, i.e take samples at the exact same time then you can correlate the signal received between dishes (which will arrive at different times due to delays in propagation), and find out the time difference of the signal which then points out to signal origin (beam forming) - this is how phased radar works once you align the signals you can use the minute differences in the signals to compute a synthetic aperture, i.e improving the angular resolution

marginalia_nu 5y ago

Wouldn't a light version of this basically just be a fancy camera obscura?

photonic37 5y ago
The light version of this would be closer to a light field camera [https://en.wikipedia.org/wiki/Light_field_camera https://en.wikipedia.org/wiki/Light_field_camera] The major difference between a microphone array and an imaging sensor is the availability of phase information for the received wave. A microphone oscillates with the sound pressure wave, and that oscillation is translated directly to a voltage. Your software can see the full time series of that wave, so the information about it is 'complete'. An optical image sensor, essentially, turns photons into electrons. The optical wave is too fast to turn into a voltage time series, so you only see the wave's amplitude at a given sample in time. Therefore, in order to turn it into an image, you need to recover some fraction of the phase information in some way. A pinhole is one way to do that. One way to think of a pinhole is that it maps every source point to a distinct imaging plane point, so the phase of the wave doesn't matter as much to the final image. It acts as a filter that cuts out ambiguous information that phase would have disambiguated. A lens performs a similar operation by interacting with the light wave's phase to bend wavefronts in a way that maps points on the object to an imaging plane. Those approaches don't recover 100% of the phase information, but they recover or filter enough to form the image you care about. Light field cameras attempt to recover more complete phase information through various ways better explored in the wikipedia link. Could you create a sound blocking plane with a pinhole that makes an acoustic camera that follows similar principles to an optical camera obscura? I bet at some level you could, but I also bet it would not be very advantageous. You still need a microphone array to act as the imagine plane. The size of the pinhole is probably very constrained by sound wave diffraction (it's a pretty long wave after all, compared to light). Using the directly available acoustic phase information is more compact and efficient.
- marginalia_nu 5y ago
  I figured if you were to create an optical camera on the same principles of an acoustic camera you would get into trouble with the very short coherence length of sunlight. It's easy enough to build something that can deal with a laser, but sunlight has a coherence length of just a couple of dozen micrometers. If you are working on a larger scale than that, the phase information effectively becomes useless.