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by Anechoic 5127 days ago
They are small, white/pale-grey boxes about 4 inches square by 2 inches deep.

Are you sure about that? That doesn't jibe with the picture I've seen (http://i.imgur.com/XBevy.jpg) and doesn't sound optimal for an acoustic transducer.
2 comments
justinph 5127 days ago
I'm 99.9% sure that is NOT a shot spotter. We have Shotspotter in some areas Minneapolis, as well as city-wide wifi. Those little cans are all over the place and are part of the WiFi System. I took a photo of one just down the street from my house: http://i.imgur.com/OqYEA.jpg
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jauer 5126 days ago
The can looks like a BelAir Networks BelAir200 WiFi Node.
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yellowbkpk 5127 days ago
As another poster mentioned, that's most likely a wifi access point.

I spent the last few minutes poking around StreetView looking for a photo of the units I'm thinking of without luck (the dates Google lists on their StreetView seem quite incorrect), but a hard, flat surface would be perfect for a sound transducer. It'd also be easier to weatherproof than something with microphones dangling outside.

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Anechoic 5127 days ago
I spent the last few minutes poking around StreetView looking for a photo of the units I'm thinking of without luck

In my city I know that they periodically move the units so it may be that they were moved when the pics were taken.

but a hard, flat surface would be perfect for a sound transducer.

I would be surprised if the mic was embedded in a flat surface, that would cause all sorts of diffraction problems for a surface that wasn't (effectively) infinite, especially since there is a lot of high-frequency energy in gunshots. I suppose they could try to equalize them out, but that seems like unnecessary work. But again, I haven't seen a (confirmed) transducer.

It'd also be easier to weatherproof than something with microphones dangling outside.

edit: actually I suppose the mic could be offset just an inch or so from the enclosure, that would be enough to reduce diffraction problems while not being noticeable from a distance. I'd love to see one of these systems up close. That problem has long since been solved with metal diaphragm mics, especially at the price points they're targeting. Microphones in airport monitoring systems last for years.

edit 2: some more searching indicates that a unit is actually made up of an array of mics. In that case, a flat surface would make perfect sense.

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