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Comment by backendanon | Hacker News Reader

At 1 million times less power than existing sound transmission systems I'd say it's probably less audio going into the water than a baby whale fart.

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that's -60dB, no?

like the reduction from gunshot (140dB) to alarm clock (80dB)... or jet plane take-off (120dB) to normal conversation (60dB).

Depending on the actual, absolute values, one-millionth can therefore be still annoying — especially if it is pervasive and omnipresent.

The innovation here is to make that unidirectional rather than omnidirectional. So seems reasonable to conclude that even if the beam is gunshot noisy, outside the beam is basically silent.

Maybe I'm not understanding, but isn't this reflecting sound back, generated by something else? It seems the device uses one millionth the power, but the thing stimulating the device uses an unspecified amount, that would increase dramatically with distance. Unfortunately this is an MIT press release, so it will be impossible to understand what's actually going on.

Does anyone have a more informative link?

There is no more information, as described in TFA, for reasons of this information having not been discovered yet by the researchers studying the new tech.

The press release links the two publications: - http://www.mit.edu/~fadel/papers/VAB-paper.pdf - http://www.mit.edu/~fadel/papers/PAB-theory-paper.pdf

> ... fix the input electrical power to 150 W ... The 20 dB DI transducers push the uplink decoding range up to several kilometers.

That probably is close to a baby whale fart, but I doubt competing systems are running at 150,000,000 W, as the above comment suggests. In their test to get 60m, they used 1.8W. 1.8 MW seems unlikely, for the same distance, with competing tech.

There's a fundamental misunderstanding of what's going on here. But, that's to be expected, with how these press releases are written.

This is asymmetric communication. The node itself uses backscatter thus operates at few micro-Watts. The remote acoustic projector is the 1.8W, and can communicate with 100s/1000s(?) of micro-watt power nodes - similar to RFIDs, but this tech works underwater.

I understanding that, but claiming that the backscatter energy, or the nodes power usage, is all the ocean life (the context of this comment chain) will see is incorrect, as the previous comment did. They primarily see the excitation energy, which appears to be ~23db greater (harvesting efficiency) than the backscatter, at the node. This is the same as something sitting between an RFID chip and the reader would.

So, the sea life near the transmitter would still have a bad time. The sea life near the nodes would see 23db more* than an active node, assuming the same power could be used to transmit from the node. Correct? This seems logical, since the energy harvesting will come at a coupling and efficiency cost, which means significantly more energy in the water at that node. If you had a battery powered node, you wouldn't need all the extra energy, and instead could just transmit.

All these numbers being thrown around are the power usage of the node, not what the sea life actually sees.

* Maybe more, since the signal path is twice as short, meaning your SNR starts higher at the midpoint (node).

And note, 1.8W is for the dock test. Their paper suggests 150W will be required for km ranges, with 20db directionality.

Does it matter if it is generating the sound or just intentionally bouncing it back? It is still a source of unnatural sound that as of now has unknown affects on the natural inhabitants of the medium the sound is traveling. While you may think the hairless apes on the surface can do whatever they want regardless of repercussions, others of the hairless apes choose to be more conscientious of their effects on the surrounding environment.

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