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So, compressed air power is a Thing. Compressed air tools are reasonably common, and compressed air has even been delivered commercially as an industrial utility at times.

But all existing compressed-air technology is, to draw an analogy with electricity, "direct current"--you have a constant pressure (voltage) at the inlet which drains to a lower pressure (voltage) outlet. With electricity, however, there are quite a few benefits to also using alternating current supplies. In a compressed air system, this would correspond to periodically cycling the inlet pressure up and down, rather than holding it constant--and transmitting "AC" compressed air power would be equivalent to transmitting sound waves down a tube.

So, is this feasible? Would it make any sense to have "sonic power plants" transmitting high-powered sound through closed pipes in a compressed-air analogy of AC electricity? If yes, how could such a power distribution system be made efficient? And if not, why not?

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    $\begingroup$ Actually, AC electricity is harder to produce - but it travels much, much, much further distances, which makes it much more economical. Edison started with DC, which is much simpler to work with. (It took a while just to invent an AC motor, DC motors are easy). However, Cyclic air pressure doesn't have that benefit (ease of transfer). In fact, it would be a lot worse over distance (it would even out to a level pressure over distance). I'm not convinced a tool could be made to work better with it. @DKNguyen is right, comparing mechanical to electrical doesn't work. $\endgroup$ Aug 22 '20 at 0:03
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    $\begingroup$ I will give you an interesting comparison. I lived once in a town with no water tower - the city's pressure was 100% by pump. The result is that the water in the pipes had cyclic pressure. You couldn't tell at the tap because the shift from high-pressure to low-pressure at the tap hid the effect - but what it would do is split pipes - especially the softer metals like copper. The cycling pressure weakened the metal. Mechanical != electrical. $\endgroup$ Aug 22 '20 at 0:10
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    $\begingroup$ "AC compressed air" is called sound. Yes, it can transmit power. Yes, transmission of power via mechanical waves is kinda sorta analogous to alternating electric current; there is even a theory of sonics describing the transmission of power via mechanical vibrations. There are even some applications. The major blocker is that there is no mechanical analogue of the magnetic field, and this means no transformers; in the absence of transformers, alternating current is not really more useful than direct current. $\endgroup$
    – AlexP
    Aug 22 '20 at 0:17
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    $\begingroup$ @JBH Is DC easier to produce? Every motor and generator I have seen seems quite naturally AC and always requires extra junk to make it DC. $\endgroup$
    – DKNguyen
    Aug 22 '20 at 0:48
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    $\begingroup$ @JBH, sure, if you're talking about Leyden jars. Dynamos, however, are inherently AC. "In the first practical electric generators, called dynamos, the AC was converted into DC with a commutator." You have to add complexity to them to get them to generate (bad) DC, and add even more to get non-pulsing DC. By any objective measure, that is not "easier to produce". I'm not disputing your claim that DC was used first, I'm disputing that AC is "harder to produce". It isn't. (I'd take this to chat if I knew how...) $\endgroup$
    – Matthew
    Aug 22 '20 at 4:20
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Having plumbed my workshop with both compressed air and vacuum piping, it certainly would've made my life easier if they could be the same pipe. If periodically the pipes were positively pressurized, and my air socket wrench could use that power, and periodically the pipes were negatively pressurized, used for the vacuum heat moulding, and cleaning.

If there was some magic gate that would allow the positive pressure to go to the positive pressure devices, and negative relative pressure to go to the vacuum devices, my life would be a little easier.

You can also use alternative high pressure / vacuum into a push pull, which theoretically can be used to drive a pneumatic cylinder, which can be converted to rotation by mounting off centre. (Same as a car engine, expanding pushes cylinder down, vacuum pulls it up, shaft rotates, car moves forward).

Sounds good in theory? It is! In practice? Not so much. Your vacuum cycle needs to totally empty the pipe. It needs to undo all the work of the compression cycle and then some. That could take several minutes, especially if delivered from a plant kms away.

Ac electricity doesnt have to "drain the pipe" when negative. The effect travels at the speed of light, and the positive and negative waves travel at the same speed.

Replace the air with hydraulic fluid, however, and you may be able to benefit from grid sized distribution of an alternating push / pull force. I want to raise a platform, I just wait for the push cycle and open the gate to allow the push to lift my platform, shutting the gate when the cycle goes negative or the platform reaches the desired height. I want to lower it, I simply wait for the pull cycle and open my gate during that cycle.

The effect would travel at the speed of sound in the fluid. Typically several km per second. Plants would need to be every few km or the cycle would need to in tens of seconds. Of course, every machine in your society would have this stop/start rhythm to it.

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  • $\begingroup$ Ummm... naked Earth electric charge is –4.5e5 C (pdf warning). Which means that the AC current doesn't exactly oscillate around an "absolute potential of 0V" (which is OK since the electric potential is defined up to an additive constant, so the "absolute electric potential" is a nonsense for any pragmatical purposes). The outcome of this - you don't need to have absolute vacuum for an oscillating air pressure to be defined as "pneumatic AC". $\endgroup$ Oct 11 at 9:00
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AC's big advantage is you can swap between voltage and current - and they have different efficiencies, with a big chunk of laminated steel and wire.

Considering the sound "pipes" would be vibrating, you'd have substantial losses and very noisy pipes.

You would also need to somehow efficiently convert this sound to "work" - there's a few technologies that would be a start - piezo crystals can convert sound to electricity at a low efficiency. Without this, its a no go, small scale or large scale.

Air pressure does work (and in a sense - a lot of ICE engines do work on air pressure changes, to the point where you can run a steam engine model off just air pressure).

You can't convert sound to other energy sources as efficiently.

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