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Comment by amluto | Hacker News Reader

One could modify this experiment to have very obvious effects. For example:

- Run the amplifier output through a banana or mud. Even if this somehow works and you can hear the sound, you’ll probably smell it as you cook and/or electrolyze your conductor :) (The banana likely works because the load impedance is very high in the experiment they did. The load impedance with an actual speaker is typically in the ballpark of 8 ohms. I admit I haven’t stuck a pair of multimeter probes in a banana lately, let alone done a proper I-V or AC impedance measurement.)

- Use really long cables. It’s not especially rare to be able to hear and even understand AM radio that gets accidentally picked up on a long cable and converted to baseband by some accidental nonlinearity in the amplifier.

- Use the actual outdoor mud on a rainy day as your conductor. I bet you can get some very loud mains hum like that.

Even audiophiles can probably identify these effects!

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A difference that long cables make can be heard in this video: https://www.youtube.com/watch?v=SorO-QpqYRU .

Therein, audio from a microphone is sent through progressively-longer cables until the length reaches ~6 miles. It gets pretty muffled-sounding... eventually.

(The longest pair of wires I've sent analog audio through was in the realm of 37 miles, stretching across the countryside. AMA, I guess.)

Hah, that AM-receiving cable was at a theater and only a couple hundred feet long.

In general, with low-level analog audio and non-ridiculous lengths, additive noise effects are likely to become audible long before attenuation or especially frequency-dependent attenuation. As a decent heuristic, as long as the DC resistance is small compared to load impedance, the cable impedance is unlikely to be a problem. For the connection from the amplifier to the speaker, additive noise is unlikely to be a problem, so the DC resistance is even a decent heuristic: keep the round-trip resistance below half an ohm or so and you should be fine with most speakers.

I used to have a fairly nice Rotel preamp that would receive CB radio transmissions sometimes. Whatever it was that was behaving as an AM detector must have been near the output of it because the intensity was unaffected by the volume control.

That was almost certainly the result of illegal transmitters, but it was annoying. One time I heard a man shouting through the stereo and the signal was hot enough in RF land that it made my X10-controlled lights flash on and off.

Later, I got DirecTV and it became differently-annoying: The noise of the satellite receiver switching bias voltage to select between different LNB polarities was sufficient to make a loud pop through the speakers (again, unaffected by the volume control). This made channel surfing very noisy. I was able to reduce it rather substantially with some very deliberate choices in audio cabling construction.

But with better gear (and with the differential[1] signalling that every avenue of pro audio seeks to use by default), this kind of stuff is usually a complete non-issue.

[1]: We popularly call this kind of connection "balanced," but we're wrong about that since there's usually hardly any concern about impedance matching. But it's definitely consistently differential, so I find this less-popular nomenclature to have the right amount of specificity to ~fit reality.

Ok I’ll bite…

37 miles?!? Why??

Land-mobile radio stuff. Analog, voice communication.

Our sales guy had sold a remote node for a voter system to improve receive coverage for a central dispatch system. (Signal-to-noise voters are pretty neat: They can continuously compare two or more related audio signals and [ideally!] pick the one that is best for use while discarding the others.)

That node wasn't all that far away as the crow flies, but it was a very long way out in telephone cabling miles. It spread across two different telco LATAs.

So we rented this very long series of bits of wire held together by scotchloks and punch blocks and whatever else in telephone world to use, and we used it. It was not a conditioned circuit: Just wire.

The specific endpoints of that wire were kind of neat, too: There was some basic EQ that could be used to help compensate, and (IIRC) some impedance adjustment to dial in the circuit itself.

And there was a continuous pilot tone used to set gain: Apparently, when wire gets really long like that, atmospheric conditions can dynamically change its attenuation.

Putting a pilot tone near the middle of the voice range (to be notched it out later) and using its level to set gain helps to improve consistency.

That wireline stuff all worked pretty well.

(The remote node was ultimately a bust. The sales guy also tried to cram too much shit into one feedline and antenna, and the gear to combine and separate all of those signals ate too much energy to make any of it an improvement over doing nothing at all.

Which is... well, that's exactly what the engineering told him would happen, but he did it anyway.

No part of this was inexpensive.)

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