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I've got this very faint memory from my childhood that my dad connected 2 pieces of old, 5 cm diameter speakers to each other with a 10 meter wire. And without any other equipment, it would transmit the sound between rooms. Not loud or anything, but volume was enough to hear what the other was saying.

Got me thinking, after all, the sound waves move the coil, that generates electricity that would move the coil in the other speaker too. If the mouth is close to the speaker, it could work.

But I never found anything like that while googling for this. Could this actually work?

user47093
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    Yes, I have done it. If the speakers are matched and you cross your fingers. In fact I just talked about this in an earlier post today. – MadHatter Dec 29 '15 at 00:10
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    The reason for the weak sound on the receiver end is that the loudspeaker will have low efficiency converting sound energy to electrical for transmission and, again, low efficiency in converting electrical to sound at the receiver. The resultant sound energy will be the product of the two efficiencies. e.g., if both conversions were 10% (0.1) efficient the result would be 0.1 x 0.1 = 0.01 (1%) of the input energy would arrive at the output. The fact that you can hear it at all is down to the ear's logarithmic sensitivity to sound. – Transistor Dec 29 '15 at 01:34
  • Even more surprising would be that we could have a conversation even without the help of electricity, if we directly connected the membranes of the two speakers with a thin stretched cord ... but this phenomenon is outside the EE section... I conducted this experiment when I was a few years old... Few years later a classmate of mine and I made an electrical connection but with the help of a carbon microphone, moving iron speaker and battery in a loop... – Circuit fantasist Aug 17 '20 at 21:21

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It can work, and you saw it work long ago. I had a setup like that (some kind of science kit) when I was young. It works just like you guessed. You could do this again today, if you can find two speakers like that. The hardest thing these days is to find a quiet place to try it!

gbarry
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It's called a sound powered telephone (wiki) and they are used: -

enter image description here
(source: soundpoweredtelephone.com)

Wiki says: -

A sound-powered telephone is a communication device that allows users to talk to each other with the use of a handset, similar to a conventional telephone, but without the use of external power. This technology has been used since at least 1944 for both routine and emergency communication on ships to allow communication between key locations on a vessel even if power, including batteries, is no longer available. A sound-powered phone circuit can have two or more stations on the same circuit. The circuit is always live, thus a user simply begins speaking rather than dialing another station. Sound-powered telephones are not normally connected to a telephone exchange.

Note the hand crank magneto on the right for generating a voltage to ring the bell to attract attention. You can get the ringer magneto replaced with a battery oscillator - I designed one back in the 80s for use on British Railways - no power available! I think I've still got the prototype in my garage - it was a leaving present: -

enter image description here

Glorfindel
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Andy aka
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Of course it works .Alexandra Graham Bell did this and it worked.He spent most of his time trying to invent things for the deaf .His wife was deaf .This telephone that made him famous didnt help the deaf .These phones were not very loud and they got really bad on long lines .His phone was a short loop thing like broadband .Thomas Edison who was young when Bell was old got a design contract to "Improve the telephone " He came up with the Carbon microphone after considerable experimentation.This gave a much louder phone that could run longer distance .It made sense in an age when transistors were not around.

Autistic
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