Any one up for a discussion?

I’m in the mood for a good engineering-based discussion. By engineering, I mean math, physics, programming, or whatever.
topics similar to:
refraction of light: why did it start, and was it first noticed in water or glass?
waves and electronics: why do microphones work, and who figured that out?

Here’s a microphone fun fact tied to a bit of music history. Microphones basically work in the same fashion as a loudspeaker, only in reverse (sound wave -> electrical signal and visa versa). It uses electromagnetic induction - sound waves move a magnet surrounded by a coil, producing a current.

But anyway, when recording Paperback Writer, Geoff Emerick (the Beatles’ recording engineer) actually used a loudspeaker wired in reverse instead of a microphone to record the low end of Paul’s bass better. That’s why that song has such a distinct bass line. Fun fact of the day!

Correct! which, by the way, I think is pretty darn cool. How do you think someone discovered that?

If I had to guess, It started with two separate discoveries. First, the discovery that a moving diaphragm could pick up and reproduce sounds (ears and drums are respective examples that are respectively naturally occurring and well known throughout history). The second was that a current traveling through a coil can move things. Anybody passing a current through a long wire that had been coiled to keep it organized could have discovered this if there was any iron or magnetic material around, a compass would work too. Then the two were put together and the transducer was born.

The first was a Thomas Edison thing - a cone with a diaphragm at the smaller end, the center of which was mounted a needle. As a wax-coated cylinder spun beneath the needle, screaming into the cone transferred the vibrations to the wax. Shellac the wax to harden it, and use the same needle-diaphragm-cone assembly to trace the wax impression, and sound comes out. But only once - the cylinder was ruined by playing it :frowning:

Of course, that system was eventually optimized a lot. Tinfoil coating, for example.

The second was a Michael Faraday thing, IIRC, but of course it wasn’t seriously implemented in practice until amplification came about. But before then* Alexander Graham Bell used a bunch of carbon granules beneath the diaphragm which, when mechanically excited** generated a small current and, conversely, vibrated when electrically excited.

  • Emile Berliner actually had the first microphone, the “loose contact microphone”, but it wasn’t as practical as the carbon granule type, which is still in use today.
    **meaning ‘have energy put in’. Don’t get excited.

Typically, sound waves move the coil. The magnet is stationary.

By the way, the fact that these two cases had separate physical interpretations was a burr under Einstein’s saddle that was a motivating factor for his 1905 paper On The Electrodynamics of Moving Bodies.

Microphones can also use capacitance (sound waves move a diaphragm which changes its capacitive coupling to a second diaphragm) and the piezoelectric effect (sound waves distort a crystal which creates a voltage).