A conversation with a friend made me suddenly recall that when I was a kid in the early 80’s, we could occasionally hear a faint rendition of the major local AM station coming from the faucet of the kitchen sink. We lived just a mile or two from the broadcast antenna.
It was very faint and had a spooky sizzling quality, but it was unmistakable. Our wall-mounted telephone also picked it up, but more distinctly. I can understand the telephone noise reason, as there’s an amplifier and speaker. But a faucet? How?
In: Physics
Run your fingers down the teeth of a comb. Or at least imagine it for now, you know that sound it makes? Now run your fingers down it faster. It seems ike the pitch changes doesn’t it? Like it’s higher pitched? It’s an illusion.
The carrier wave of an AM signal stays at the same pitch much like the teeth of the comb. If you could directly hook an AM signal to a speaker you would hear a similar illusion.
Since you can hear that illusion, you could hear the actual transmission if only you had something to turn the electromagnetic waves in to oscillating air pressure (that’s what sound is).
Your sink was behaving as the speaker.
I believe there are 3 things you were missing when you asked this question:
1: Yes, if you could hear an AM signal it could resemble something like the source without being decoded. There is still one step between the source sound and carrier wave though, so it doesn’t sound great without being “decoded.”
2: All you need to do to turn an electromagnetic wave into physical sound is make a thing that vibrates in response to to the electromagnetic waves. Usually a magnet with the signal pushes and pulls a diaphragm that moves air – thats the big round thing on a speaker. It doesn’t *need* to be purpose built for sound though. Ever hear the hum of an electeical transformer? You can hear the AC power bevause the casing of the transformer is behaving just like your sink. Its a big metal thing that is “playing the sound of the electrical grid” (and our power grid is just a super low frequency electromagnetic wave).
3: AM stands for “amplitude modulation” – this could be restated as “we change (modulate) how loud (amplitude) the signal is really fast.” As a consequence of this, to be able to actually use AM we need the signal to be stronger, or “louder” if you will. Long story short, this is why you don’t *need* an amplifier to get somethig audible to the human ear. The signal is enough to power a small speaker.
Remember how I said our power grid is just low frequency electromagnetic waves? Well radio is then just high frequency wireless power.
There are other nuances to all of this – electromagnetism isn’t something you can really intuitively grasp in one sitting without visual aids.
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