The [Wikipedia Article](https://en.wikipedia.org/wiki/Cassette_tape_adapter#Usage) explains that it works like an mp3 adapter but it doesn’t really explain how those work. I can’t wrap my head around how it converts mp3 or bluetooth signal to a physical signal fast enough to listen to music comfortably, let alone hold a phone-call through the adapter.
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The concepts you’re talking about all have to do with storing and transferring sound information in different ways: storing it magnetically (the cassette tape), transferring it electronically (your phone, the mp3 adapter, the stereo), transmitting it on an electromagnetic frequency (bluetooth), and conducting it through air (as in hearing it from the speaker).
First off, sound is conducted through air as a series of pressure waves. It’s sort of like making waves in water; you can make one wave, or a series of waves. You can make waves bigger or smaller, or make them close to each other or more spread apart. Bigger and smaller waves correspond to louder or quieter sounds, while waves that are closer together make higher pitched sounds, and waves that are spread apart make lower pitched sounds. A song is lots and lots of pressure waves in series, that vary both in size and spacing, or ***amplitude*** and ***frequency****.* This series of waves of different amplitudes and frequencies is called a ***waveform****.*
As for a tape cassette and player, it works like this. The cassette player senses small magnetic fields. A regular cassete *film* is sort of like a long spool of tape with little magnetic particles in it. The waveform of the music is stored on that tape by varying the way those magnetic particles are aligned. In this way, you can simulate the waveform of *sound* with a waveform of the magnetic field along the tape. As the tape scrolls past the “read head” of the cassette player, the head sense how the magnetic field changes. It does this by having a tiny coil of wire inside- as the magnetic fields change, they cause electricity in that coil of wire that varies in the same way as the waveform and the magnetic fields.
A cassette *adapter* makes the same changing magnetic field for the read head. But instead of a moving magnetic tape, the adapter uses electricity to generate the changing magnetic field. By varying the *voltage* of the electrical signal in the same way that the amplitude and frequency of the waveform change, you can transmit the waveform as an electrical signal.
That tiny electrical signal made in the read head is then amplified by some complex circuitry. Sometimes this circuitry can be slow, but if it’s well designed, it’s almost instant; after all, electrical signals travel almost at the speed of light (see footnote). The amplified electrical signal has a high enough power to drive a speaker. By varying *how much* the electrical signal is amplified, you can control the volume. This is how a volume knob works. The speaker then moves back and forth, moving air. It can move back and forth a lot, or a little, and quickly or slowly. By doing so, it makes the pressure waves in the air that you hear as sound.
Bluetooth can work in a couple ways. The waveform can be transmitted on a radio frequency, as a series of changes in amplitude and frequency on the *electromagnetic spectrum.* Basically, it can send the waveform as invisible light. *Alternatively,* the computer in your phone can break the song file up into chunks of digital information, and send those chunks one at a time. Then the bluetooth receiver in the cassette adapter interprets those chunks back into a waveform, and uses electricity to make the changing magnetic field needed for the cassette player to work.
Now, you asked how it all works fast enough to listen to music comfortably, or to have a phone call. The answer is simple, and perhaps unsatisfying. Each step in the process, starting with the song file going from your phone to the cassette adapter, to the amplifier, to the speaker, and to your ear, *doesn’t take long enough for you to notice.* The physics of it just don’t take very much time to happen. As I stated before, electrical signals travel almost at the speed of light. Magnetic fields *do* travel at the speed of light. And the electromagnetic signal that your bluetooth operates *is light*. So the slowest part of the system is the sound traveling from your speaker to your ear, which happens at the speed of sound, which is 767mph, or 343 meters per second. Even that, being a tiny, tiny, tiny fraction of the speed of light, is too fast for you to notice.
As for a phone call, there are some more things going on, but it’s sufficient to say that it’s the reverse process from what I’ve already described. The sound of you talking moves a diaphragm on a microphone, which generates a tiny electrical signal, which is broken into digital chunks, transmitted to your phone by bluetooth, and then sent on a different EM frequency to a cell tower and on to it’s destination. Except for the time it takes for the sound to reach the microphone, all the other steps in the process happen at or near the speed of light.
I hope this is ELI5-ey enough for you. I know it’s kind of long.
Footnote: Because some people may be confused by the speed of electrical signals, I’ll clarify. Electricity *does not* travel at the speed of light. That is to say, electrons do not travel down a wire at near the speed of light. However, an electrical *signal* does. It’s like a tube packed with marbles running single file, all touching each other. If you push another marble in at one end, a marble will come out the other end *at the same time.* The marble you just put in, however, won’t come out the other end until you push the enough marbles in after it.
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