[Quote]:

If you ever connected to the Internet before the 2000s, you probably remember that it made a peculiar sound. But despite becoming so familiar, it remained a mystery for most of us. What do these sounds mean?

As many already know, what you’re hearing is often called a handshake, the start of a telephone conversation between two modems. The modems are trying to find a common language and determine the weaknesses of the telephone channel originally meant for human speech.

Below is a spectrogram of the handshake audio. I’ve labeled some signals according to which party transmitted them, and also put a concise explanation below.

(The audio is on Wikimedia Commons)