The name MODEM is a contraction of MOdulator-DEModulator. Modems transmit data by Modulating a signal (tone), and receive data by Demodulating the signal (tone). The sound they generate is the modulated signal. By using a tone, they can transmit a digital signal over an acoustic (sound) channel. The original modems with the cups for the headset were also known as acoustic couplers.
The original modems used a simple signalling system. As speed increased, the signalling system became more complex. Noise on the line degrades the available speed. Higher speed signalling mechanisms have error correction, and speed adjustment mechanisms built into the protocol.
North American phone systems used to charge a premium for data conditioned lines. However, any phone line which was function within specifications was capable of carrying a 1200 bit/s signal. Higher speed signals did require a much better signal, and some lines which were fine for voice use would fail to carry the signal at the full rate.
Modern digital phone systems carry sound using a digital signal. In North America, the signal is at 56 kbit/s. This is the upper limit on modem signals traversing a digital switch in North America. Last I knew, European used a 64 kbit/s channel. I don't know if Modems in Europe are/were capable of carrying a 64 kbit/s modem signal.
17Those were the days.. – Thomas – 2012-09-20T12:01:45.630
2You really needed that noise when you were paying for the phone calls, too. – Lee Kowalkowski – 2012-09-20T12:20:44.363
7@Thomas: Indeed. I wonder how many kids nowadays have even heard that sound? FWIW, I miss that sound a lot (modems/routers nowadays just seem 'soulless' in comparison), although one thing I certainly don't miss is desperately trying to muffle the darn speaker with a pillow when connecting at night, before I thankfully figured out the AT command mentioned by tylerl below! (Oh, and I ran my own BBS for a while - how many people remember those? Ah, fun times.) – Karan – 2012-09-20T15:29:48.670
1@JanDoggen that's not the question, I was wondering why they only did it at the beginning. – Celeritas – 2012-09-20T15:39:06.160
6I miss the days when mainframes used to make a noise. A healthy ICL 2900 mainframe would chirrup like a bird. If you got a couple of seconds silence followed by by a sort of whirring sound you knew the system had crashed and was dumping its memory to disk. The next sound would be the bang-bang-bang of the line printer printing out the core dump. Happy days! – Paul Cager – 2012-09-20T18:10:39.210
Celeritas, are you saying that dial-up modems no longer make a sound? Can you name a make and model? Or are you thinking of DSL boxes or cable modems? Anything that works over a phone line (i.e. "dial up") had better monitor the audio. – Kaz – 2012-09-23T05:15:47.633
@Kaz no that's not what I meant but thanks anyways. – Celeritas – 2012-10-23T08:12:44.030