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Not all modems from back in the day had speakers, for example an early popular modem was the Hayes Micromodem II (available for Apple ][ and S-100 machines) and it did not have a speaker.

But the speaker served a few purposes that I can think of:

1. Early modems were not very good at detecting various states, e.g. busy line, voice pickup, disconnected number notifications, etc. Without the modem supporting the detection of these states, the terminal program wouldn't do a very good job reporting it.


2. You could quickly tell if the line was busy or if someone picked up the phone. Even with later modems, detecting voice was not quick or necessarily accurate.


3. You could detect any dialing errors easier. Obviously, this would be more appropriate for complicated numbers rather than ones in your local area code / exchange.

Personally, I always left the speaker on though perhaps turned down low enough that I could hear the dial and busy tones. If anything, it did give me the ability to whistle a carrier tone by hearing it so many times :-)

So why were modem speakers such a persistent feature, and "fixture" of the time?

Three basic reasons:

1. Adding a simple amplifier and a speaker is the most easy way to handle unexpected situations


2. In many countries/networks having a speaker active while establishing a connection was mandatory to make it legal/get a validation


3. Adding it was as well the least cost intensive way to handle complex situations.

Keep in mind, the kind of modems most people used were not for dedicated lines, like today's cable or DSL installations, but operating over regular dial up. To start with, anything could happen - starting from some angry mom picking up the phone were her son has hooked up his BBS, wrong numbers, all the way to signal tones outside the spec the modem was built for (for example ring tones in various countries used different rhythms and frequencies).

So having a speaker was a quite good debugging tool - after all, whatever happened on a phone line would be within 3000 Hz, clearly noticeable by humans listening.

This is also the reason why phone regulators/companies required them to be installed as well. They wanted to minimize interruption of third parties (due to mishandled/directed calls) as well as having the user able to detect special situations on his own, without calling for service. In fact, in many cases it was required that each and every modem had to have a handset (or a complete phone) connected, so the operator can pick it up and talk to any (unintended) party on the other side to apologize and/or first talk to the other side before switching to data. In reality most private users ignored that part (as well).

These are the reasons why feeding the speaker, until a valid connection was established, was the default configuration - though, it wasn't really enforced. If you didn't like it, it could be configured away - at least with most modems.

Last but for sure not least: modem producers liked it as well. After all, with a speaker they didn't have to invest much money in detecting all kinds of situations. Let the user handle it like he was used to doing with a regular phone call.

I suspect it could be a holdover from the days when a modem was a box that connected between your terminal and your phone. The phone handset was used to dial the remote number; when you heard the whistle from the far modem, you'd press the 'online' button on the modem, and then replace the phone handset.

And, of course, if it wasn't a modem whistle you heard, it was either a confused human, or else some telephone-system signal telling you why you hadn't been connected (either tones or voice announcement).

So audible signals were an intrinsic part of knowing where you were in the connection sequence.

The photo is from Handbook of Data Communications, UK Post Office, 1975. The modem's the box under the telephone.

I seem to recall that the PC-board modems could be configured to not enable the speaker, but I for one always left the audio on, for reassurance that things were progressing in the desired manner.

All the answers above have concentrated on outgoing calls but modems also took incoming calls and, because phone lines were expensive, it was unusual to have multiple lines for fax, data, and voice.

Therefore the modem was usually installed in-line with the phone (i.e. the circuit came into the modem, and then continued onward to the handset).

Usually a modem would be set to answer after a set number of rings, allowing time for a human to pick up the phone. Moreover, the modem could be set to answer in quiet mode and not do negotiation until it heard a remote modem, so as not to fry the ears of a voice caller.

In this case, the speaker was very useful, as the caller would hear the cessation of ring-tone as the call was answered but no sound. They'd usually go "hello" or similar, and you could then pickup the handset and talk as usual.

On an older modem, if one tried to dial a number, one of two things would happen:

1. Before a configurable timer expired, the modem received a carrier tone. In this case, it would output "CONNECT" and then start exchanging data with the far end.




2. The timer expired without the modem having received a carrier tone. In this case, the modem would output "NO CARRIER".

Older modems had circuitry to detect the carrier tone, but not to detect anything else that might happen on the line, since a modem had no real reason to care about such things. All the modem cared about is whether the connection succeeded or failed. A human might want to know more, but the simplest way of giving such information to a human was to add a speaker.

BTW, I remember a modem which had a configuration option to mute the speaker as soon as it heard the carrier pilot tone, rather than at the end of negotiation; I don't know why that wasn't a more common option.

I listened for a dial tone - told me my phone line was working. Phone outages were common in the 80's.

My guess is that's its mainly for easy human understandable notification.

Probably it can be compared a little bit with electrical cars. They do not make sound. Because people are not used to them, it can be dangerous if one suddenly approaches you (especially from a side you cannot see). To prevent this problem, some cars make sounds (like a motor) to 'warn' people there is a car near.

Of course this is more from a safety point of view, but when someone sees a car, it expects to hear one too. Since people were used to hear a sound from a modem, they need to hear the sound; if it is silent they might think it is nonfunctional (even with some terminal program indicating otherwise).

Originally, you had to dial the phone by hand and listen for the carrier. When you heard it, you would place the handset into the acoustic coupler. The telcos and PTTs did not allow direct connection to the communications equipment.

Later, you could connect directly: you would pick up the handset and manually dial and listen for the carrier. When you heard it, you would toggle the ONLINE switch and hang up the phone.

When the marketing people and the pointy-haired bosses wanted to "dial in" to what the geeks were doing, they wanted to have that squeaky sound coming out of their IBM too so everyone knew they were ahead of the game and working hard. After that, it just became the way things were and always had been.