An acoustic coupler is a device which sends and receives computer data through a telephone line using sounds rather than electrical signals. The devices were popular in the early days of internet use when traditional modems were not always practical. Today, they are mainly seen in countries with less advanced telecommunications networks, and in equipment used by deaf people to make telephone calls.
At one time, it was impossible in many places to use a standard modem which plugged into a telephone wall socket. This could sometimes be a physical problem where telephone cables went directly into the wall, meaning there was no socket. In other cases, there were laws banning anyone from connecting electrical equipment directly into the phone system.
The solution for people wanting to connect to the internet was the acoustic coupler. The most common type was a device onto which you placed the handset of your telephone. There were usually padded seals so that no noise could leak into or out of the mouthpiece and earpiece.
The device then transmitted and received tones which the computers at each end could translate into data. You can get an idea of how this works when you press buttons on a phone today and each digit makes a slightly different noise. Even if you aren’t old enough to remember them, you may have seen an acoustic coupler in movies such as WarGames or Ferris Bueller’s Day Off where Matthew Broderick’s characters used them to access computers through the phone network.
Today, few computer users in developed countries have any need for an acoustic coupler. However, they can be useful when visiting countries where some phones are still hardwired and where there is no cellphone network to allow mobile broadband use. Some travelers even use them to connect to the internet through public payphones. As even today’s faster acoustic couplers are still only about half as quick as a standard dial-up connection, this technique is far from ideal, but it can be passable for less data-intensive tasks such as checking e-mails.
Acoustic couplers are also still used in some devices used by deaf people, known by terms such as minicom or teletypewriter. Each user types a message which is then converted to sounds, sent through the handsets, and then converted back into text which appears on a screen on the device at the other end. In some cases, the deaf person is connected to an operator who has a device and then relays the messages to and from a fully-hearing caller on a normal telephone.