Loudspeakers or microphones are used to produce sound waves via electrical devices such as music systems or telephones or to convert sound waves into electrical signals. In the case of microphones, sound brings about voltage changes, thus allowing sound waves to be converted into electromagnetic waves. Loudspeakers generally operate on electrodynamic principles and convert electromagnetic waves into sound waves with the help of membranes and coils.
In telephony, the frequency range of the transmitted sound waves is limited to between 300 and 3,400 Hz. (with the exception of HD telephony). The sound waves are converted into electrical signals by a microphone and transmitted in analog or digital form via the telephone network. For digital transmission, the analog signal has to be transformed into a digital signal by means of sampling, quantisation and coding. Digital signals can also be transmitted via mere data networks. Voice-over IP (VoIP), for example, uses the Internet for transmitting digital voice data. This makes it possible to access the services of a cloud telephone system via any Internet connection.