Sound has propagation features quite different from that of light, which makes it an excellent alternative in situations where light does not work well. Specialized imaging applications, e.g., ultrasonic microscopy and echolocation, make sophisticated use of sound. But for the most part, the use of sound in computers and communications applications has been limited to music, simple sound effects, and speech synthesis and recognition. However, modems and faxes are pseudoacoustic devices and cannot effectively communicate through air. Many electronic devices in and around the human-made environment are designed to speak or listen.
Many devices have a speaker or beeper to relay status to a human.
For example, the existing speakers in even the most low-end television could be used to communicate with toys and appliances if only they had the capability to listen. Many devices, such as telephones or faxes, have microphones. Concern over noise includes annoyance, communication interference, work interference, hearing loss, and other health issues, such as interference with sleep, impact on vasoconstriction, vertigo, and change in galvanic skin response. If ultrasound is extremely intense, it may cause heating and scattering of hard objects. Noise at frequencies lower than 100 kHz may be unpleasant for pets.