The two main types of echo are acoustic echo and electrical echo caused by hybrid impedance mismatch. Usually, in a two-party call, only the speaker hears an echo but the listener does not. However, in a conference call, many parties might hear an echo.

Acoustic echo occurs when the voice of the speaker traverses through the airpath in the acoustic environment of the listener and reflects back to the microphone of the terminal of the listener. The severity of the echo effect depends on the acoustic properties of the room of the listener, such as, room size and wall reflection characteristics.

Electrical echo is also a reflection effect but is due to an impedance mismatch between four-wire and two-wire systems or in the interface between a headset and its adapter.

The perception of echo for the listener increases with delay. Usually, human ears ignore echo received within 30 ms. However, if the level of the received echo signal is extremely high, even 2 ms of delay causes a perception of echo. Echo received after 30 ms is usually perceived as annoyance. The perception of echo can be greater in the IP Telephony system because the end-to-end latency in some IP Telephony implementations exceeds the latency in some circuit-switched systems.

To reduce echo, customers must deploy echo cancellers at strategic places in telephones or network equipment. Echo cancellers, which have varying amounts of memory, store incoming voice streams in a digital form in a buffer and compare the received voice with the previously transmitted voice patterns stored in memory. If the patterns match, the echo canceller attempts to remove the newly received voice stream, but a residual level of echo is left even in optimal operating conditions.

Echo cancellers function properly only if the one-way delay between the echo canceller and the echo source, for example, the acoustic airpath at the telephone set or electrical hybrid, is not larger than the capacity of the echo canceller. Otherwise, the echo canceller does not find a pattern to cancel.

The Avaya G430 and G450 Branch Gateways, Avaya Aura® Media Server (MS), the Avaya IP SoftPhone, and all IP Telephones incorporate echo cancellation designed for IP Telephony to improve voice quality.