Echo Cancellation - Acoustic Echo

Acoustic Echo

Acoustic echo arises when sound from a loudspeaker—for example, the earpiece of a telephone handset—is picked up by the microphone in the same room—for example, the mic in the very same handset. The problem exists in any communications scenario where there is a speaker and a microphone. Examples of acoustic echo are found in everyday surroundings such as:

• Hands-free car phone systems
• A standard telephone or cellphone in speakerphone or hands-free mode
• Dedicated standalone "conference phones"
• Installed room systems which use ceiling speakers and microphones on the table
• Physical coupling (vibrations of the loudspeaker transfer to the microphone via the handset casing)

In most of these cases, direct sound from the loudspeaker (not the person at the far end, otherwise referred to as the Talker) enters the microphone almost unaltered. This is called direct acoustic path echo. The difficulties in cancelling acoustic echo stem from the alteration of the original sound by the ambient space. This colours the sound that re-enters the microphone. These changes can include certain frequencies being absorbed by soft furnishings, and reflection of different frequencies at varying strength. These secondary reflections are not strictly referred to as echo, but rather are "reverb".

Acoustic echo is heard by the far end talkers in a conversation. So if a person in Room A talks, they will hear their voice bounce around in Room B. This sound needs to be cancelled, or it will get sent back to its origin. Due to the slight round-trip transmission delay, this acoustic echo is very distracting.