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Room Acoustics
G.167 testing is performed in real rooms. If the product meets the
requirements in these rooms, it is compliant. A device that is
G.167 compliant in one room, however, might not be compliant in
another. This is because the acoustics of all rooms are different.
This flexibility allows manufacturers to test their products in the
types of rooms they were designed for and claim compliance.
However, this also means that the customer has the responsibility of determining whether the AEC will
operate in his or her particular environment. An AEC solution that was designed to operate in an office may
not work properly in a conference room. If an echo canceller were compliant in one room and not another,
it would most likely be due to a tail length that was too short for the second room. Since tail length is not
specified by G.167, it must be evaluated separately.
Because of differences in room
acoustics, a device may be G.167
compliant in one room and not
another!
Tail Length
The tail length of an AEC is the length of time over which it can cancel echoes. The tail length of the echo
canceller should meet the requirements of the room it is to be operated in. This is directly related to the
reverberation time of the room. As the room reverberation time increases, a longer tail length will be
needed in that room. If the reverberation time is much longer than the tail length, a significant amount of
the echo will remain audible. However, excess tail length will not improve or degrade the performance of
the canceller. Determine the minimum tail length requirements for an application based on the typical
acoustics of rooms the product will be used in. Any products that do not meet or exceed that tail length
should not be used in that application.
There are two main factors that affect the reverberation time of a room. They are room size, and the
materials used to construct the walls and objects in the room. Most sound is absorbed when it strikes walls
or other surfaces. If materials are used that absorb sound well (such as carpet, curtains, or acoustic tile),
the reverberation will die out more quickly than if the room contains mostly reflective materials (hard wood,
glass, or plaster). If a room is small, the sound waves will bounce off the walls more frequently, and will be
absorbed more quickly.
The following formula is useful in determining the necessary tail length for an environment. It relates the
tail length to the room size and the number of cancelled reflections.
T = (N + 1) * d / c
• T is the tail length of the echo canceller
• N is the number of reflections cancelled
• d is the longest distance between walls
• c is the speed of sound (343 meters per second or 1125 feet per second at room temperature).
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