Echo cancellation—This reduces the chance of sound output through a loud-
speaker being reamplified into the sound input. Reamplification results in
the well-known squealing feedback we have all heard when a microphone
and a speaker are too close together.
Limiters—As the name implies, they limit the amplitude of the sound, restrict-
ing the volume to a predetermined setting.
Compressors—Compressors are like limiters but instead of just limiting the
loudest sound they “compress” the audio signal to a certain volume range
avoiding both the loudest and the quietest sounds.
Expanders—These simply expand or increase the volume of the audio signal to
a predetermined sound level.
Gates—Gates block or eliminate audio sounds below a certain frequency level.
Automatic gain control—This automatically controls the volume of the signal
at a specific frequency level.
Delay—Delay of an audio signal is typically used in large venues where natural
sounds can be heard after processed sounds and delay is used to synchro-
nize the two.
Figure 15.2 Interactive whiteboards.
172 Smart Building Systems for Architects, Owners, and Builders