on the angle of a sound's approach. If the sound comes from the front (relative to the unit's primary axis), the microphone receives it loud and clear. If it comes from behind, the microphone attenuates it (that is, it reduces the amplitude of the signal), and reduces transmitted reverberation. The drawback of unidirectional microphones is that they require conferees to speak within the microphone's capture range. An omnidirectional microphone can be superior, but only if the system counters direct acoustic coupling with good echo cancellation. Walls and ceilings are launching pads for echoes; windows are even worse. In practice, speakers are often located too high or too low and, once sound waves bounce off of a couple of walls, the listener's microphone (which also is generally at about the same level as the participants' faces) captures those sound waves, transmits them to the sound system, and creates an undesirable effect called indirect acoustic coupling. Echo cancellation addresses problems caused by indirect acoustic coupling (also called multipath echo), in which a microphone captures sound waves that bounce off of objects. Because it involves sound bouncing off of surfaces and eventually approaching a microphone even from the front, unidirectional microphones can only do SO much to control indirect acoustic coupling. The earliest approach to acoustic coupling was to avoid it altogether. Echo suppression, in which loudspeakers and microphones were attenuated on an alternating basis to prevent feedback, was the first real attempt to control indirect acoustic coupling. This caused words to be clipped as microphones switched back and forth. Interruptions, a natural part of any meeting, are not possible with echo suppression, which compromises the natural rhythm of speech. Video Communications: The Whole Picture Echo cancellation has largely replaced echo suppression. Speech is modeled to create a digitized replica of an echo. Using a digital signal processor (DSP) the synthesized echo of the signal is subtr