The frequency limit of piezoelectric devices is much higher. The higher limit means that the same devices can use ultrasound whenever interdevice communication might be distracting or not be relevant to human listeners. Sound signals have several convenient mathematical representations. Unfortunately, whenever air or any objects in a given environment move, the parameters of the system change, and as a result, the impulse response may change in time. The noise component in Equation 1 represents both the deviation of the system behavior from the regular convolution formula caused by nonlinearities and all other sources of sound in the environment.
Noise is uncontrollable and largely unpredictable.
An impulse function gives the most complete description of an environment. Unfortunately, calculating the impulse response of the system in real time is a complicated task even if ample computational resources are available. It is possible to use a simpler representation of the received signal. The original signal can reach the receiver either directly or after reflection from one or more surfaces.