The manner in which a device (usually a transducer of some sort) behaves after the initial energy input (sound) has stopped. An impulse is a signal or sound that has a very short, in fact, a vanishingly small, duration. A true mathematical impulse has zero duration and infinite amplitude, but still a finite amount of energy. The energy in an impulse is spread evenly over a wide frequency band, and this means that it can be used as a test signal to measure the characteristics of an audio device. The impulse response convolved with an input signal gives the output response.