The importance of the impulse as a concept in engineering work has come about not only because of the extensive use of pulses in many types of communication systems, but also because many sources of interference or radio noise are "impulsive" in nature. By an impulsive source one generally means a source of electromagnetic energy which can be represented by a series of discrete disturbances of low duty cycle. Usually, it will have a relatively broad frequency spectrum. The importance of the impulse is related to the fact that where the duration of the pulse generated by a given source is sufficiently short1 in comparison with the reciprocal of the center or tuned frequency of a "narrow-band" network responding to it, the waveform at the output of the network is of a very definite shape practically independent of the input waveform, and has a peak value proportional to its "impulse strength." Because of these relations, an impulse generator is useful for calibrating the network response, and the networks themselves may be characterized in terms of their equivalent "impulse bandwidth." Because the impulse has a broad spectrum, a quantitative measure of the "spectrum amplitude" is also a useful quantity.