The root mean square (rms) value of a time varying signal is obtained by squaring the function at each instant, obtaining the average of the squared values over the interval of interest, and then taking the square root of the average value. The purpose of this is to convert an a.c. waveform into its equivalent d.c. value such that the variations can be read on a sound-measuring instrument. The rms value represents the effective energy value and is the best measure of steady continuous sounds. The period of interest, or averaging time, in sound measuring instruments is sometimes called the time weighting and is standardized in international regulations to specific values called the Slow, Fast and Impulse responses.