Abstract: A narrow band noise gives a definite pitch, the height of which depends on its central frequency (f) and the brightness or tonality on its band width (B). In normal subjects, for a given f, the perception threshold of its modulation (△f) is found to be a function of B and can be expressed as △f=ABⁿ, where A is a theoretical value of △f when B=1 Hz, quite near to the △f for pure tone. Value n varies within a range of 0.22 -0.45 for different f's below 4000 Hz, being somewhat smaller for 4000 and 8000 Hz. The △f-f correlation curve of narrow band noises is quite similar to that of pure tones, provided △f values are taken for the same B/f ratio. Intensity of the carrier noise (20-80 dB) exerts very little effect on △f. △f of narrow band noise also varies in inverse proportion to the square root of modulation duration (up to 172ms), as FM-perception threshold of pure tone does.