envelope
n. in acoustics, a slowly varying or “smoothed” change in amplitude. Usually it refers to temporal changes, such as those produced by amplitude modulation or beats, but it can also refer to the shape of a spectrum, as in a spectral envelope, or to spatial changes, as in the envelope of the traveling wave (see basilar membrane). “Slowly varying” is not precisely defined but generally refers to fluctuations whose rate is much less, by a factor of at least 0.5, than the highest rates in the temporal, spectral, or spatial representations. For example, the beats created by adding 1000 Hz and 1005 Hz tones have a quasi-sinusoidal envelope with a periodicity of 5 Hz, but there is no envelope created by the addition of 1000 Hz and 2005 Hz. Temporal and spectral envelopes are important in auditory perception.