So pitch is a function of wave frequency, and the harmonic series determines the texture or timbre of the sound. But sounds are not static, they develop and change over time. A violin note for example has a bow strike sound, followed by the development of full string vibration which sustains fairly constantly for the duration of the bowing. A guitar though has a plectrum strike sound with a very rapid onset of full string vibration which dies away fairly rapidly.
We map the progression of a sound in time by using the concept of an 'envelope.' We can identify an envelope relating to loudness as well as an envelope relating the changes in the harmonic content. There may also be an envelope describing 'noise' (i.e. unpitched sound like bow scrape, wind noise and piano hammer noise) in the sound.
The loudness of sound over time is called the 'volume envelope.'
The start of a sound is generally called 'Attack' and the way it fades away is called the 'Decay'. When a sound carries on at a constant loudness this is referred to as 'Sustain'.
Sounds can change over time not only in loudness but also in timbral character. A sound (such as a piano note or a picked guitar string) may have a loud, bright attack, and a slow decay which becomes decreasingly 'bright' until it finally fades away.
The decreasing high-frequency content over time happens because the high frequency partials contain less energy than the low, and therefore fade sooner.
In singing, the Envelope of the sound we make is one of the major factors which give a voice power and energy.