Apple Soundtrack Pro - Manual (Page 289)

Appendix C Audio Basics 289
Envelopes
Another aspect of sound that helps us to distinguish between instruments and voices
playing the same pitch is a sound’s envelope. Every note played on a musical
instrument has a distinct curve of rising and falling volume over time. Sounds
produced by some instruments, particularly drums and other percussion, start at a high
volume level but quickly decrease to a much lower level, and die away to silence
quickly. Sounds produced by other instruments, for example, a violin or a trumpet, can
be sustained at the same volume level, and can be raised or lowered in volume while
being sustained. This volume curve is called the sound’s envelope, and acts like a
signature to help our ears recognize what instrument is producing the sound.
Phase Relationships
When two instruments or voices are playing the same pitch, the sound waves may
have the exact same frequency and amplitude, but the peaks and troughs of the wave
reach our ears, or a microphone recording the sound, at slightly different times. This is
referred to as a difference in the phase of the sound waves. When two sound waves are
completely in phase, the volume of the sound is doubled. When two sound waves are
completely out of phase, they cancel each other out and we hear silence. Certain
effects, such as phase shifters, make use of these properties of phase relationships to
alter the sound of an audio signal.
Recording Sound
When a sound is recorded, the sound waves traveling through the air are converted to
an electrical signal, using a device called a transducer. Sound can be recorded using
either analog or digital recording technology.
Sustained envelope Percussive envelope
Time