Apple Soundtrack Pro User Manual download pdf (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 envelopePercussive envelope

Time

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Apple Soundtrack Pro User Manual Page 289