Apple Soundtrack Pro 2 - Manual (Page 467)

Appendix B Audio Fundamentals 467
Musical sounds also typically have a volume 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 instruments,
start at a high volume level but quickly decrease to a much lower level and die away to
silence. 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 the ear recognize what instrument is producing the sound.
Measuring Sound Intensity
Human ears are remarkably sensitive to vibrations in the air. The threshold of human
hearing is around 20 microPascals (μP), which is an extremely small amount of
atmospheric pressure. At the other extreme, the loudest sound a person can withstand
without pain or ear damage is about 200,000,000 μP: for example, a loud rock concert
or a nearby jet airplane taking off.
Because the human ear can handle such a large range of intensities, measuring sound
pressure levels on a linear scale is inconvenient. For example, if the range of human
hearing were measured on a ruler, the scale would go from 1 foot (quietest) to over
3000 miles (loudest)! To make this huge range of numbers easier to work with, a
logarithmic unit—the decibel—is used. Logarithms map exponential values to a linear
scale. For example, by taking the base-ten logarithm of 10 (10
1 ) and 1,000,000,000 (10 9 ), this
large range of numbers can be written as 1–9, which is a much more convenient scale.
Percussive volume envelope Sustained volume envelope