Apple Final Cut Pro 7 - Manual (Page 833)

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.
Percussive volume envelope Sustained volume envelope
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.
Because the ear responds to sound pressure logarithmically, using a logarithmic scale
corresponds to the way humans perceive loudness. Audio meters and sound measurement
equipment are specifically designed to show audio levels in decibels. Small changes at
the bottom of an audio meter may represent large changes in signal level, while small
changes toward the top may represent small changes in signal level. This makes audio
meters very different from linear measuring devices like rulers, thermometers, and
speedometers. Each unit on an audio meter represents an exponential increase in sound
pressure, but a perceived linear increase in loudness.
833 Chapter 52 Audio Fundamentals