Measuring sound intensity, What is a decibel – Apple Final Cut Express HD User Manual
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Measuring Sound Intensity
Our 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, such as 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.
Since our ears respond to sound pressure logarithmically, using a logarithmic scale
corresponds to the way we perceive loudness. Audio meters and sound measurement
equipment are specifically designed to show audio levels in decibels. 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 linear increase in perceived loudness.
Important:
When you mix audio, you don’t need to worry about the mathematics
behind logarithms and decibels. Just be aware that to hear incremental increases in
sound volume, exponentially more sound pressure is required.
What Is a Decibel?
The decibel measures sound pressure or electrical pressure (voltage) levels. It is a
logarithmic unit that describes a ratio of two intensities, such as two different sound
pressures, two different voltages, and so on. A bel (named after Alexander Graham Bell)
is a base-ten logarithm of the ratio between two signals. This means that for every
additional bel on the scale, the signal represented is ten times stronger. For example,
the sound pressure level of a loud sound can be billions of times stronger than a quiet
sound. Written logarithmically, one billion (1,000,000,000 or 10
9
) is simply 9. Decibels
make the numbers much easier to work with.