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 (101) and 1,000,000,000 (109), 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.

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 109) is simply 9. Decibels make the numbers much easier to work with.

In practice, a bel is a bit too large to use for measuring sound, so a one-tenth unit called the decibel is used instead. The reason for using decibels instead of bels is no different from the reason for measuring shoe size in, say, centimeters instead of meters; it is a more practical unit.

Number of decibels
Relative increase in power
0
1
1
1.26
3
2
10
10
20
100
30
1000
50
100,000
100
10,000,000,000

Decibel Units

Audio meters are labeled with decibels. Several reference levels have been used in audio meters over the years, starting with the invention of the telephone and evolving to present day systems. Some of these units are only applicable to older equipment. Today, most professional equipment uses dBu, and most consumer equipment uses dBV. Digital meters use dBFS.

  • dBm: The m stands for milliwatt (mW), which is a unit for measuring electrical power. (Power is different from electrical voltage and current, though it is related to both.) This was the standard used in the early days of telephone technology and remained the professional audio standard for years.
  • dBu: This reference level measures voltage instead of power, using a reference level of 0.775 volts. dBu has mostly replaced dBm on professional audio equipment. The u stands for unloaded, because the electrical load in an audio circuit is no longer as relevant as it was in the early days of audio equipment.
  • dBV: This also uses a reference voltage like dBu, but in this case the reference level is 1 volt, which is more convenient than 0.775 volts in dBu. dBV is often used on consumer and semiprofessional devices.
  • dBFS: This scale is very different from the others because it is used for measuring digital audio levels. FS stands for full-scale, which is used because, unlike analog audio signals that have an optimum signal voltage, the entire range of digital values is equally acceptable when using digital audio. 0 dBFS is the highest-possible digital audio signal you can record without distortion. Unlike analog audio scales like dBV and dBu, there is no headroom past 0 dBFS. For more information about digital audio metering, see About Audio Meters.