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Sound
Sound Intensity and Level
Physics Textbooks Boundless Physics Sound Sound Intensity and Level
Physics Textbooks Boundless Physics Sound
Physics Textbooks Boundless Physics
Physics Textbooks
Physics
Concept Version 7
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Decibels

The decibel is a logarithmic unit used to quantify sound levels, by comparing a physical quantity to a reference level.

Learning Objective

  • Identify how decibel is quantified in acoustics


Key Points

    • In acoustics, the decibel is quantified relative to a reference which has been set at a sound pressure level of 20 micropascals, and is called a 0 dB.
    • The following equation is used to calculate the sound pressure level, or amplitude: $amplitude[dB]=20log_{10}\frac{s}{s_{o}}$so is the reference pressure which is 20 micropascals or 0 dB, and s is the observed sound pressure.
    • The human ear has a standard sound threshold of 120 dB, which expressed logarithmically is around 1012. This is a standard threshold, but it also depends on frequency. Loudness is a measure of sound intensity taking frequency into account, and is called a A-weighted decibel, dB(A), or a phon.

Terms

  • decibel

    A common measure of sound intensity that is one tenth of a bel on the logarithmic intensity scale. It is defined as dB = 10 * log10(P 1/P 2), where P1 and P2 are the relative powers of the sound.

  • phon

    A unit of apparent loudness, equal in number to the intensity in decibels of a 1,000-hertz tone judged to be as loud as the sound being measured.


Full Text

The decibel, dB, is commonly used to quantify sound levels, although it is not a unit of sound, but a unit of pressure. The decibel is a logarithmic unit that indicates the ratio if a physical quantity to a reference level. It is one tenth of a Bel, which was named after the inventor of the telephone, Alexander Graham Bell. The word decibell comes from the prefix, deci, that is 1/10 of the word it precedes For more information on how to convert units, refer to the unit conversion atom. Although the decibel can be used to talk about a number of different subjects, in this atom we are going to cover its use in acoustics and sound level.

In acoustics, the decibel is quantified relative to a reference which has been set at a sound pressure level of 20 micropascals, and is called a 0 dB. This reference level is a typical threshold of human hearing perception. The following equation is used to calculate the sound pressure level, or amplitude:$amplitude[dB]=20log_{10}\frac{s}{s_{o}}$so is the reference pressure which is 20 micropascals or 0 dB, and s is the observed sound pressure.The human ear has a standard sound threshold of 120 dB, which expressed logarithmically is around 1012. This is a standard threshold, but it also depends on frequency. Loudness is a measure of sound intensity taking frequency into account, and is called a A-weighted decibel, dB(A), or a phon. This figure shows The Fletcher Munson Chart, which demonstrates the different sound frequencies and decibels that the human ear perceives as the same.

The Fletcher Munson Chart

The Fletcher-Munson equal-loudness contours. Phons are labelled in blue

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