Some sound measuring instruments use the letter 'Z' as an indication of linear SPL. Unweighted sound pressure level is called 'linear sound pressure level' and is often written as dB L or just L. B-weighted sound pressure level is written either as dB B or L B, and C-weighted sound pressure level is written either as dB C or L C. In order to distinguish the different sound measures, a suffix is used: A-weighted sound pressure level is written either as dB A or L A. Because the frequency response of human hearing changes with amplitude, three weightings have been established for measuring sound pressure: A, B and C. Humans do not perceive low- and high-frequency sounds as well as they perceive sounds between 3,000 and 4,000В Hz, as shown in the equal-loudness contour. Human hearing does not have a flat spectral sensitivity ( frequency response) relative to frequency versus amplitude. Д®qual-loudness contour, showing sound-pressure-vs-frequency at different perceived loudness levelsД®ars detect changes in sound pressure. if the thermodynamic properties of the air are disregarded, in reality the sound waves become progressively non-linear starting over 150В dB), larger sound waves can be present in other atmospheres or other media, such as underwater or through the Earth. While 1 atm ( 194 dB peak or 191 dB SPL) is the largest pressure variation an undistorted sound wave can have in Earth's atmosphere (i.e. The lower limit of audibility is defined as SPL of 0 dB, but the upper limit is not as clearly defined. Most sound level meters provide readings in A, C, and Z-weighted decibels and must meet international standards such as IEC 61672-2013. The main instrument for measuring sound levels in the environment is the sound level meter. These references are defined in ANSI S1.1-2013.

In other media, such as underwater, a reference level of 1 ОјPa is used. Most sound-level measurements will be made relative to this reference, meaning 1 Pa will equal an SPL of 94 dB. The proper notations for sound pressure level using this reference are L p/(20 ОјPa) or L p (re 20 ОјPa), but the suffix notations dB SPL, dB(SPL), dBSPL, or dB SPL are very common, even if they are not accepted by the SI. Which is often considered as the threshold of human hearing (roughly the sound of a mosquito flying 3В m away).

Db sound reference