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Collaboration, Voice & AV · Foundational

Sound Pressure Level (SPL)

The acoustic pressure of a sound expressed in decibels relative to 20 micropascals, the threshold of human hearing.

Definition

dB SPL = 20 log10(p / 20 µPa). Normal conversation is about 60 dB SPL at 1 m, and the threshold of pain is roughly 120 to 130 dB SPL. In free field SPL falls about 6 dB for each doubling of distance from a point source. Weightings such as A and C adjust measurements to approximate human hearing.

Example

An SPL meter set to A-weighting reads 45 dBA background noise in an empty conference room.

Exam tip

Use 20 log for SPL changes with distance ratio: doubling distance loses about 6 dB.

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