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Sound Pressure Level

Definition

The sound pressure level (SPL) is a logarithmic measure of the effective pressure of a sound relative to a reference value. It is defined as

\[ L_p = 20 \cdot log_{10} \left( \frac{p}{p_0} \right) \]

where

  • \(p\) is the root mean square sound pressure
  • \(p_0\) is the reference sound pressure

The commonly used reference sound pressure in air is

\[ p_0 = 20 \mu \text{Pa} \]

which is often considered as the threshold of human hearing. (In other words, 20 µPa corresponds to 0 dB SPL)

Note that different values of \(p_0\) may be used for different transmission media.

Weighted Sound Level

When measuring the sound level for certain purposes, a weighting curve (in frequency) may be applied to emphasize or de-emphasize the sound energy in different frequency band. For example, when measuring the loudness of noise, the "A-weighting" curve may be applied to reflect the human ear response (high response around 1k-10kHz, low response elsewhere).

A-weighting Sound Level

The A-weighting sound level is the sound level measured with the A-weighting curve. The measurement is denoted in the unit of "dB(A)" or "dBA"

Reference