The **sound level pressure** (SPL) or **acoustic pressure level** is a logarithmic measure of the effective pressure of a sound relative to a reference value: $ L_{p} = 20 \log_{10}\left(\frac{p}{p_{0}}\right) $ where $L_{p}$ is the sound level pressure in [[deciBel]], $p$ is the root mean square of the sound pressure and $p_{0}$ is a reference sound pressure. The reference sound pressure corresponds to $0dB$ is the [[threshold of hearing]]: a pure tone of $1 kHz$ with a pressure of $p_{0}=20\ \micro Pa$. A typical sound pressure captured by a close-talking microphone at 2.5 cm from the talker is $p=1Pa$ that corresponds to $L_{p} = 94 dB$ SPL. Assuming a spherical wave, the sound's intensity is inversely proportional to the square of the radius, $r^2$, so that every time the distance is doubled, the sound level pressure is reduced by $6dB$. This means that at a distance of 25 cm, the sound level pressure is $L_{p}=74\ dB$.