Sound power and acoustic properties of a room determine sound pressure and thus the perceived loudness in the room. Auditory impressions in the human ear are caused by sound pressure p which affects the eardrum. The threshold of hearing is located around 2 · 10−4 μbar; the threshold of pain is located around 2 · 10+2 μbar. Due to the complex range of 6 decimal powers and also due to the physiological perception of loudness, the sound pressure level was introduced as a logarithmic scale. The logarithmic ratio of a noise source’s sound pressure p to the hearing threshold pressure p0 is expressed as the sound pressure level LP (often also referred to as sound level) in decibels (dB).
Sound pressure (level)
![](uploads/tx_beleuchtungspraxis/formula.5523.gif)
with the reference pressure (hearing threshold) of
![](uploads/tx_beleuchtungspraxis/formula.5532.gif)
![](https://www.trilux.com/en/lighting-practice/fileadmin/user_upload/Tab_Schalldruck_EN.pdf.png)
Table 3.153: Values for the sound pressure level of typical sound sources