What is added to the dB to find the Sound Pressure Level (SPL)?

The correct answer involves understanding the relationship between sound intensity levels expressed in decibels (dB) and sound pressure levels (SPL). When converting from a reference level, specifically from a signal level to SPL, it is standard practice to add 20 dB to the log of the amplitude to obtain the SPL in dB.

The rationale behind this addition is rooted in the formula used to express sound pressure levels: SPL = 20 log10(p/p0), where 'p' is the sound pressure of the sound wave being measured, and 'p0' is the reference sound pressure, usually taken as 20 micropascals in air (the threshold of hearing). This formula essentially emphasizes the logarithmic nature of sound intensity and pressure, which is fundamental in acoustics.

In summary, the addition of 20 dB when calculating the SPL from a sound level is because it adjusts for the logarithmic scale of sound pressures, establishing accurate and meaningful measurements in acoustic terms.