Sound Pressure Level (SPL) is a measure of the pressure variation in a sound wave compared to a reference level. It is usually expressed in decibels (dB), which is a logarithmic scale. The reference level for SPL is typically 20 micropascals, the threshold of hearing for humans. SPL helps quantify how loud a sound is perceived. For example, normal conversation might be around 60 dB, while a rock concert can reach levels of 110 dB or more. Understanding SPL is important in fields like acoustics, audio engineering, and environmental noise control.