SPL
A logarithmic measure of sound pressure relative to the threshold of human hearing, expressed in decibels.
Sound Pressure Level (SPL) quantifies the strength of a sound wave as the pressure variation it creates in air, measured in decibels (dB). The decibel scale is logarithmic because human hearing responds logarithmically to pressure changes.
The reference pressure (0 dB SPL) is 20 micropascals, approximately the quietest sound a healthy young person can hear. Each 10 dB increase represents a 10× increase in sound intensity, but is perceived as roughly "twice as loud."
Common SPL levels: • 0 dB: Threshold of hearing • 30 dB: Quiet room • 60 dB: Normal conversation • 85 dB: Prolonged exposure risk begins • 100 dB: Power tools, loud music • 120 dB: Threshold of pain
SPL measurements often use weighting filters (A, C, or Z) to match different applications. A-weighting approximates human hearing sensitivity and is common for noise regulations. C-weighting is flatter and better for music and peak measurements.
Formula
SPL = 20 × log₁₀(p / p₀)- SPL = Sound Pressure Level (dB)
- p = Measured Pressure (Pa)
- p₀ = Reference Pressure (20 µPa)
Practical Example
Measuring studio monitor output
If pressure is 0.2 Pa: SPL = 20 × log₁₀(0.2 / 0.00002) = 80 dB80 dB SPL is a comfortable mixing level, loud enough for detail but safe for extended sessions.
Standards: IEC 61672-1
Related Terms
Frequency Response · RT60
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