Sound Pressure Level (SPL)
Definition
Sound Pressure Level (SPL)
Sound Pressure Level is the logarithmic ratio of measured acoustic pressure to the reference pressure of 20 micropascals, expressed in decibels. Defined by IEC 61672-1:2013, SPL is the fundamental quantity for all acoustic measurements. SonaVyx measures SPL in real time through your browser microphone with A, C, and Z frequency weighting.
SPL = 20 × log₁₀(p / p₀) dB, where p₀ = 20 μPa (threshold of hearing)
How SPL Is Measured
SPL is measured using a calibrated microphone connected to a sound level meter that applies frequency weighting (A, C, or Z) and time weighting (Fast 125 ms, Slow 1 s, or Impulse 35 ms attack). IEC 61672-1:2013 Clause 5 specifies the performance requirements for Class 1 (laboratory) and Class 2 (field) instruments. SonaVyx implements the complete IEC 61672 signal processing chain in Rust WASM, including digital IIR A-weighting and C-weighting filters.
Practical Example
A venue manager needs to verify that a live concert does not exceed the 96 dBA LAeq,15min limit imposed by local noise ordinances. Using SonaVyx with a calibrated microphone at the mixing position, the engineer monitors A-weighted Leq in real time. When the running average approaches 94 dBA, a visual alert warns the FOH engineer to reduce system level before the limit is breached.
The Decibel Scale
Because human hearing spans an enormous pressure range — from 20 micropascals (threshold of hearing) to 200 pascals (threshold of pain) — the decibel scale compresses this ten-million-to-one ratio into a manageable 0 to 140 dB range. Each 6 dB increase represents a doubling of sound pressure, while each 10 dB increase is perceived as roughly twice the loudness. This logarithmic relationship matches the nonlinear sensitivity of human auditory perception.
Frequency Weighting
Raw SPL measurements treat all frequencies equally, but human hearing is less sensitive to low and very high frequencies. A-weighting applies a filter curve that approximates the inverse of the ear's frequency response at moderate levels, attenuating frequencies below 500 Hz and above 6 kHz. C-weighting is nearly flat, with slight roll-off below 31.5 Hz and above 8 kHz, used for peak measurements and low-frequency assessments. Z-weighting (zero weighting) applies no filtering.
Time-Weighted Metrics
IEC 61672-1 defines three time weightings. Fast (125 ms time constant) tracks rapid level changes and is standard for general measurements. Slow (1 s time constant) smooths fluctuations for reading stability. Impulse (35 ms attack, 1500 ms decay) captures peak transients and is used for impact noise assessment. The equivalent continuous level Leq integrates energy over the entire measurement period, providing a single representative value.
Statistical Levels
Statistical analysis of SPL over time yields percentile levels: L10 is the level exceeded 10% of the measurement period (indicating loudest events), L50 is the median level, and L90 is exceeded 90% of the time (indicating background noise). These metrics are essential for environmental noise assessment per ISO 1996 and help characterize the variability of noise exposure in workplaces and communities.
Calibration
Accurate SPL measurement requires calibrating the microphone against a known reference level. IEC 61672-1 Clause 5.7 specifies calibration using a sound calibrator producing 94 dB or 114 dB at 1 kHz. SonaVyx supports 94 dB calibration with per-frequency correction curves loaded from standard .cal files, enabling measurement accuracy approaching Class 2 specifications when used with an external calibrated microphone.
Try It Now
Measure Sound Pressure Level — free SPL meter in your browser