Sound Pressure Level

Air-pressure wobbles leave the cone at 343 m/s, push the eardrum, and the SPL meter reads their size in dB above the 20 uPa reference.

What it is

Sound Pressure Level (SPL) is how loud a sound is in air, measured in decibels relative to a fixed reference pressure.

Key facts

• Unit: decibel SPL (dB SPL), a LOGARITHMIC ratio, not a linear unit
• Reference p0 = 20 micropascals (20 uPa = 0.00002 Pa) = quietest audible sound = 0 dB SPL
• Formula: Lp = 20 x log10(p / p0). Lp = level in dB, p = measured RMS pressure, p0 = 20 uPa reference, log10 = base-10 logarithm
• +6 dB = DOUBLE the sound pressure; +3 dB = DOUBLE acoustic power; -3 dB = HALF power; +10 dB = roughly TWICE as loud to the ear (and 10x intensity)
• Speed of sound in air = 343 m/s at 20 degrees C (about 1235 km/h)
• Inverse-square law: every DOUBLING of distance from a point source loses 6 dB (1m to 2m = -6 dB)
• 0 dB = threshold of hearing; 120-130 dB = threshold of pain
• WorkSafe AU limit: 85 dB(A) for 8 h max; every +3 dB HALVES safe time (88=4h, 91=2h, 94=1h); peak cap 140 dB(C)
• Reference levels: conversation approx 60 dB; loud gig 100-110 dB; jet at 30 m approx 120 dB
• Weightings: dB(A) mimics ear sensitivity (noise limits); dB(C) flatter (peaks/bass). Intensity is proportional to pressure squared, so 20log(pressure) = 10log(power)

How it works

1. Sound = air pressure wobbling above and below normal atmospheric pressure.
2. A mic or SPL meter measures the RMS size of those pressure wobbles in pascals.
3. That pressure is divided by the 20 uPa reference, then run through 20 x log10.
4. The log scale squashes a huge range (1 to 1,000,000x) into a readable 0-120 dB.
5. Set the meter to dB(A) slow for venue limits, dB(C) peak for transient/bass checks.
6. Read the number against the venue cap or the 85 dB 8-hour exposure rule.

Real examples

• Quiet recording studio ambience: about 20-30 dB SPL.
• Normal conversation at 1 m: about 60 dB SPL.
• Busy bar / loud restaurant: about 85-90 dB SPL.
• Front-of-house at a live gig: 100-110 dB SPL (where ear-damage timers start).
• Standing about 25 m from a jet at takeoff: around 140 dB SPL, instant pain and damage.

How it helps in live sound

• Carry a calibrated SPL meter (or a checked phone app) and log dB(A) Leq across the show, not just peaks.
• Many WA venues cap around 100 dB(A) over 15 min; know the cap BEFORE doors open or you get shut down.
• Use the 6 dB-per-doubling rule: measure at the mix position, not the front row.
• Hand out earplugs and protect your own ears: 100 dB is safe for only 15 min before risk.
• Watch dB(C) for sub/bass complaints from neighbours; low end carries furthest and trips noise limits.
• Tune for +6 dB headroom before clipping, then you can push transients safely.