Decibel Shield
FAQ › Glossary
What is sound pressure level (SPL)?
Sound pressure level, written SPL or Lp, is 20 log₁₀(p/p₀) in decibels, where p is the measured sound pressure and p₀ is 20 micropascals, the quietest sound a young ear can detect. Doubling the pressure adds 6 dB, ten times the pressure adds 20 dB, and about 10 dB feels twice as loud. Every reading on a decibel meter is an SPL.
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The formula
SPL = 20 log₁₀(p / p₀), with p₀ = 20 µPa in air. The reference is the approximate threshold of hearing at 1 kHz, so 0 dB is not silence but the quietest audible sound. Because the scale is logarithmic, the range from a whisper to a jet engine, a pressure ratio of about a million, fits into 120 dB. OSHA defines the term the same way in its technical manual.
| Change | Sound pressure | Sound intensity | Perceived loudness |
|---|---|---|---|
| +3 dB | ×1.4 | ×2 | Just noticeable |
| +6 dB | ×2 | ×4 | Clearly louder |
| +10 dB | ×3.2 | ×10 | About twice as loud |
| +20 dB | ×10 | ×100 | About four times as loud |
Sources: OSHA Technical Manual, Section III Chapter 5; NIOSH, Noise and Hearing Loss. See the decibel levels chart for sourced examples at each level.
SPL, weighting and time
A plain SPL counts every frequency equally. Meters usually apply a weighting filter, which is why readings carry a suffix: dBA or dBC. They also average over time, fast (125 ms) or slow (1 s), and can report an equivalent continuous level, Leq, over a whole measurement. Our online decibel meter shows an A-weighted SPL with slow response.
What is 0 dB SPL?
The reference pressure of 20 micropascals, close to the softest 1 kHz tone a healthy young ear can hear. It is not the absence of sound; a very quiet room sits around 20 to 30 dB and negative SPL values are possible in an anechoic chamber.
What is the difference between SPL and dB?
A decibel is a ratio; SPL is a decibel value with a stated reference, 20 µPa. Sound power level and sound intensity level use different references (1 picowatt and 1 picowatt per square metre), so the same source has different numbers on each scale.
Why does doubling the sound pressure give 6 dB and not 3 dB?
Intensity goes with the square of pressure. The formula uses 20 log for pressure and 10 log for intensity so that both describe the same change: doubling pressure quadruples intensity, and both work out to 6 dB.
What SPL is dangerous?
NIOSH recommends keeping the 8-hour average below 85 dBA, halving the allowed time for every 3 dB above it, and treats 140 dB peak as a ceiling for impulsive sound. See the safe decibel levels page for the full table.