Short answer: Sound pressure level (SPL) is the strength of a sound’s pressure change expressed in decibels, relative to 20 micropascals, the quietest sound a healthy young ear can hear. The formula is SPL = 20 × log10(p ÷ 20 µPa), so 0 dB SPL is the threshold of hearing, 60 dB SPL is conversation (0.02 Pa) and 94 dB SPL is 1 pascal.
SPL is the number a sound level meter shows, and the basis of every decibel chart. This guide explains what it means, how to calculate it, how it differs from loudness, intensity and sound power, and how to measure it. You can convert any value between dB SPL and pascals with the SPL converter.
What Does Sound Pressure Level Mean?
Sound travels as tiny, rapid changes in air pressure above and below atmospheric pressure. Sound pressure is the size of those changes, measured in pascals (Pa) as an RMS (root-mean-square) value. Because audible pressures range from 0.00002 Pa to over 60 Pa, a factor of about three million, acoustics uses a logarithmic scale instead: the sound pressure level in decibels. Every 20 dB multiplies the pressure by 10; every 6 dB doubles it. The logarithmic decibel scale guide explains why.
The SPL Formula
- SPL (dB) = 20 × log10(p ÷ p₀), where p is the RMS sound pressure and p₀ = 20 µPa in air (1 µPa underwater).
- p = p₀ × 10^(SPL ÷ 20) to go back from decibels to pascals.
- Example: 0.2 Pa → 20 × log10(0.2 ÷ 0.00002) = 20 × 4 = 80 dB SPL.
SPL Table: From 0 to 140 dB
| dB SPL | Sound pressure | Example |
|---|---|---|
| 0 | 20 µPa | Threshold of hearing |
| 20 | 200 µPa | Rustling leaves |
| 40 | 2 mPa | Quiet library |
| 60 | 20 mPa | Normal conversation |
| 70 | 63 mPa | Vacuum cleaner |
| 80 | 0.2 Pa | Heavy traffic |
| 90 | 0.63 Pa | Petrol lawn mower |
| 94 | 1 Pa | Microphone calibrator |
| 100 | 2 Pa | Car horn at 5 m |
| 120 | 20 Pa | Siren up close, threshold of discomfort |
| 130 | 63 Pa | Threshold of pain |
| 140 | 200 Pa | Jet engine at 30 m |
Select a level to read what it means for hearing and how long it is safe, or see many more everyday sounds on the decibel chart.
SPL vs Loudness
SPL is physical; loudness is how strong a sound seems to you. Human hearing is most sensitive between about 2 and 5 kHz, so a 100 Hz tone needs a much higher SPL than a 1 kHz tone to sound equally loud. Loudness is measured in phons and sones. To approximate loudness in a single number, meters apply A-weighting and show dB(A); see dB vs dBA.
SPL vs Sound Intensity vs Sound Power
| Quantity | Unit | What it describes |
|---|---|---|
| Sound pressure level (SPL) | dB re 20 µPa | The sound at one spot, what a meter measures; it falls with distance |
| Sound intensity level | dB re 1 pW/m² | Energy flow per square metre; in open air it is almost equal to SPL |
| Sound power level (Lw) | dB re 1 pW | Total sound a source emits, independent of distance; used on machine and appliance labels |
That is why an appliance rated “55 dB sound power” often measures less at your listening position. For the full family of decibel units, see dB vs dBm vs dBW.
SPL, dBA and dBFS: Which Is Which?
- dB SPL: unweighted sound pressure level in air.
- dB(A): SPL with A-weighting, used for noise limits and hearing safety.
- dBFS: a digital level in a recording or on a sound card, relative to full scale, with no fixed link to SPL until the microphone’s sensitivity is known; see dB vs dBFS.
Peak, RMS and Average SPL
Sound meters show several versions of SPL. The RMS level with Fast (125 ms) or Slow (1 s) time weighting is the normal reading. Lmax is the highest RMS level in a period, Lpeak the highest instantaneous pressure (important for gunshots and hammering), and Leq the energy average over the whole measurement. To combine or average several SPL readings correctly, use the decibel calculator.
How to Measure SPL
- Open the free sound decibel meter or use a sound level meter.
- Hold the microphone where the listener’s ears are, away from your body and walls.
- Choose dB(A) for general noise, or dB(C)/dB(Z) for bass and peaks.
- Read the average over 30 seconds or more, not a single moment.
Phones can read within a few dB of a real meter in the everyday range after you calibrate the microphone, ideally with a 94 dB (1 Pa) calibrator. For legal or workplace measurements you need a Class 1 or Class 2 instrument; see sound level meter vs decibel meter and decibel meter vs sound pressure meter.
SPL in Audio Equipment
- Speaker sensitivity is the SPL at 1 m with 1 W (or 2.83 V) input, e.g. 88 dB SPL; every 3 dB more sensitivity halves the amplifier power needed.
- Maximum SPL on a microphone datasheet is the level at which distortion reaches a stated limit; above it, readings are unreliable.
- Headphone sensitivity is often given as dB SPL per mW or per volt, which is why some headphones play much louder than others at the same volume setting.
Frequently Asked Questions
What is a normal SPL?
Indoors, 30–40 dB SPL is quiet, 50–60 dB is typical for a home or office in use, and conversation is about 60 dB. Long exposure above 85 dB(A) risks hearing damage.
What does dB SPL mean?
Decibels of sound pressure level: the level relative to 20 µPa. “SPL” distinguishes it from other decibels such as dBV or dBFS.
Can SPL be negative?
Yes. Sounds quieter than 20 µPa have a negative SPL; some young listeners can hear a few decibels below 0 dB SPL around 3–4 kHz.
What is the maximum possible SPL?
About 194 dB SPL in air at sea level, where the pressure swing equals atmospheric pressure; beyond that the wave becomes a shock wave.