Decibel Shield
SPL calculator (sound pressure level)
Sound pressure level is 20 × log10 of the pressure divided by 20 micropascals, so 1 Pa is 94 dB SPL and 20 Pa is 120 dB. For a loudspeaker, add 10 × log10 of the watts to its sensitivity and subtract 20 × log10 of the distance in metres: an 88 dB speaker on 10 W gives 98 dB at 1 m and about 88 dB at 3 m. Enter pressures, or your speaker, amplifier and seat.
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Pressure and decibels
94.0 dB SPL is 1.002 Pa RMS (re 20 µPa)
Loudspeaker level at a distance
Free-field level at the listener
88.5 dB SPL
Needs about 143 W into that speaker.
Free field, one speaker, no room gain or power compression. A sensitivity quoted at 2.83 V is the same as 1 W only for an 8-ohm load.
Pascals to dB SPL
| Pressure | dB SPL |
|---|---|
| 20 µPa | 0.0 |
| 200 µPa | 20.0 |
| 2 mPa | 40.0 |
| 20 mPa | 60.0 |
| 0.2 Pa | 80.0 |
| 1 Pa | 94.0 |
| 2 Pa | 100.0 |
| 10 Pa | 114.0 |
| 20 Pa | 120.0 |
| 63.2 Pa | 130.0 |
| 200 Pa | 140.0 |
Speaker level by power and distance
| Power | 1 m | 2 m | 4 m | 8 m | 16 m |
|---|---|---|---|---|---|
| 1 W | 88 | 82 | 76 | 70 | 64 |
| 10 W | 98 | 92 | 86 | 80 | 74 |
| 100 W | 108 | 102 | 96 | 90 | 84 |
| 1,000 W | 118 | 112 | 106 | 100 | 94 |
The formulas
Lp = 20 × log10(p / p₀) dB, with p the RMS pressure and p₀ = 20 µPa, the reference pressure for sound in air. A speaker's level at distance d is its sensitivity + 10 × log10(P / 1 W) − 20 × log10(d / 1 m): power is a 10 log quantity and pressure falls with distance as a 20 log one (6 dB per doubling of distance, as on the sound distance calculator). Many speakers quote sensitivity at 2.83 V, which is 1 W only into 8 ohms.
Sources: ISO 1683:2015, Preferred reference values for acoustical levels (sample); UNSW Physclips, dB: What is a decibel?; UIUC Physics 406, Absolute Calibration of Pressure Microphones; University of Salford, Loudspeakers tutorial; Amundson, Theory & Practice: SPL Computation.
To see what these levels sound like, the decibel chart lists everyday sounds from a whisper to a jet, and the decibel meter measures the level where you are. For powers in dBm or dBW, use dBm to watts.
What is 94 dB SPL in pascals?
94 dB SPL is 1 pascal RMS (93.98 dB to be exact), which is why sound level meter calibrators play 94 dB. 114 dB is 10 Pa, 120 dB is 20 Pa and 0 dB is 20 micropascals, the reference pressure.
How loud will my speakers be?
Start from the sensitivity, the level the speaker makes at 1 m with 1 W. Add 10 × log10 of the amplifier watts and subtract 20 × log10 of your distance in metres. An 88 dB speaker on 100 W gives 108 dB at 1 m and about 98 dB at 3 m in free field. The estimate leaves out the room, the speaker's directivity and power compression near full power.
Why does doubling amplifier power add only 3 dB?
Because decibels count ratios: twice the power is 10 × log10(2) = 3 dB. To add 10 dB, the change most people hear as twice as loud, the amplifier needs ten times the power. Moving twice as close to the speaker adds 6 dB for free.
What is the difference between dB SPL and dB(A)?
dB SPL is the unweighted level of the pressure. dB(A) weights it by frequency the way hearing is less sensitive to low and very high sounds, so a bass-heavy sound reads well below its dB SPL in dB(A). Hearing-safety limits such as NIOSH's 85 dB are set in dB(A).
