Sound pressure level (SPL) is a logarithmic measure of the effective (RMS) sound pressure at a point, expressed in decibels relative to a reference pressure of 20 micropascals in air, which approximates the threshold of human hearing at 1 kHz. In acoustics it is the standard way to state how strong a sound is where a listener or microphone is placed.
The defining formula is Lp = 20 · log10(p / p0), where p is the RMS pressure and p0 is 20 µPa. The factor 20 rather than 10 appears because pressure is a field quantity: sound intensity and power scale with the square of pressure, so a doubling of pressure corresponds to about 6 dB.
How sound pressure level is defined
Sound is a small, rapid fluctuation of air pressure around the static atmospheric pressure of about 101,325 Pa. SPL describes only the fluctuating part, averaged as an RMS value over a stated time. Because audible pressures span roughly one million to one between the quietest and the loudest bearable sounds, a logarithmic scale compresses that range to about 0 to 130 dB.
Some reference points help calibrate intuition. The values below are typical and vary with distance and the exact source.
| Situation | Approximate SPL | RMS pressure |
|---|---|---|
| Threshold of hearing at 1 kHz | 0 dB | 20 µPa |
| Quiet recording studio | 20-30 dB | 0.2-0.6 mPa |
| Normal conversation at 1 m | about 60 dB | 20 mPa |
| Busy road at the curb | 75-85 dB | 0.1-0.4 Pa |
| Microphone calibrator reference | 94 dB | 1 Pa |
| Loud rock concert near the stage | 105-115 dB | 3.6-11 Pa |
| Threshold of pain | about 120-130 dB | 20-63 Pa |
The 94 dB = 1 Pa reference is worth remembering. Microphone sensitivity is specified at this level, and acoustic calibrators produce exactly 94 dB or 114 dB at 1 kHz so that measuring systems can be checked in the field.
SPL compared with related acoustic quantities
SPL is often confused with other decibel quantities. The decibel itself is only a ratio; the reference and the measured quantity give it meaning, so a value is incomplete without them.
| Quantity | What it describes | Depends on position | Reference |
|---|---|---|---|
| Sound pressure level | Pressure at a point | Yes | 20 µPa |
| Sound intensity level | Energy flow per area in a direction | Yes | 1 pW/m² |
| Sound power level | Total acoustic output of a source | No | 1 pW |
| Loudness | Perceived strength in phons or sones | Yes, and on the listener | Equal-loudness contours |
| Amplitude | Size of the waveform in any unit | Yes | None by itself |
The difference between sound power and sound pressure is the most practical one. A loudspeaker has a fixed sound power, but the SPL it produces depends on distance, room reflections and direction. A specification such as 98 dB SPL is meaningless without a distance and drive level, for example at 1 m with 1 W.
Frequency weighting and time weighting
Sound level meters apply frequency weighting because human hearing is less sensitive at low and very high frequencies, especially at moderate levels. The standard filters are defined in IEC 61672.
| Weighting | Shape | Typical use |
|---|---|---|
| A-weighting (dBA) | Strong cut below about 500 Hz | Noise regulation, occupational exposure |
| C-weighting (dBC) | Nearly flat, slight roll-off at extremes | Peak levels, concerts with heavy bass |
| Z-weighting (dBZ) | Flat within the meter’s range | Technical analysis, spectrum work |
Time weighting decides how fast the meter responds: Fast (125 ms) follows rapid changes, Slow (1 s) smooths fluctuating noise. For varying sounds, the equivalent continuous level Leq gives the constant level that carries the same energy over a period, for example LAeq,15min for a concert venue limit.
Adding levels and the effect of distance
Decibel values do not add arithmetically. Two uncorrelated sources at equal level produce a total about 3 dB higher, not twice the decibel value; two coherent, in-phase signals can add up to 6 dB at points where they align.
- +3 dB doubles the acoustic power or intensity.
- +6 dB doubles the sound pressure.
- About +10 dB is perceived as roughly twice as loud.
- −6 dB per doubling of distance applies to a point source in a free field (inverse square law).
- −3 dB per doubling of distance applies to a long line source such as a busy road.
Indoors, the inverse square law holds only close to the source. Beyond the critical distance, reflected sound dominates and the level stays nearly constant, which is controlled by the room’s absorption and reverberation.
Measuring SPL correctly
A reliable measurement needs a calibrated meter, a stated weighting and time constant, and a defined position. Mistakes in any of these produce numbers that cannot be compared.
| Mistake | Consequence | Better practice |
|---|---|---|
| No weighting stated | Values differ by 10 dB or more for bass-heavy sound | Always report dBA, dBC or dBZ |
| Meter not calibrated | Unknown offset | Check with a 94 dB calibrator before use |
| Measuring too close to walls | Reflections inflate the reading | Keep distance from surfaces |
| Wind on the microphone | Low-frequency noise raises the level | Use a windscreen outdoors |
| Single reading of varying sound | Unrepresentative value | Use Leq over a defined period |
Phone apps can give rough indications, but their microphones and processing are not class 1 or class 2 instruments. For regulatory or hearing-protection decisions, a meter conforming to IEC 61672 should be used.
In plain words
Sound pressure level tells you how strong a sound is at a particular spot. Sound is tiny, fast changes in air pressure, and SPL measures how big those changes are, using the decibel scale so that whispers and jet engines fit on one manageable ruler.
The scale is not like a normal ruler. Every 10 decibels sounds roughly twice as loud, and moving away from a sound source makes it quieter quickly outdoors. A quiet room sits around 30 dB, normal speech around 60 dB, and long exposure above about 85 dB can damage hearing.
A real-world example
A small music club must keep its average level at the mixing desk below a local limit of 100 dB LAeq over 15 minutes. The sound engineer mounts a class 2 measurement microphone at the desk, checks it with a 94 dB calibrator before doors open and logs both A-weighted and C-weighted levels during the show.
In the first set, the LAeq reaches 101 dB while the C-weighted level is much higher, showing that the bass content dominates. The engineer lowers the subwoofers by 3 dB and trims a harsh vocal range, bringing the LAeq to about 98 dB without making the band sound weaker to the audience.
The log provides documented compliance for the venue, and the staff on the floor stay within a safer exposure range.