Sound pressure is the local deviation from ambient atmospheric pressure caused by a sound wave passing through the air. When a loudspeaker cone pushes forward, it compresses the air molecules in front of it, creating a region of slightly higher pressure. When it pulls back, it creates a region of slightly lower pressure. These alternating compressions and rarefactions propagate outward as a pressure wave — and the magnitude of that pressure variation is what we call sound pressure.
Sound pressure is what your eardrum detects, what microphones measure, and what ultimately determines how loud a sound seems. It is the most fundamental physical quantity in acoustics, measured in Pascals (Pa) and almost always expressed on a logarithmic scale as decibels of sound pressure level (dB SPL).
Real-World Analogy
Imagine standing at the edge of a lake on a windy day. The wind creates waves — the water surface rises and falls around its average level. The height of those waves above (or below) the average level is analogous to sound pressure — it is the deviation from the calm baseline.
A gentle breeze creates tiny ripples (low sound pressure — quiet). A strong gust creates large waves (high sound pressure — loud). In both cases, the average water level has not changed — it is only the variation around the average that differs. Sound pressure works identically: the average atmospheric pressure (about 101,325 Pa at sea level) does not change, but the sound wave creates tiny fluctuations above and below that average.
Technical Definition
Sound pressure (p) is measured in Pascals (Pa), where 1 Pa = 1 Newton per square metre. The range of sound pressures that the human ear can detect is enormous:
- Threshold of hearing: approximately 0.00002 Pa (20 micropascals, written 20 muPa)
- Normal conversation at 1 m: approximately 0.02 Pa
- Loud rock concert: approximately 2 Pa
- Threshold of pain: approximately 20 Pa
- Jet engine at 1 m: approximately 200 Pa
Sound Pressure Level (SPL)
Sound pressure level converts the linear Pascal scale into a logarithmic decibel scale:
L_p = 20 x log10(p / p_ref)
Where p is the measured sound pressure and p_ref is the reference sound pressure of 20 muPa (the approximate threshold of human hearing at 1000 Hz), standardised in IEC 61672-1:2013.
This gives us the familiar dB SPL values:
| Sound Pressure (Pa) | SPL (dB) | Example |
|---|---|---|
| 0.00002 | 0 dB | Threshold of hearing |
| 0.0002 | 20 dB | Quiet rural area |
| 0.002 | 40 dB | Quiet library |
| 0.02 | 60 dB | Normal conversation |
| 0.2 | 80 dB | Busy traffic |
| 2 | 100 dB | Nightclub |
| 20 | 120 dB | Threshold of pain |
Every 20 dB increase corresponds to a tenfold increase in sound pressure. Every 6 dB increase approximately doubles the sound pressure.
RMS and Peak Values
Sound pressure oscillates continuously. The standard measurement uses the root-mean-square (RMS) value, which represents the effective average pressure over time. Peak sound pressure can be 1.4 times the RMS value for a pure tone, or much higher for impulsive sounds like gunshots or hammering.
Why It Matters for Design
Sound pressure level is the primary metric for:
Noise criteria compliance. Standards like NR, NC, and RC curves specify maximum allowable sound pressure levels in octave bands. HVAC systems, traffic noise, and industrial sources are all assessed by the SPL they produce at the receiver position. AcousPlan's noise criteria checker evaluates measured or predicted SPL against these thresholds.
Background noise assessment. The background noise level in a room — the SPL present when no intentional sound source is active — determines the signal-to-noise ratio available for speech. The Speech Transmission Index (STI) depends directly on the difference between speech level and background noise level at each frequency.
Hearing conservation. Occupational noise exposure limits (85 dB(A) for 8 hours per most regulations) are specified in terms of sound pressure level. Acoustic design for workplaces, music venues, and entertainment facilities must consider both the SPL experienced by occupants and the duration of exposure.
Room acoustic predictions. When AcousPlan calculates the sound field in a room, the underlying physics operates in terms of sound pressure and sound energy density. The reverberation time describes how quickly the sound pressure level decays after a source stops — by definition, RT60 is the time for SPL to drop by 60 dB.
Measurement and verification. Sound level meters measure SPL, and every acoustic measurement — from RT60 testing to noise surveys — ultimately reduces to measuring sound pressure and expressing it in decibels.
How AcousPlan Uses This
AcousPlan works with sound pressure level throughout its calculation pipeline. The RT60 definition itself is based on a 60 dB decay in SPL. The noise criteria module evaluates octave-band SPL values against NR, NC, and RC curves. The STI calculator uses the signal-to-noise ratio (in dB) at each octave band to determine speech intelligibility.
When you enter background noise levels into the calculator, you are entering sound pressure levels. When the results show compliance or non-compliance with a noise criterion, that assessment is based on comparing your SPL values against the standard's threshold curve.
The speech privacy calculator uses SPL directly — it calculates the received speech level after transmission loss and distance attenuation, then compares it to the background noise SPL to determine the Articulation Index and privacy rating.
Related Concepts
- What is a Decibel (dB)? — The logarithmic scale used to express sound pressure level
- What is Noise? — Unwanted sound, quantified by its sound pressure level
- What is Frequency in Acoustics? — Sound pressure varies with frequency
- What Are Octave Bands? — How SPL is analysed across the frequency spectrum
- What is Transmission Loss? — The reduction of SPL through a barrier
Calculate Now
Ready to evaluate sound pressure levels in your room? Use the AcousPlan Room Calculator to assess noise criteria compliance and speech intelligibility based on your measured or estimated SPL values.