94
dB SPL
0.00242131
W/m²
50,000
94
dB SPL
0.00242131
W/m²
50,000
The Sound Pressure Level (SPL) Calculator converts a measured sound pressure value into its decibel representation relative to the standard reference pressure. SPL is the most commonly used quantity in acoustics for describing the loudness of sounds, noise levels in workplaces, and the output of audio equipment. It is measured using sound level meters and forms the basis of noise regulations and hearing conservation standards worldwide.
Sound pressure level is defined as:
$$L_p = 20 \cdot \log_{10}\left(\frac{p}{p_0}\right) \text{ dB SPL}$$
where $$p$$ is the measured root-mean-square (RMS) sound pressure in pascals, and $$p_0 = 20 \times 10^{-6}$$ Pa (20 μPa) is the standard reference pressure, corresponding approximately to the threshold of human hearing at 1 kHz.
The factor of 20 is used because pressure is a field quantity (amplitude), and power is proportional to pressure squared. Equivalently, this can be written as:
$$L_p = 10 \cdot \log_{10}\left(\frac{p^2}{p_0^2}\right) \text{ dB SPL}$$
The calculator also estimates the equivalent sound intensity using the plane-wave relationship:
$$I = \frac{p^2}{\rho c}$$
where $$\rho c \approx 413$$ rayl (Pa·s/m) is the specific acoustic impedance of air at standard conditions ($$\rho = 1.204$$ kg/m³, $$c = 343$$ m/s).
Common SPL reference points: 0 dB SPL is the threshold of hearing (20 μPa), 30 dB is a quiet room, 60 dB is normal conversation, 85 dB is the threshold for hearing damage with prolonged exposure, 120 dB is the threshold of pain, and 194 dB is the theoretical maximum for a sound wave in air (where the pressure amplitude equals atmospheric pressure). A 1 Pa RMS sound pressure corresponds to 94 dB SPL — this is often used as a calibration standard for microphones.
Inputs
Results
SPL = 20 × log₁₀(1/0.00002) = 20 × log₁₀(50000) = 94 dB. This is the standard calibration level used for microphone sensitivity specifications.
Inputs
Results
SPL = 20 × log₁₀(0.02/0.00002) = 20 × log₁₀(1000) = 60 dB. Typical of normal conversation at about 1 meter distance.
The reference pressure of 20 μPa (2 × 10⁻⁵ Pa) was chosen to approximate the threshold of human hearing at 1 kHz — the frequency where our ears are most sensitive. This standardization (per IEC 61672) ensures that 0 dB SPL represents the quietest detectable sound for a typical young person with healthy hearing.
The theoretical maximum SPL in air is about 194 dB, where the pressure amplitude equals atmospheric pressure (~101,325 Pa). Beyond this, the wave becomes a shock wave rather than a linear sound wave. In practice, levels above 185 dB cause nonlinear distortion effects.
A 10 dB increase in SPL is perceived as roughly a doubling of loudness. However, this relationship is frequency-dependent. Human ears are less sensitive to low and very high frequencies, which is why A-weighting (dBA) is used for noise measurements to approximate human perception.
dB SPL is an unweighted measurement of sound pressure. dBA applies an A-weighting filter that reduces the contribution of low frequencies (below ~500 Hz) and very high frequencies (above ~6 kHz), mimicking the frequency response of human hearing. Most noise regulations specify limits in dBA.
Prolonged exposure above 85 dBA can cause noise-induced hearing loss. OSHA limits workplace exposure to 90 dBA for 8 hours, with halving of allowed time for each 5 dB increase. Brief exposure above 120 dB can cause immediate damage. Always wear hearing protection in loud environments.
RMS (root-mean-square) provides a meaningful average of the oscillating pressure waveform. Peak pressure fluctuates rapidly, but RMS represents the effective value that corresponds to the time-averaged energy of the sound wave. For a sine wave, the RMS value is the peak divided by $$\sqrt{2}$$.
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