Sound intensity and decibel level
A decibel value needs a reference and a specified measured quantity.
Key knowledge
Sound intensity is average acoustic energy flux per area. A progressive sinusoidal plane wave in a lossless fluid has I=p_peak²/(2ρc_s)=p_rms²/(ρc_s). Sound intensity level is L_I=10log₁₀(I/I_ref), commonly with I_ref=10⁻¹² W m⁻². A tenfold intensity increase is 10 dB; intensity level, sound-pressure level and perceived loudness are distinct.
Values and equations
I=p_peak²/(2ρc_s)=p_rms²/(ρc_s); L_I=10log₁₀(I/I_ref)
- I,I_ref
- Acoustic intensity and positive reference intensity W m⁻²
- p_peak,p_rms
- Peak and rms pressure fluctuations Pa
- ρ
- Equilibrium fluid density kg m⁻³
- c_s
- Sound speed m s⁻¹
- L_I
- Intensity level dB
Conditions and scope
- Linear progressive plane wave, stated averaging interval, fluid density ρ>0 and sound speed c_s>0; p_peak is the pressure-fluctuation amplitude and p_rms=p_peak/√2 for a sinusoid.
- The level requires I>0 and I_ref>0. Standing waves/near fields do not generally satisfy the progressive-wave intensity–pressure relation; the reference is a convention, not a universal hearing threshold.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read §17.3 intensity17.8, sinusoidal pressure/intensity17.11 and level17.12. RMS relation is the sinusoidal mean-square identity; exclude universal hearing-threshold and universal loudness claims.
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Entry details
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- foundation-physics-sound-intensity-level
- Revision
- foundations-refresh.1
- Structured reference record
- Foundational reference additions