sound pressure level
PrintThe level, in decibels, of acoustic pressure waves.
Sound pressure is the dynamic variation of the static pressure of air and is measured in force per unit area. Sound pressure is normally represented on a logarithmic amplitude scale, which gives a better relationship to the human perception of hearing. Typical values on this open-ended scale are a sound level of 0 dB, which is the average threshold of human hearing, and 60 to 70 dB for normal conversation, 110 dB at an extremely loud concert, and 150 dB for the noise of a rocket takeoff or a jet engine at close range.
You can measure the sound pressure level with various “weighting filters” whose characteristics approximate the sensitivity of the ear at various frequencies and levels based on the Fletcher-Munson curves. Most commonly used is the “A Weighting,” which is most sensitive in the mid-frequencies. Other, less common weightings are B, C, and D.
The Sound Pressure Level (SPL or LP) in dB is defined as:
SPL = 20 log10(p/pref)
Where p = the actual sound pressure
pref = 20 µPa, which roughly corresponds to the threshold of human hearing
Important Note: Because of the many factors potentially influencing a decibel reading, be careful when reporting acoustical data. Without a complete description of all the instrument settings during the measurement, the information could be ambiguous. A complete statement would include the following elements:
- The level as a numeric, as NN.N (Note that one decimal place is sufficient precision for almost any test.)
- dB for decibels or B for Bels
- SPL for sound pressure level, PWL for sound power level, SIL for sound intensity level
- The reference quantity used in the level computation, as appropriate (for example, 20 ìPa)
- Frequency weighting filter used
- Time weighting used
- Octave or one-third octave band filter used
For example, a complete description of an acoustics measurement is: “85 dB SPL re 20 ìPa A-weighted, Slow”, “47 dB PWL re 1 pW flat-weighted in the 250 Hz one-third octave band, equivalent continuous level.”
Units:
Pa or dB re 20 µPa
Additional References
Related NI Products:
International Standards:
Helpful Web Sites:
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