Sound Levelin decibels, is typically the RMS (root-mean-squared) of the sound pressure level which was measured and averaged over a stated period of time. The measurement may also be A-weighted for example to correlate with the subjective human response and unless some time-weighting or other rule of average is specified the measurement is now commonly known as the Leq - equivalent continuous sound level as measured by modern integrating sound level meters.
Sound Level Definition IEC 801-22-14,logarithm of the ratio of a given sound pressure to the reference sound pressure of 20 μPa, the sound pressure being obtained with a standard frequency weighting and with a standard exponential time weighting. Sound level in decibels is twenty times the logarithm to the base ten of that ratio
● Note 1 : standard frequency weightings A, B and C and standard exponential time weightings fast (F), slow (S) and impulse (I) are given in IEC 651 Sound level meters.
● Note 2 : time and frequency weightings employed should be specified, but if not stated explicitly, Fast (F) exponential time weighting and A-frequency weighting are understood.
Sound Level is also known the weighted sound pressure level
Time-average Sound Level Definition IEC 801-22-16,logarithm of the ratio of a given time-mean-square standard-frequency-weighted sound pressure for a stated time period, to the square of the reference sound pressure of 20 μPa. Time-average sound level in decibels is ten times the logarithm to the base ten of that ratio. Also known as the equivalent continuous sound level and the Leq.
● Note 1 : If a frequency weighting is not specified, the A-frequency weighting is understood
● Note 2 : In principle, exponential time weighting is not involved
Not be confused with the Time-average Sound Pressure Level Definition, IEC 801-22-11. To compare these definitions directly click here
Sound Level Terms in Common Use
Integrating Sound Level Meters, a little history. In earlier times the instantaneous sound pressure levels were exponentially averaged in a simple RMS detector of limited dynamic range and the results were further dependent on the selected meter time-constant settings, so a series of measurements could not be combined to get the overall result. The only way to 'measure' and 'record' time histories, at that time, was to connect the meter output voltage to a logarithmic level recorder and plot the dB level vs time histories.
With the advent of digital techniques it was possible to integrate the sound levels over a wide dynamic range, use linear averaging circuits over long measurement periods and the integrating averaging sound level meter was born, now commonly known as the Leq or equivalent continuous sound level meter
Integrating circuits also greatly enhance the accuracy and dynamic range of L10, L90, Ln etc., statistical noise measurements.
Sound Level Meter Classes: sound level meters are graded into classes, originally known as 'types'.
There are also many cheap sound level meters for sale, that do not meet even the Class 2 requirements - buyers beware.
See also • noise dose • sound exposure • sound intensity • sound power