Acoustic Glossary

Sound Level in decibels, is the RMS (root-mean-squared) of the sound pressure measured and averaged over a stated period of time.

Measurements are usually A-weighted to correlate with the human response and are now commonly known as the LAeq when measured by modern integrating sound level meters - see the time-average sound level definition.

Older and more basic sound level meters comply with the earlier:

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 61672 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.

Also known as 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.
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
Also known as the equivalent continuous sound level - not be confused with the time-average sound pressure level (IEC 801-22-11).

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.

To see a range of integrating sound level meters click here.

We also have full pages dedicated to sound level meter basics and Leq basics.

Sound Level Meter Classes - originally known as Types.
Sound Level Meter Class 0 - the requirement for laboratory measurements
Sound Level Meter Class 1 - precision instruments with stricter tolerances than -
Sound Level Meter Class 2 - sometimes referred to as Industrial Grade Meters.

There are also many cheap sound level meters for sale, that do not meet even the Class 2 requirements - buyers beware.

Why are good sound level meters so expensive?