Signal Processing/Statistics

How is SEL calculated?

Sound Exposure Level (SEL) is calculated by summing the RMS Sound Pressure levels for each sampling interval over the selected time duration and referencing back to a 1-second duration. Therefore, SEL is related to RMS_SPL by: SEL = RMS_SPL + 10·Log10T where T is in seconds for the selected time duration. SEL is the sound level that has the same acoustic energy in one second as the sound over the selected time duration. Note that the RMS level is also known as Leq (equivalent continuous sound level) is some fields of acoustics.

How are the Exposure measures calculated?

Clicking on the ‘Exposure’ button brings up the Exposure calculation window.

Exposure levels and durations are calculated using the RMS sound pressure levels (weighted or unweighted) from the LVX files (processed wave files).

The Sound pressure levels are first converted to energy levels, and linear interpolation is used over the LVX sampling interval to calculate durations and levels.

Note

The LVX sampling interval can affect results, especially for T90 measures of individual impulsive sounds. To obtain the T90 values from short pulse, use the Signal module.

Why don’t the 1/3 Octave band levels add to the RMS level for a particular time slice?

The most important reason for this for an ‘unweighted’ signal is that the third octave bands presented may not cover the whole spectrum of the input signal.

There will be signal energy below the minimum and maximum third octave bands.

The degree of this is influenced by the low and high pass prefiltering set in dBWav’s ‘Set Up’ dialog window.

For an ‘unweighted’ signal the RMS value (and the Lpk value) is calculated after the low and high pass prefiltering.

Note

For any weighted signal the RMS value is reconstructed from the third octave bands, and Lpk is not calculated.