Reference level calculation procedure
A reference level calculation procedure is implemented for use with various reference measurement situations. The procedure is initiated by pressing the Perform computation… button, which opens up a browser window and the user can specify a set of reference impulse response files. These responses must have the same length and sampling frequency. The responses are windowed according to the choice in Window type, and scaled according to the choice in Source distance. Thereafter the energy of each is calculated in bands, and the reference level is found by averaging the energy across all the specified responses. The results are displayed in the Direct energy corrections dialog box, and should be saved for later use and reference. The calculation might take a while.
For a true omni-directional sound source a reference measurement in a single direction would be sufficient. Most practical sources are 12-faced speakers that start to show lobes in the polar pattern around 1kHz. The ISO 3382 standard therefore specifies that a proper reference level measurement shall be made in a free-field, and data from at least 29 directions shall be energy averaged. WinMLS can handle such a reference measurement, as well as any other less accurate reference that might be chosen.
Alternatives may range from the full set measured in a free-field, via fewer directions, increasingly reflective environments, to actual in-situ single direction responses. The user must choose a reference from what is practically possible, and be aware of the resulting accuracy.
The windowing options are shown and explained below.
