The Cal files Preferences list all the outputs and inputs that have previously been selected in REW with any calibration files that have been specified for them. Choosing an output device and output or input device and input will cause REW to automatically load the associated calibration file. Entries can be deleted if they are no longer needed. Individual calibration files can be added using the Browse buttons or removed using the Clear Cal buttons. The calibration data will be applied to all new measurements taken after it has been loaded and will be shown on the SPL & Phase graph for the measurements. The currently active entries are highlighted. The calibration files that will be used for a new measurement can be seen by using the Cal files button on the Measure dialog.
Soundcard calibration files are specific to the sample rate being used. If the sample rate changes a file for that sample rate will be loaded, if available.
Mic calibration has a C weighting option. Select Input device is a C Weighted SPL Meter if you are using a C weighted SPL meter as the input to REW, measurements will then be corrected to remove the low and high frequency roll-offs of the C weighting characteristic. If a cal file is also loaded the correction will only be applied outside the frequency range covered by the cal file.
Mic cal files can apply to all inputs on a device or, if Separate cal file for each input is selected individual cal files can be specified for the separate inputs of the device. For example, the left and right inputs of a miniDSP EARS headphone measurement system have separate cal files. Note that most USB microphones are mono devices and provide the same data on both left and right channels, a single cal file for the device is appropriate for those.
To apply or remove a calibration file for an existing measurement, use the Change Cal... button in the measurement panel.
To calibrate the REW SPL meter reading against an external meter use the Calibrate button in the SPL Meter controls.
The calibration file is a plain text file which by default has the extension .cal, though other extensions are also accepted. It should contain the actual gain (and optionally phase) response of the meter or microphone at the frequencies given, these will then be subtracted from subsequent measurements. The values in the calibration file can be separated by spaces, tabs or commas. Typically the values are relative to the level at some reference frequency, e.g. 1kHz, so the gain value there is 0.0.
SPL Meter Calibration data 20 -15.38 50 -3.69 100 -1.34 200 -0.62 500 -0.26 1000 0.0 2000 1.80 5000 3.95 10000 -0.71 20000 -6.28
USB microphone cal files will typically have a line at the start of the file that
defines their sensitivity, of the form:
"Sens Factor =1.2345dB, SERNO: XXXXXXX"
The sens factor figure is usually the input dBFS reading the mic will produce when driven by a 94 dB calibrator with the input volume set to maximum.
Analog microphone cal files can also include sensitivity information. Add a line at the
start of the file of the form:
Sensitivity -12.34 dBFS