Meter menu

Compensate for C Weighting
SPL measurements use weighting curves to shape the signal they receive and emphasise those regions that are of interest for certain requirements. The most common curves are shown in the figure below.
Frequency weighing filter characteristics
C weighting gives the broadest response (apart from the "Flat" characteristics which many meters do not offer) but it has a substantial low frequency roll off due to a pair of first order poles at 20.6Hz. The Wizard can compensate for the effects of C weighting during its measurements by taking account of the weighting curve characteristic and adding the curve's attenuation at the frequency being measured to the SPL value being displayed.

This feature is enabled by default, to disable it (for example, if you are using a microphone and preamplifier with a flat response, or your meter has a Flat setting) uncheck the entry in the Meter menu. C weighting compensation is only applicable during sine wave measurement and is not active when the signal generator is turned off. If a mic/meter calibration file has been loaded C weighting compensation is ignored within the range of the cal file data but taken into account above and below the cal data range.

Note that the analogue output of the popular Radio Shack Analog meter passes through the meter's weighting network and so C weighting compensation should be turned on when using it. However, there have been some reports that the Radio Shack Digital meter's analog output does not pass through the weighting network, in which case C weighting compensation should be turned off that conflicts with the published schematics of the meter, however.

The InverseC indicator below the SPL reading is lit when C weighting compensation is active.
Track Generator Frequency
The accuracy of SPL measurements at a particular frequency can be improved by selecting the Track generator frequency item in the Meter menu. This changes the way the SPL figure is calculated, switching from direct calculation of the RMS value of the input data samples to carrying out a single-point Discrete Fourier Transforms at the current measurement frequency over sequence lengths that are exact multiples of the measurement signal period. The SPL figure is the magnitude of the DFT results.

This has several benefits: Most loudspeakers generate significant harmonic distortion when reproducing low frequencies, rejecting those provides a more accurate measurement of the room's response at the measurement frequency.

This feature is enabled by default, to disable it uncheck the entry in the Meter menu. Tracking is only applicable during sine wave measurement and is not active when the signal generator is turned off.

The Tracking indicator below the SPL reading is lit when generator frequency tracking is active.
Load Meter/Microphone Calibration File
Loads a set of calibration data for a calibrated microphone or SPL Meter, or calibration values you would like to be applied to your measurements. 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 microphone or SPL meter being used 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. ETF5 calibration files are in a suitable format.

Here is an example of a valid file format:
                Microphone calibration data

                516.797  0.000  0.000
                538.330  -0.012  0.078
                559.863  -0.024  0.163
                581.397  -0.035  0.251
                602.930  -0.045  0.339
                624.463  -0.055  0.426
                645.996  -0.065  0.512
                667.529  -0.076  0.598
                689.063  -0.087  0.688
                710.596  -0.098  0.782
                732.129  -0.109  0.882
                753.662  -0.119  0.985
                775.195  -0.129  1.092
            
After a calibration file has been loaded it wil be applied to all subsequent measurements. Loading the calibration file does NOT affect any data already measured and does not affect any measurement data that is imported. The graph display is updated to show the calibration curve, offset to lie at the centre of the graph window.

Linear interpolation is used between calibration points. Outside the range of the calibration data the behaviour depends on whether C weighting compensation has been selected. If C weighting compensation is selected, C weighting curve figures will be used for frequencies above or below the range of frequencies in the calibration data. If not, the calibration values for the lowest frequency in the file will also be applied for all lower frequencies and the calibration values for the highest frequency in the file will be applied for all higher frequencies.

The calibration file name and path are remembered for the next startup, the file will be loaded automatically when the Wizard is opened. A message confirming loading of the file is given.

To stop calibration data being applied, use the "Clear calibration data" option.

Useful tip: To apply or remove a mic/meter calibration file to a measurement after it has been taken, simply load or clear the cal data as required and press the Apply Windows button in the Impulse Response Controls graph group to recalculate the frequency response.
Clear Calibration Data
Clears the calibration data structures, all subsequent measurements will not have any meter calibration corrections applied to them and the Wizard will not load any previously specified meter calibration file on the next startup.

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