The SPL Meter displays a value based on the RMS level of the selected
input channel, offset to correspond to an SPL reading that is calibrated
against an external meter. Valid readings are not displayed until
SPL Calibration has been carried out,
the meter figures are red if the SPL reading has not been calibrated.
The SPL reading is filtered to make it easier to read fluctuating values,
the filter is controlled by the SPL Update selector. For general use the
"Slow" setting is best, when measuring subwoofer levels with pink noise the
"Very Slow" setting should be used to average out the random fluctuations of
the subwoofer calibration signal.
The soundcard input channel which is measured is selected in the Soundcard settings. Setting the audio input is described in Getting Started.
The SPL reading can differ from the reading that would be recorded by an
external meter due to additional factors the Wizard takes into account. There are
indicators below the SPL value to highlight these occasions.
The InverseC indicator is lit when the input has been selected as a C Weighted SPL Meter in the Mic/Meter settings and compensation is currently being applied. In that case REW will calculate the amount of attenuation the C weighting network contributes at the frequency being measured and add that to the displayed value so that it shows the unweighted SPL at the measurement point. C weighting compensation is only applied to the SPL reading when the test signal is a sine wave, as the compensation is calculated at the frequency being measured.
The Tracking indicator is lit when REW is tracking the generator frequency. When tracking is active a Discrete Fourier Transform is applied to the input signal to accurately measure the level at that frequency even under noisy conditions or with a soundcard that has poor signal-to-noise ratios.
When the meter is tracking the generator frequency the relative amplitudes
of the 2nd and 3rd harmonics of the test frequency are displayed as percentages
of the level of the fundamental. For low frequency measurements the subwoofer
is usually the main contributor to distortion. If there is clipping somewhere
in the signal path this will typically show up as high 3rd harmonic distortion.
High noise levels in the system will also raise the displayed distortion figures.
The SPL reading may be calibrated against an external SPL meter by pressing the Calibrate SPL button. The text on the button is red if the meter has not been calibrated