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
Input Calibration has been carried out,
the meter figures are red if the input 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 using the
buttons below the record button. The default is to use the Right channel,
but if you are using a mono microphone input it may be necessary to
select the Left input.
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 Wizard has been configured to Compensate for C-Weighting in the Meter menu and compensation is currently being applied. If the frequency being measured is within the region that is attenuated by the C-weighting network of the attached SPL meter the Wizard will add the amount of the attenuation 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 the Wizard has been configured to track the generator frequency. If 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.
The peak and RMS values of the selected input channel are
displayed in dB relative to digital full scale. Below -12dB the values are
green, between -12 and -6dB they are amber and above -6dB they are red.
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 levels. High noise
levels in the system will also raise the displayed distortion figures.
The level meter below the record button shows the meter input
rms level in dB below full scale, with the peak level shown as a red line.
The Input Volume is set by the Wizard as part of Input Calibration. The process is started by pressing the Set Input Volume button. After setting the input volume the SPL reading may be calibrated against an external SPL meter by pressing the Calibrate button below the SPL display.
Setting the audio input is described in Getting Started.