Calibrating the Soundcard

Once the measurement level has been set, the input volume has been set and the SPL reading has been calibrated, the Wizard is ready to make a calibration measurement of the soundcard's frequency response. This will be used to remove the soundcard's response from room measurements.

Making the Soundcard Calibration Measurement

  1. Connect the soundcard's line output directly to its line input - use the channels that will be used to make system measurements
  2. Select "Measure Soundcard Response..." from the Soundcard menu
  3. When prompted, set the input level at which the measurement should be made. For good signal-to-noise ratio whilst avoiding clipping and compression the level should be around -6dB (the default), lower levels may be used to check the response at that level but will give noisier results, that noise will be added to any subsequent measurements that use the resulting cal file
    Measurement input level selection

  4. The measurement result should be fairly flat but will probably roll off at the lowest and highest frequencies and will typically have some ripple at the high end. This measurement is from a laptop's internal card which rolls off fairly rapidly below 20Hz (graph vertical scale is 1dB/division):
    Laptop soundcard measurement 1dB/div

  5. Some results from the measurement are shown on the soundcard tab:
    Laptop soundcard measurement results

  6. Here is a measurement from an external USB card for comparison
    : External USB soundcard measurement 1dB/div

    External USB soundcard measurement results

  7. If the measurement looks like this:
    Bad soundcard calibration result

    it is probably due to a feedback loop from the Line In to the output. This can happen if the soundcard has some feature for record monitoring - for example, on the Soundblaster Live 24-bit External there is a "Monitor" feature for the Line In that must be turned off to get correct results, on some other Creative soundcards (e.g. Audigy 2 ZS) there is a Record Advanced Controls setting for "Record without monitoring" that must be selected.
  8. Having obtained a measurement it needs to be saved as a calibration file. Select the "Save Measurement as Soundcard Cal File..." option in the Soundcard menu and choose a name and location for the file. The file is saved and then loaded as a calibration file to use for all subsequent measurements. On the next startup the file will be loaded automatically with a confirmation message like this one:
    Soundcard Cal confirmation message

  9. To check that the calibration has been successful, select the Left channel and press the button to make another measurement and check the result - it should be flat to better than 0.1dB except perhaps at the lowest frequencies, where there may still be 0.2dB or so of roll-off if the soundcard has poor low frequency range, and close to the sampling limit (half the sample rate) where the results are highly variable. N.B. Turn off C weighting compensation in the Meter menu before making the check measurement otherwise the result will show the U shape of the inverse C weighting curve.
    Laptop Soundcard Response After Cal

  10. The calibrated external USB card is much flatter at the lowest frequencies:
    External USB Soundcard Response After Cal

  11. Reconnect the soundcard to the SPL meter and AV processor/equaliser

The Wizard is now ready to make room response measurements.

Note that all measurements made on the Soundcard channel use the full sweep range to half the soundcard sample rate, regardless of the sweep end frequency setting, and the soundcard calibration file is NOT applied to measurements made on the soundcard channel.

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