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.
- Connect the soundcard's line output directly to its line
input - use the channels that will be used to make system measurements
- Select "Measure Soundcard Response..." from the Soundcard menu
- 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

- 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):

- Some results from the measurement are shown on the soundcard tab:

- Here is a measurement from an external USB card for comparison
:


- If the measurement looks like this:

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.
- 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:

- 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.

- The calibrated external USB card is much flatter at the lowest frequencies:

- 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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