Once the audio input and output have been selected (or left as default
if using the default OS settings) REW 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 channel that will be used to make system measurements,
which should be the same one that has been selected in the
Input Channel control.
- Press the Measure... button on the Soundcard Settings panel
and follow the instructions in the help panel
- The measurement result should be fairly flat but will probably roll
off at the lowest and highest frequencies and will often 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):

This is a measurement from another laptop soundcard, showing better low
frequency response and a smoother high frequency response:

- Some results from the measurement are shown in the measurement notes:

- 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. Press the Make Cal... button in the Soundcard Settings and
choose a name and location for the file. The file is saved and then
automatically re-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:
- Leave the loopback connection from line output to line input
in place
- Uncheck the C Weighted SPL Meter box in the
Mic/Meter Settings and press the
Clear Cal button in the Mic/Meter Settings to clear any
mic/meter calibration file that has been loaded. These steps
are required because the loopback connection has no C weighting
or mic/meter response to compensate for
- Press the Measure button

to bring up the Measurement panel

- Click the expand button
if necessary to show the measurement settings and set the End
Freq to 20,000Hz
- Press Start Measuring to make a measurement and check
the result - it should be flat to better than 0.1dB except perhaps
below 10Hz, where there may still be slight roll-off
if the soundcard has poor low frequency range. The plot below
has a vertical scale of 0.1dB/division, the green line is the
measurement result and the dashed grey line is the soundcard
calibration curve:

- If the C Weighted SPL Meter
box has not been unchecked or the mic/meter calibration data
has not been cleared the measurement will show the U
shape of the inverse C weighting curve or the inverse of the
mic/meter calibration data:

- Re-select C Weighted SPL Meter if using one and reload
any mic/meter calibration file that was cleared for the check
- Remove the loopback connection and connect the soundcard to the
SPL meter and AV processor/equaliser
Note that soundcard measurements made from the Soundcard Settings
panel 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 such soundcard measurements
Note also that the soundcard calibration file is only valid
for the sample rate at which it was measured, if the sample rate is changed
the soundcard should be re-measured at the new sample rate
Setup information and example measurements for the Creative Soundblaster
Live! 24-bit USB External soundcard can be found
here.
The next step is to
Check levels
Help Index