Setting the signal level REW uses during measurement involves
generating a pink noise calibration signal and adjusting the AV
processor's volume control and/or the calibration signal level so that at
the measurement point (usually ear height at your main listening position)
your SPL meter shows a level of around 75dB. The soundcard's input volume
then needs to be adjusted to get a good signal level from the SPL meter
or mic preamp when the cal signal is playing.
Open the Soundcard Settings panel and
choose whether to set the levels using your subwoofer or a main speaker,
making the appropriate selection in the drop-down box in the
Levels panel. This tells REW whether
to use a subwoofer or speaker calibration signal. If you have connected
the soundcard output directly to your subwoofer or to an equaliser that is
connected to your subwoofer then choose Check/Set Levels with Subwoofer
here, if you are connected to an AV processor input you can use the subwoofer
or main speaker settings.
Press the Check Levels... button and follow the instructions on screen.
The test signal defaults to an RMS Level of -12dB FS. If connected to an AV
processor, start with the volume fairly low and increase it until the meter
is reading around 75dB. The exact level is not critical. If connecting directly
to an equaliser such as the BFD, use the Sweep Level
control to change the level of the generated signal. In either case, the
final Sweep Level will be used for subsequent measurements - remember
to use the same AV processor volume setting whenever measurements are made.
The output volume control will usually have no effect on the
level of line outputs, but will affect a headphone output or the
PC's speakers - if using a true line output for the measurement
signal the output volume control should be muted to prevent the signal
being heard from the PC's own speaker(s).
The following notes are relevant if an output device and output have been selected
in REW and the Control output mixer/volume
box is ticked.
- REW sets the soundcard's wave volume to full scale and
the output volume to half scale. On exit REW restores the wave and volume
levels that were in use when it started up if they are no higher than
the levels currently set (this is to reduce the possibility of creating
a feedback loop).
- The range of the REW controls is 0..1, but the underlying
gain controls in the soundcard are generally logarithmic. If you use
REW to alter the volume control settings using the arrows on
the spinners, the settings will change in increments corresponding
to 0.5dB, however the soundcard's own controls will usually have
lower resolution, so changes may have no effect on the output until
the next step in the soundcard's control is reached.
- Changes in volume settings outside REW (e.g. in the
Windows mixer) are automatically detected and reflected in REW's
controls.
- REW automatically mutes all playback sources other than Wave when
it starts up, to eliminate noise or sounds from other sources. It is particularly
important to mute the input used for the signal from the SPL meter (usually
Line In), otherwise a feedback loop results as the signal picked up by
the SPL meter is fed directly back to the output. The original mute states
are restored when REW exits. Here is an example of correct playback
control settings on a Windows system, only Wave is unmuted, the Wave
volume is full scale and the output volume is half scale.
- Some soundcards do not provide an input volume control (e.g.
some USB cards). As long as the signal RMS level is within the -30 to -12dB
range when the speaker/subwoofer cal signal is playing everything
should work OK. If the level is below -30dB try reducing the meter range
to raise the level, but be careful not to reduce the range to the point
the meter is overloaded. If the level is above -12dB try increasing the
meter range to lower the level.
- Some soundcards do not provide programmatic access to their input
volume control so REW may not find a volume control or it may
not be able to alter it. In that case you can use the controls in the
soundcard's mixer or the OS audio level controls to make the required
adjustments manually.
- On some soundcards REW may not be able to directly select
the required input via its device and input selectors - for example, on
Audigy 2 selecting the Line In is done by selecting "Analog Mix" in the
Record panel of the Basic tab of the Creative Surround Mixer then going
to the Source panel and muting all the sources except for Line In. If
it does not seem possible to select the required input via REW's
device and input selectors, or REW does not seem to be making
the correct setings, leave the input device set to
Default Device and make the input selection
and input volume adjustments via the soundcard's mixer or the OS audio
level controls. An example of suitable Windows volume control settings
is shown below, with Line In selected

- Unfortunately REW is a bit clunky on the Mac, Apple have
their own class library for accessing the audio hardware and provide
only very limited support for the core JavaSound implementation. If
you set the sample rate to 44.1k you should be able to choose Mac OS
X Core Audio as the input device. Set the input volume by using the
tool in the Mac's Sound Preferences or the level controls in the Audio
Midi Setup utility as there is no programmatic access for REW.
Setup information and example measurements for the Creative Soundblaster
Live! 24-bit USB External soundcard under XP can be found
here.
After checking levels the next step is to
calibrate the SPL reading.
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