Getting set up for measuring
When using REW to make measurements it is best to exit any other applications, disconnect from the internet and disable any wireless networking. Interference on audio inputs from the wireless interface or high processor demand from other applications, anti-virus updates and the like can cause gaps in the generated or captured audio signals leading to incorrect measurement results.
The initial steps required to make room measurements are:
Choose the audio input and output
Calibrate the soundcard
Check levels
Calibrate the SPL reading
The various calibrations usually only need to be done once. If running REW for the first time it is best to read through these initial help chapters in sequence rather than jumping directly to the individual setup steps, however if your computer has already been set up using other acoustic measurement software you may be able to skip directly to Making Measurements.
Choosing the Audio Input and Output
REW defaults to using the audio input and output which have been set as the defaults in your OS. If you wish to use other audio inputs or outputs they can be selected in the Soundcard Preferences panel, click the Preferences button in the toolbar to display the panel. The device lists show all soundcards that REW has detected, when a soundcard has been selected the input and output lists show the available inputs/outputs on that soundcard. Note that if a USB soundcard is plugged in after REW has been started it may take up to 1 minute for it to appear in the list of devices - this is a feature of the Java Runtime Environment.
The lists include both internal and external devices and default drivers offered by the operating system.
Where possible, select the soundcard itself rather than the OS drivers "Primary Sound Capture Driver", "Primary Sound Driver", "Java Sound Audio Engine" or similar. REW needs direct access to the controls on the soundcard if it is to automatically adjust levels, this may not be possible if the OS drivers are selected. Java Sound Audio Engine is also prone to pops and clicks during playback whch degrade measurements.
Under OS X the free M3D Output Mixer from
www.mandolane.co.uk. is highly recommended, after installation select M3DMixer as the output device.
Once the devices have been chosen, the input and output can be selected. Typically the input will be called "LINE_IN" and the output will be "SPEAKER" or "LINE_OUT", however these names may be different for USB soundcards - for example, the input may be labelled "Digital Audio Interface".
Trouble-shooting tip: To prevent REW from accessing soundcard controls, leave the input and output devices set to "Default Device". The actual input and output used and any level control settings will then need to be made using the OS volume controls and/or the soundcard's mixer.
Note that on a Mac it may be necessary to set the Sample Rate to 44.1k to access audio inputs and outputs. There is no programmatic access to soundcard controls on the Mac, use the controls in Applications - Utilities - Audio Midi Setup.
Calibrating the Soundcard
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 and is a useful check that the soundcard is configured correctly.
Connect the soundcard's line output directly to its line input - use the channel that will be used to make measurements, which should be the same one that has been selected in the Input Channel control.
Press the Calibrate... button on the Soundcard Preferences panel and follow the instructions in the help panel at the bottom
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. It can also happen if "Line In" is not muted in the Playback mixer for the soundcard.
Having obtained a measurement it needs to be saved as a calibration file. Press the Make Cal... button in the Soundcard Preferences 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:
The calibration file will be applied to all new measurements made after it has been loaded. To apply or remove a soundcard calibration file for an existing measurement, use the Change Cal... button in the Show More part of the measurement panel.
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 Preferences and press the Clear Cal button in the Mic/Meter Preferences 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 Preferences 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.
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