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:
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.
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.
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.
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.
The next step is to Check levels