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:

  1. Choose the audio input and output
  2. Calibrate the soundcard (not applicable when using a USB microphone)
  3. Check levels
  4. Calibrate the SPL reading (not when using a USB microphone that has REW-compatible sensitivity cal data)

The various calibrations and level checks 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

On Windows platforms REW can use either the Java soundcard drivers or ASIO drivers, on other platforms only Java drivers are supported. If using the Java drivers REW defaults to using the audio input and output which have been set as the defaults in your OS. Those defaults may change as you connect or disconnect audio devices, so it is best to select the specific devices, inputs and outputs you wish to use for measurement 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. ASIO devices appear in the list if they have an installed driver even if the soundcard is not connected, connect the soundcard to populate the list of supported sample rates. Note 1: 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. Note 2: if ASIO4All is being used and a change is made in the ASIO4All control panel to select other devices or inputs/outputs REW will need to reload the ASIO4All driver before it can display the newly available channels. To do that switch the Driver setting to Java, then back to ASIO and re-select ASIO4All as the ASIO device.

When using the Java drivers the lists may include both internal and external devices and default drivers offered by the operating system.

Audio input devices

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 which degrade measurements.

Once the devices have been chosen, the input and output can be selected. When using Java drivers the input will typically be called "LINE_IN" or "MICROPHONE" 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". ASIO devices have more specific names for the available inputs and outputs and each mono channel will be listed separately.

When using a USB mic with a cal file that contains a sensitivity figure REW needs to read the input volume setting to correctly show SPL, to allow that the input device and input for the mic must be selected (they must not be left as "Default Device").

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.

Multichannel Output

When using Java drivers a channel may be selected after choosing an input or output. The Java drivers typically provide stereo inputs and outputs, so the measurement input can be Left or Right and the output can be Left, Right or, to send the measurement signal to both channels, Left+Right. On Windows the Java drivers currently (March 2018 using Java 8 update 162) only offer stereo outputs even if the soundcard connected is configured for or supports multi-channel, for example an HDMI output offering multi-channel PCM audio. On OS X (and perhaps Linux) a device configured for multi-channel can make all its channels available. An example of selecting from any of the 8 outputs of a multichannel card on OS X is shown below, the card is configured for 8-channel, 16-bit data at 48 kHz as shown in the Audio Midi Setup image. Note that OS X has an unusual channel order for this card, with the back channels appearing in the list before the side channels. REW labels the channels in order as Left, Right, C, LFE, SL, SR, SBL, SBR but on this card selecting SL (for example) in REW produces output on the left channel of the connector marked Rear on the soundcard and SBL produces output on the left channel of the connector marked Side. That oddity aside the channels are individually selectable, and any unusual channel assignments can be dealt with using the Output Channel Mapping control.

8-channel output selection

OS X 8-channel output

Sample Rate Selection

When using ASIO drivers sample rates up to 192 kHz will be offered, if the soundcard supports them. The Java drivers offer sample rates up to 96 kHz, though devices may only be available for 44.1 kHz and 48 kHz depending on the OS and the devices. Make sure the input and output devices are configured in the OS to operate at the rate selected in REW, otherwise the OS will resample between the selected rate and the rate at which the input or output device is actually running.

It is best to use 44.1 kHz or 48 kHz for acoustic measurements unless the test specifically requires measurement results above 20 kHz (studying tweeter resonances, for example). Higher sample rates increase memory use, slow down processing and do not improve accuracy.

Calibrating the Soundcard

This step is not applicable when using a USB microphone as the input, skip directly to Check levels

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 is important to check that the soundcard is configured correctly and the result can be used to remove the soundcard's response from measurements.

  1. Connect the soundcard's line or headphone 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.
  2. Press the Calibrate... button on the Soundcard Preferences panel and follow the instructions in the help panel at the bottom
  3. The measurement result should be fairly flat (varying by much less than 1dB over most of the range) but will roll off at the lowest and highest frequencies and will often have some ripple at the high end. If the result shows excessive variation between 20Hz and 20kHz a warning message will be shown, such a measurement should not be used. Below is a valid measurement from a laptop's internal card which rolls off fairly rapidly below 20Hz (graph vertical scale is 1dB/division):
    Laptop soundcard measurement 1dB/div

    This is a measurement from another laptop soundcard, showing better low frequency response and a smoother high frequency response:
    Laptop soundcard measurement 1dB/div

  4. Some results from the measurement are shown in the notes:
    Laptop soundcard measurement results

  5. If the measurement looks like this:
    Bad soundcard calibration result

    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, or if "Listen to this device" has been selected in the Windows soundcard properties for the input.
  6. 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.
  7. 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 measurement panel.

    Change Cal button

  8. To check that the calibration has been successful:
  9. Remove the loopback connection and connect the soundcard to the mic or 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.

The next step is to Check levels

Help Index