## Making Measurements

Once the audio input and output have been chosen, the soundcard has been calibrated, the levels have been checked and the SPL reading has been calibrated REW is ready to make room response measurements.

Connections should be as explained in Getting Started, if connected to an AV processor, select the input to which the soundcard's output is connected.

### Making a Measurement

- Press the Measure button (or Ctrl+M) to bring up the Measurement dialog
  REW can have 30 measurements loaded at once. If there are already 30 measurements when a new measurement is requested a warning is given as the first measurement would need to be removed to make room for the new one:
- Make sure the SPL/Impedance selector at the top of the Measurement panel is set to SPL (see Impedance Measurement for information about measuring impedance)
- A delay of up to 60s can be selected before the measurement sweep starts, use the Sweep Delay control to configure this
- Click the expand button if necessary to show the measurement settings
- Set the Start Freq to the lowest frequency for which you wish to see the response and End Freq to the highest. The sweep will span the range from half the start frequency to twice the end frequency (with an overall limit of half the soundcard sample rate) to provide accurate measurement over the selected range
- Level controls the rms signal level at which the sweep is generated, relative to digital full scale. The maximum value is -3 dBFS, unless the View preference Full scale sine rms is 0 dBFS has been selected, in which case the maximum is 0 dBFS. Using the maximum value places the peaks of the signal at digital full scale. The default value is -12 dBFS. This control is normally preset to the Sweep Level established during the Check Levels process. If you will be comparing measurements from several speakers, or comparing a series of measurements from a speaker, make sure they are measured with the same Sweep Level.
- Length controls the duration of the sweep signal, specifying the number of samples in the sweep sequence. The default is 256k. Dividing the number of samples by the soundcard's sample rate gives the sweep duration in seconds. The overall duration includes silent periods before and after the sweep
  Longer sweeps provide higher signal-to-noise ratio (S/N) in the measurements, each doubling of the sweep length improves S/N by almost 3 dB. However, the time required to perform the processing after each sweep will more than double for each doubling of sweep length. If REW is running on a computer which does not have a fast processor and a lot of memory, measurements will be much faster using the shortest sweep length (128k samples), at a small S/N penalty of about 3dB compared to the default. At least 2 GB of RAM and a fast processor are recommended if using the 1M sweep, invalid measurements may occur on computers which have insufficient RAM or processor speed
- REW allows multiple sweeps to be averaged, although best results are generally obtained by using single, longer sweeps rather than multiple, shorter sweeps. Do not use multiple sweeps if the input and output are on different devices (for example, if the input is a USB mic). If Sweeps is more than 1 REW uses synchronous pre-averaging, capturing the selected number of sweeps per measurement and averaging the results to reduce the effects of noise and interference. The pre-averaging can improve S/N by almost 3 dB for each doubling of the number of sweeps. Averaging can be useful if the measurements are contaminated by interference tones, whether electrical or acoustic, as they typically will not add coherently in the averaging and hence will be suppressed by the process
  Warning: some soundcards do not maintain sample synchronisation between the successive sweeps which produces a corrupt measurement that has multiple, closely-spaced peaks of approximately the same level in its impulse response, 1 peak for each sweep. This can also happen if the input and output are on separate devices. If the frequency response with multiple sweeps is significantly different from the response with a single sweep, stick with single sweeps
- Total Time shows the overall duration of the sequence of sweeps
- The Check Levels button generates a few seconds of a pink noise signal that depends on the frequency range selected for measurement and checks that the input level is not too high or too low. Pressing Cancel while the pink noise signal is playing will turn it off (it turns off automatically after 3 seconds). The rms level that was measured is shown on the measurement panel, with a warning if the level is too high or too low
- Press Start Measuring to make a measurement. If a delay has been configured time remaining before the sweep begins is shown
- When the sweep starts progress is shown on the measurement panel along with a display of the measurement headroom

The result of the measurement is displayed in the graph area, information about the measurement appears in the Measurements Panel. Measurements are given a default name of the date and time at which they are made, a more appropriate name can be entered in the box at the top of the measurements panel

Notes relating to each measurement can be entered in the notes area, click the Notes button if the notes area is not visible

For details of the various ways of viewing the measured data, including averaging multiple measurements, refer to the Graph Panel help.

### Measurement Headroom

The headroom figure on the measurement panel shows how far away the input is from clipping, and hence how much the sweep level could be increased before clipping would occur. The figure is red if there is less than 6dB of headroom (warning that the input is close to clipping), green between 6 and 40 dB (or between 6 and 60 dB for USB mics). A message is shown if the headroom is more than desirable, as increasing the Sweep Level or the AV processor volume would improve the signal-to-noise ratio in the measurement which in turn increases the accuracy of the impulse and frequency responses. Note that after making such a change subsequent measurements will be at a higher SPL on the graphs than those made before the change.

If the room's resonances are very large the input signal level may exceed the input range and cause clipping. If this occurs a warning is displayed, as input clipping will cause errors in the derived frequency response. The sweep level, AV processor volume or input volume should be reduced and the measurement repeated. Note that after making the change subsequent measurements will be at a lower SPL on the graphs than those made before the change.

If the signal levels are very low this may indicate a connection problem:

After measuring the response of a channel you can look at adjusting EQ immediately, or make other measurements first.

Note that some resonances which are very pronounced when measuring a speaker alone do not appear if a pair of speakers (e.g. Left and Right) are run together - this is because the positioning of the speakers in the room can prevent some resonances being excited (in particular, the odd order width modes will not be excited by content which is the same on Left and Right speakers if they are symmetrically placed across the width). Such resonances can often be left uncorrected, to identify them compare measurements from individual channels with those made with two channels driven at the same time (achieved on AV32R DP or AV192R by setting the Repeat Sig. entry in the TMREQ filter menu to Yes and selecting the channel which is to repeat the test signal, or on other processors by connecting both left and right soundcard outputs to the selected AV processor input or using a Y lead to drive two inputs at once).

### Measuring with a timing reference

REW can make use of a timing reference when it measures, according to the setting on the measurement panel or in the Analysis preferences. The timing reference selection controls whether REW uses a loopback on the soundcard as a timing reference, or an acoustic timing reference, or no reference. Using a timing reference allows REW to eliminate the variable propagation delays within the computer and soundcard so that separate measurements have the same absolute timing.

If a loopback is selected the reference channel signal must be looped back from output to input on the soundcard and measurements will be relative to the loopback timing. Usually this means measurements will have a time delay that corresponds to the time it takes sound to travel from the speaker being measured to the microphone.

If an acoustic timing reference is used REW will generate a timing signal on the output that has been selected to act as the reference before it generates measurement sweeps on the channels being measured. The level of the timing reference is set separately from the measurement level using the control at the top of the measurement dialog. The timing signal is a high frequency sweep to allow accurate timing, a subwoofer cannot be used as the reference channel. Measurements will have a time delay that corresponds to the difference in their distance from the microphone compared to the distance of the reference speaker - if the reference speaker is further away the delay would be negative. When an acoustic timing reference is used individual measurements taken from the same mic position will have the same relative timing, allowing trace arithmetic to be carried out on the traces in the All SPL graph. Note that multiple sweeps cannot be used when using an acoustic timing reference.

If using a timing reference REW can calculate the delay through the system being measured relative to the reference and show it in the measurement Info panel as "System Delay" in milliseconds, with the equivalent distance in feet and metres shown in brackets. Note that delay values are not accurate for subwoofer measurements due to the limited bandwidth of the subwoofer response, the delay estimate is based on the location of the peak of the impulse response and subwoofers have a broad peak and a delayed response.

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