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

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
Measurement Name

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

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 18dB. A message is shown if the headroom is more than 18dB, 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 it will be necessary to use "Set Target Level" to establish the new reference level for filter setting, and subsequent measurements will be at a higher SPL on the graphs than those made before the change

Low Signal Level

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 or AV processor 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.

Clipping warning

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

Very Low Signal Level

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).

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