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.
- Press the Measure button
(or Ctrl+M) to bring up the Measurement panel

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 dB FS, which would place the peaks
of the signal at digital full scale. The default value is -12dB FS.
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
- 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 improves S/N by almost 3 dB for each
doubling of the number of sweeps. Averaging is particularly 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.
Note that 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.
In general it is recommended to use single, longer sweeps rather than
multiple, shorter 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 spans 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.
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 18 dB. A message is shown if the headroom is more than 18 dB, 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

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.

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 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).
Help Index