Making Measurements

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

Connections should be as explained in Getting Started, if connected to an AV processor, select the input to which the interface'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. The measurement name can be changed if desired by entering a new name in the box at the top of the measurement 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 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.

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, 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.

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 interface 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. The timing reference selection controls whether REW uses a loopback on the interface 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 interface so that separate measurements have the same absolute timing. A Timing offset can be set that will shift the response relative to the timing reference, allowing the time of flight delay to be removed if using a loopback connection, for example. The timing offset can be set manually or from the figure calculated using the Estimate IR Delay function.

If a loopback is selected the reference channel signal must be looped back from output to input on the interface 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, which can be removed for subsequent measurements using the Timing offset.

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 relative to the measurement level using the Ref level trim control, it may need to be higher or lower depending on whether the speaker used as the timing reference is further away or closer than the speaker being measured. 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. The delay can be shifted using the Timing offset. 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. Any Timing offset is shown below the System Delay. For speakers the delay estimate is based on the location of the peak of the impulse response. Subwoofers have a broad peak and a delayed response due to their limited bandwidth so the delay is instead measured relative to the start of the impulse response. The start of the impulse response cannot be located as precisely as the peak, however, so delay values are less accurate for subwoofer measurements.

Measuring with file playback

It sometimes is not possible to connect to an input on the system being measured for it to play back the sweep. If the system can play back from files a sweep file can be used instead.

File measurement

A Measurement Sweep file must be generated and saved from the REW signal generator. The sweep file must include the acoustic timing reference signal, REW uses that to synchronise input capture to playback on the system being measured. A copy of the file is placed on the system being measured and the file is also loaded into REW so that it can configure the correct the measurement frequency range, sweep settings and sample rate, which are retrieved from the file.

Sweep file loaded

After loading the file press Start, REW will then capture the noise floor and pause, showing Waiting for timing reference...

Waiting for timing reference

Now play back the file on the system being measured, once REW detects the timing reference it will capture the sweep and generate the measurement.

Number of measurements

REW can have up to 99 measurements loaded at once according the setting for Maximum measurements in the View preferences, the default is 30. If the maximum number has been reached 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:

No Free Positions

Help Index