Making Measurements

Once the measurement level has been set, the SPL input has been calibrated and the soundcard response has been measured and saved as a calibration file the Wizard 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.

Selecting the Test Signal Input on AV32R DP or AV192R

Select the AV processor input to which the soundcard's output has been connected then go to the TMREQ menu. If the channel you wish to measure already has filters defined, they can be disabled temporarily by pressing the eject key on the remote control (the record key is used to re-enable them).

Go into one of the filter menus for the channel and set the Test Signal to Current R. Set Repeat Sig. to No (this is only used when you want to measure the effect of running two speakers at the same time). Set Bass Redir. to No, which prevents the subwoofer being activated when measuring a Bass Limited speaker - the signal which would normally be redirected to the subwooofer is discarded. After making corrections, re-measuring with Bass Redir. set to Yes will allow the integration between the speaker and the subwoofer to be checked.

Select the Channel to be Measured

Choose the channel via the tabs on the Filter Panel. It is best to measure and correct the subwoofer first, then move on to Left, Centre and Right speakers and finally the surrounds and surround backs.

Set the Target Level

The Target Level for the channel must now be set using the button in the Filter Panel. This will provide a reference level against which to compare the subsequent measurements, before setting the level make sure the channel has been selected via its tab on the filter panel. Once the Target Level has been set, the channel can be measured.

The Target Level is important for getting the right correction filter gains and widths. When measuring the subwoofer, the target level will usually measure higher than it should be due to the effect of the room's resonances, after making a measurement of the subwoofer's frequency response you can manually reduce the target level setting to something that looks more sensible, or go through the filter setting process, apply the filters to correct the peaks to your initial target, then re-measure the target level to get a more accurate figure - use the new level to repeat the filter setting process.

Measure the Channel

Measurements can be made automatically across a range of frequencies or manually at user-selected frequencies, pressing a button each time the wizard should take a reading.

Automatic Measurement - Sweep

Automatic measurement will measure the channel's frequency response across the range set in the controls below the button. The "Sweep" option (selected by default) uses a logarithmically swept sine signal to measure the response. It is much faster than stepped sine measurements (around 10s in total), more accurate, less likely to suffer from clipping at resonances and allows the impulse response of the room to be determined, which in turn is the basis for many additional features. When measuring with a sweep the signal generated will start at DC with an initial linear sweep to 10Hz, then continue with a log sweep that ends an octave higher than requested, to ensure the response is accurately captured throughout the requested range.
Automatic Measurement controls

If the channel being measured already has measured data you will be warned before the measurement proceeds.

Channel already has measured data

A progress indicator is shown while the sweep is being generated and captured.

Measurement Progress Indicator - Sweep

After capturing the sweep, FFT processing is carried out to derive the system's impulse response and the corresponding frequency response. There are controls to adjust the position and widths of pre and post impulse windows that define the portion used to derive the frequency response, these controls may be accessed by selecting the "Impulse Response Controls" graph group. The reference position for the windows is usually the impulse peak (at time = 0).

Impulse Response Controls

The default settings for the windows will usually be suitable. In smaller rooms it may be necessary to use a shorter post-impulse window duration, around 300-500ms - if the frequency response plot appears noisy and jagged try reducing the post-impulse window period and hit "Apply Windows" to recalculate the frequency response. In very large rooms the window can be increased to improve the frequency resolution. The frequency resolution corresponding to the current total window duration (pre and post combined) is shown above the Apply Windows button.

The peak level in each sweep measurement is checked and a message appears if it is lower than ideal. The measurement level set via the "Set Measurement Level" button leaves headroom to allow for resonances, but if the resonances are small (or if the measurement is being made after resonances have been countered by filters) the headroom may be excessive. Increasing the measurement level (via the arrows on the Measurement Level spinner at the bottom of the signal generator panel) or the AV processor volume will raise the signal level and improve the signal-to-noise ratio in the meaasurement 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. If this occurs a warning is displayed, as input clipping will cause errors in the derived frequency response. The measurement level or AV processor volume should be reduced and the measurement repeated. Note that after making the 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 lower SPL on the graphs than those made before the change.

Low Signal Level

Measured data traces may be smoothed by fractional octave averaging using the selector below the trace name in the Filter Adjustment trace group. This is mainly for full range measurements, as reflections can cause severe comb filtering which makes it difficult to see the underlying trend of the response.
Trace Smoothing

After measuring the response of a channel you can start adjusting the filters immediately, or measure another channel first. Notes relating to this measurement can be entered using the Measurement Notes entry in the View menu or by right-clicking on the channel's tab in the filters pane. For details of the various ways of viewing the measured data, including averaging multiple measurements, refer to the Graph Panel help.

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 AC32R 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).

Automatic Measurement - Stepped Sine

Automatic measurement using stepped sine measures the frequency response at a series of frequencies across the set range by measuring the level of a sine wave test signal which begins at the start frequency then is increased by the selected step size after each level measurement. For linear steps select one of the "Hz" values, for log steps select an "oct" value. The oct values below 1/3 are based on evenly spaced subdivisions of the ISO one-third octave intervals, following the frequencies available on the BFD Pro.

Bear in mind the the level is measured at specific frequencies only, if the measurement steps are too broad peaks which lie between measurement steps will be missed. The recommended step size for low frequency measurements is 1Hz.

If the channel being measured already has measured data you will be given the option to either merge these new measurements with the existing measurements, or remove the old measurements.

Channel already has measured data

Stepped sine measurements are plotted on the graph as the measurement proceeds, the progress indicator gives an estimate of the time remaining to complete the measurement. If the measurement is cancelled any readings taken up to that point will remain.

Measurement Progress Indicator

After measuring the response of a channel you can start adjusting the filters immediately, or measure another channel first.

Manual Measurement

Manual Measurement allows readings to be taken at frequencies and intervals of your choosing. After starting manual measurement, the text on the button changes to End Measurement. Press the button again to stop measuring.

To take a reading at the current signal generator frequency, press the Take a Reading button button. Note that the SPL meter display changes to green when the wizard detects that the value has stabilised and it is a good time to take a reading. After taking areading the frequency is automatically incremented by the step value in the automatic measurement controls, but you can change the frequency using the arrow buttons on the generator display, or by typing in a new value, or by selecting the Frequency tracks cursor option and moving the graph cursor to the next frequency at which you wish to measure.

Using Controls while Measuring

Most of the wizard controls are disabled during measurement. Useful controls which remain active are:

The next step is Filter Adjustment.

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