All SPL graph

The All SPL graph is an overlay graph that shows all measurements (SPL and/or Impedance) that have been made. It allows an average to be generated of all selected traces, arithmetic operations to be carried out on pairs of traces to generate a new trace, pairs of traces to be aligned and Inter-Aural Cross Correlation to be calculated for a pair of traces.

All SPL actions

Right clicking on the graph brings up a menu of actions which are applied to all measurements selected in the graph legend:

• A smoothing action that allows the fractional octave smoothing setting for all the selected traces to be changed. The chosen setting is applied using the Apply button
• Align SPL..., which adjusts all the selected SPL traces so that they have the same average SPL over a selected span.
• Undo SPL alignment, which reverts to the original measured levels.
• Remove IR delays, which removes the estimated IR delay for all the currently selected measurements which have an IR.
• Undo t=0 changes, which resets t=0 to the as measured state.
• Time align, which brings all the currently selected measurements into time alignment. If the measurements have been made with a timing reference (a loopback connection or an acoustic timing reference) the impulse response is shifted according to the measurement delay value, taking into account any IR timing offsets which have been applied since the measurement delay was calculated. Measurements which have been made without a timing reference are shifted according to the estimated IR delay. Time alignment can only be applied to measurements that have an impulse response.
• Align IR start, which aligns all the selected measurements using their IR start times.
• Cross corr align, which time aligns the selected measurements by cross correlation of their windowed impulse responses, using the measurement which appears first in the list of those selected as the reference (the order of measurements can be changed by dragging them up or down the list). This may be useful to align measurements from a mic array or measurements of the same source from different positions prior to vector averaging.
• Clear the ref clears any use of a measurement as a refernece for other measurements
• RMS average, which calculates an rms average of the SPL values of those traces which are selected when the button is pressed. This does the same as the Average The Responses button, the dB values are converted to linear magnitudes, those magnitudes are then squared, summed and divided by the number of measurements, the square root of the result is taken, then the value is converted back to dB. Phase is not taken into account, measurements are treated as incoherent. If only a single trace is selected the result has the magnitude data from the source measurement and no phase data. If the measurements were made at different positions (spatial averaging) it is usually best to first use the Align SPL... feature to remove overall level differences due to different source distances. When measurements with distortion data are averaged their distortion data is also averaged, providing the data cover the same frequency range at the same resolution.
• dB average, which averages the dB SPL values of the currently selected traces. This produces results that are closer to what one might intuitively expect from looking at the traces, but gives equal weight to peaks and dips which masks the magnitude difference between them. For example, a +20 dB peak has a magnitude that is 100 times larger than a -20 dB dip. dB averaging them produces 0 dB, 10 times smaller than the peak and 10 times larger than the dip. dB averaging may be useful when averaging smoothed traces to derive an EQ target, with unsmoothed data the dips would have a disproportionate effect on the result. When measurements with distortion data are averaged their distortion data is also averaged, providing the data cover the same frequency range at the same resolution.
• Vector average, which averages the currently selected traces taking into account both magnitude and phase. It can only be applied to measurements that have an impulse response and is most appropriate for multiple measurements taken from the same position, or measurements which have been time and level aligned before averaging them. When measurements with distortion data are averaged using Vector average their distortion data is also averaged, providing the measurements were made at the same sample rate and with the same sweep settings.
• RMS + phase avg. and dB + phase avg., which produce an RMS or dB average of the magnitudes and use vector average for the phases. They can only be applied to measurements that have an impulse response. Both are appropriate for multiple measurements of a source. Measurements should be time and level aligned before averaging them, cross correlation alignment is recommended for the time alignment. These averaging methods do not exhibit the magnitude dips due to phase cancellations that can occur with vector averaging. The resulting impulse response may have significant acausal content as the relationship between magnitude and phase that would normally hold is broken. As a result the average requires larger left windows than usual. The effects of any calibration files of the individual measurements are merged into the resulting impulse response.
• Vector sum, which sums the currently selected traces taking into account both magnitude and phase. It can only be applied to measurements that have an impulse response.
• Export IR WAV, which saves the impulse responses of the selected measurements to WAV files.
• Export IR txt, which saves the impulse responses of the selected measurements to text files.
• Export as txt, which exports the selected measurements to text files.

The remaining action is:

• Alignment tool, which brings up the Alignment tool dialog

Actions for individual selections

Right clicking on the selector brings up additional actions that depend on the current graph, for the All SPL graph there are controls to adjust the SPL offset and to set a measurement as a reference for other measurements.

If a measurement is set as a reference it has an orange band on the selection list, measurements using another as a reference have a purple band. Those measurements using the reference change their SPL plots to show the difference between them and the reference.

Trace Arithmetic help is here.

Alignment Tool

The Alignment tool allows gain, polarity and delay adjustments to be made to a pair of measurements with a live preview of the summed result updated as the parameters are changed (the preview uses log spaced data at 96 PPO so may differ slightly from the final result). There is also a dotted trace which shows the result of a summation of the measurements ignoring phase. When the alignment tool is opened the All SPL graph selections change to show only the traces being aligned. Other traces may be selected in the graph legend if desired. The original trace selections are restored when the alignment tool is closed. There are two sliders for delay adjustment of the B measurement, one for coarse adjustment and one for fine to provide a wide manual adjustment range. The delay range the alignment tool will use when trying to align measurements is set by the Delay range (ms) controls. The tool can operate in phase or impulse alignment modes.

The Aligned copy buttons generate a copy of the selected measurement with the alignment gain, delay and polarity selections applied. Aligned sum generates a new measurement with the summed results of the alignment settings.

Alignment Tool - phase alignment

When Show phase traces is selected in phase alignment mode the All SPL graph is split with the SPL traces at the top and phase traces for the two selected measurements at the bottom. When Show summed phase is selected the phase of the aligned sum is also shown. If Remove time delay is selected the impulse responses are shifted so that the first measurement IR starts at t=0, that can reduce wraps in the phase traces.

The Level phase at cursor button calculates the delay required to bring the first measurement phase trace to be approximately horizontal around the frequency of the cursor in the phase trace graph. Both measurements have the same delay applied so the relative delays required to align them are not affected. Shifting the phase traces in that way makes it easier to align them. The delay is only used to update the phase traces, it is not applied when using Aligned copy or Aligned sum. Undo levelling removes the levelling delay from both measurements.

The Align phase slopes at cursor button calculates the measurement 2 delay required to give both phase traces approximately the same slope around the frequency of the cursor in the phase trace graph, so they have approximately the same group delay. Note that the phase traces may still be far apart, particularly if one measurement is out of phase with the other. In that case inverting one of the measurements would bring the phase traces closer together.

The Align phase at cursor button calculates the measurement 2 delay required to bring both phase traces as close as possible to each other around the frequency of the cursor in the phase trace graph. Note that a better overall alignment might be achieved by inverting one of the measurements and then aligning phase, and/or by manually adjusting the delay.

Alignment Tool - impulse alignment

When Show impulse responses is selected in impulse alignment mode the All SPL graph is split with the SPL traces at the top and impulse responses for the two selected measurements at the bottom. To perform impulse alignment the responses are filtered at the cursor frequency with a 1/3 octave zero phase filter, the filtered responses are then aligned. The time span of the filtered impulse response graph adjusts to suit the alignment frequency.

The Align IRs at cursor button applies the filters at the cursor frequency and cross-correlates the responses to determine their best alignment. The alignment result may require inversion of one of the responses, which is evident in the impulse response display above. Here is the response after inverting the polarity of the subwoofer trace.

That ability to ignore polarity in finding the best alignment of the responses is an advantage of the impulse alignment mode, but it may struggle to align the filtered responses in an environment with strong modal resonances near the chosen alignment frequency as the resonance may dominate the filtered response.

All SPL Controls

The control panel for the All SPL graph has these controls:

If Show points when zoomed in is selected the individual points that make up the SPL and phase responses are shown on the graph when the zoom level is high enough for them to be distinguished (which may only be over part of the plot)

If Show frequency bands is selected the audio frequency bands are shown in a stripe above the graph. The bands are:

• Infrasound: below 20 Hz
• Sub bass: 20 to 60 Hz
• Bass: 60 to 250 Hz
• Low midrange: 250 to 500 Hz
• Midrange: 500 Hz to 2 kHz
• Upper midrange: 2 to 4 kHz
• Presence: 4 to 6 kHz
• Brilliance: 6 to 20 kHz

If Show modal frequencies is selected the theoretical modal frequencies for the room dimensions entered in the Modal Analysis section of the EQ Window for the currently selected measurement are plotted at the bottom of the graph.

When impedance is being plotted the axis has a span from 0 to 1 kohm. If a larger impedance range is required the axis can be switched to logarithmic with a range up to 1 Mohm using the Use a log axis for impedance check box. If selected a log axis will be used wherever impedance is plotted.

The IEC 263 ratio control allows the graph to be locked to a specified dB/decade aspect ratio. The aspect ratio is maintained by adjusting the margins around the graph as required.

The Trace options button brings up a dialog that allows the line type of the graph traces to be changed.