SPL and Phase Graph

SPL and Phase Graph

The SPL and Phase plot (or Impedance and Phase for an Impedance measurement) shows the frequency and phase responses of the measurement. The frequency response is labelled with the measurement name, the phase response uses a dotted trace and the right hand plot Y axis. The left hand Y axis can be set to show dB SPL, dBFS, dBr, dB V/V, dBu, dBV, dBW, volts, watts or, for impedance traces, ohms. The dBr and dB V/V values are effectively a transfer function view, showing either the relative input to output dBFS levels (dBr) or voltage levels (dB V/V). Values for dBW and watts are derived from the measured voltages using the reference impedance value in the RTA Appearance settings.

For stepped level measurements the graph will show a plot of the input level versus the generator level, and a linearity plot showing the ratio of input level to generator level.

Note that to have valid phase information it is necessary to remove any time delays from the Impulse Response. A time delay causes a phase shift that increases with frequency - for example, a delay of just 1 ms results in a phase shift of 36 degrees at 100Hz but 3,600 degrees at 10kHz, because 1 ms is 1/10th of the 10 ms period of a 100Hz signal but is 10 times the 0.1 ms period of a 10kHz signal, and each period is 360 degrees. The time delay of a measurement can be adjusted by changing the zero position of the time axis using the Offset t=0 controls, or by using the Estimate IR delay control, both described below.

In addition to the measured phase, the plot can show minimum and excess phase plots that result from generating a minimum phase version of the response, described further below. The plot also shows any mic/meter or soundcard calibration data for the measurement. The calibration data can be changed or removed by selecting Change Cal... on the measurement panel.

Minimum Phase/Excess Phase

If the Generate minimum phase control has been used to produce a minimum phase version of the response the minimum and excess phase traces are activated. They show the minimum phase response and the difference between the measured phase and the minimum phase (the "excess"). Note that it is best to make full range measurements if the minimum phase response is to be generated as a good result relies on measuring beyond the bandwidth of the system being measured. For more about minimum and excess phase and group delay see Minimum Phase.

Mic/Meter Cal

The Mic/Meter Cal trace shows the frequency response of the Mic calibration data for this measurement (the calibration file to use for new measurements is specified in the Cal files Preferences). If C Weighted SPL Meter was selected this curve will show the effect of C weighting (outside the range of the calibration data file, if there is one). The trace is not shown if there is no mic/meter calibration data. The trace is drawn relative to the middle of the graph.

Soundcard Cal

The Soundcard Cal trace shows the measured frequency response of the soundcard relative to its level at 1kHz (if a calibration file has been loaded via the Soundcard Preferences). The trace is not shown if cal data has not been loaded. The trace is drawn relative to the middle of the graph. Fractional octave smoothing can be applied or removed via the Graph menu and its shortcut keys. The smoothing is applied to the SPL, phase and Group Delay traces. This is mainly used for full range measurements, as reflections can cause severe comb filtering which makes it difficult to see the underlying trend of the response. Smoothing should rarely be used for low frequency measurements as it obscures the true shape of the response. When smoothing has been applied an indicator appears in the trace legend.

Graph metrics

If a frequency range is selected by holding the shift key then pressing and dragging with the left mouse button a best-fit line for the selected range is displayed, with lines above and below it to show +3 dB and -3 dB. The slope of the line in dB/octave is shown (if the frequency axis is logarithmic) along with the span of the data relative to the best-fit line and the minimum, maximum and average of the range. The range can be adjusted after selection by dragging the start or end of the area, or the whole range can be moved.

SPL and Phase metrics

SPL and Phase Controls

The control panel for the SPL and Phase graph has these controls:

SPL and Phase Controls

The phase trace normally wraps at +180/-180 degrees. This is because phase is cyclic over a 360 degree range (+90 is the same phase as -270). The trace can, however, be displayed without wrapping which is what the Unwrap Phase control does. A difficulty with unwrapped phase is knowing where the correct zero phase is, another is being able to view parts of the trace where the unwrapped value has become very large. The unwrapped phase is offset (by a multiple of 360 degrees) so that it is within the range -180..180 degrees at the cursor frequency. The +360 and -360 buttons will also shift the phase trace in 360 degree steps.

Wrap Phase changes the phase trace back to a conventional wrapped view with vertical lines where the trace crosses 180 or -180 degrees.

Invert polarity inverts the polarity of the measurement which shifts the phase values by 180 degrees.

Generate minimum phase will produce a minimum phase version of the measurement using the current IR window settings. The minimum phase trace then shows the lowest phase shift a system with the same frequency response as the measurement could have, while the excess phase trace shows the difference between the measured and minimum phase. Using this control also generates a minimum phase impulse plot and minimum and excess group delay plots, which can be viewed on the respective graphs.

Note that the IR window settings are important as the minimum phase response is derived from the frequency (magnitude) response of the measurement, which in turn is affected by the IR window settings. If the window settings are subsequently changed Generate minimum phase should be used again to reflect the new settings. Note also that the shape of the left side window (the window applied before the peak) affects the minimum phase result, a rectangular window will produce a response with lower phase shift than, for example, a Hann window.

If the system being measured was inherently minimum phase (as most crossovers are, for example) the minimum phase response is the same as removing any time delay from the measurement. Room measurements are typically not minimum phase except in some regions, mainly at low frequencies. For more about minimum and excess phase and group delay see Minimum Phase.

Offset t=0 allows the position of time zero in the impulse response to be altered, with a live preview of the effect the offset will have on phase. The measurement is not changed unless either the Apply or Apply & close button is pressed. Fine adjustment of the offset can be made with the left and right arrow keys after clicking on the slider knob. If a timing reference was used the System Delay figure (which can be viewed in the measurement Info panel) is shifted by the same amount as the zero time. The cumulative shift that has been applied to the impulse response is shown at the top of the dialog.

t=0 offset dialog

Estimate IR delay calculates an estimate of the time delay in the measurement by comparing it with a minimum phase version. The delay it calculates can be removed from the impulse response by pressing the Shift IR button on the panel shown after the delay is calculated. Note that shifting the impulse response will clear any spectrogram which had been generated as the plot would no longer be valid.

The Generate minimum phase, Estimate IR delay and Offset t=0 controls also appear in the Impulse graph control panel.

The trace offset value moves the graph position, but does not alter the data so the legend values do not change. If the Add to data button is pressed the current offset value is transferred to the measurement data and the legend readings will update accordingly.

The Scale FR Peak control adjusts the SPL offset to achieve a desired maximum SPL figure in the frequency response. This may be useful to adjust levels for an imported impulse response, for example.

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:

Show room panel is only applicable for measurements generated from the Room simulator. If selected a plan view of the room configuration used to generate the simulated response is shown.

Room plan view

If Show modal frequencies is selected the theoretical modal frequencies for the room dimensions entered in the Modal Analysis section of the EQ Window (or in the Room simulator for responses generated from there) 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 Trace options button brings up a dialog that allows the colour and line type of the graph traces to be changed. If a change is made it will be used for all measurements shown on this graph. Traces can also be hidden, which will remove them from the graph and from the graph legend.

The Component model button is only enabled for impedance measurements of inductors or capacitors. REW identifies the measurement as being of a capacitor if the phase at 20 Hz (or the start of the measurement, if higher) is below -60 degrees, or an inductor if the phase at the end of the measurement is above 45 degrees. For those measurements pressing the button will carry out a curve fit over the range from 10 Hz to 20 kHz for inductors or 100 Hz to 20 kHz for capacitors (or the measurement range if smaller) to derive equivalent circuit component values. The equivalent circuit is shown below the button. The equivalent circuit impedance and phase are shown as dashed lines overlaying the measured traces. When measuring components make sure the component lead length is close to the length it will have in circuit, otherwise the measurement will include lead resistance and inductance which will not be present when the part is used.

For capacitors the equivalent circuit is a series combination of a resistor (the ESR), a capacitor with a parallel resistor (leakage) and an inductor (likely to be very small at audio frequencies, not shown if less than 1 nH).

Capacitor equivalent circuit

For inductors the equivalent circuit is an LR2 model consisting of a series resistor, a series inductor and a resistor and inductor in parallel.

Inductor equivalent circuit

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