The Impulse graph shows the impulse response for the current measurement. It can also show the left and right windows and the effect of the windows on the data that is used to calculate the frequency response; a minimum phase impulse; the impulse response envelope (ETC) and the step response.
The Y axis used for the impulse response can be selected as % FS or dBFS
(FS = Full Scale) via a control in the top left corner which appears when
the mouse cursor is inside the graph area. The dBFS scale is equivalent to
a "log squared" view of the impulse.
Dashed vertical black lines show the extents of the impulse response windows, a dashed red line shows the reference position. If the window settings are changed the region outside the new area is shown shaded until the settings are applied. It is best to set the Y axis to dB to adjust the windows as it is then much easier to see where the response has decayed into the noise.
After each measurement the left window width is automatically set up. For full range measurements (and down to end frequencies of 1kHz) the width is 125ms, below that it increases to allow for pre-ringing effects of using a limited sweep range. To change the window settings for a measurement click the IR Windows button:
The impulse response is that of the whole system, including the mic/meter and the soundcard. The mic/meter and soundcard calibrations are only applied when calculating the frequency response.
If the Generate Minimum Phase control has been used to produce a minimum phase version of the current measurement's magnitude response a minimum phase impulse trace is activated, showing the impulse response the minimum phase system would have.
The envelope of the impulse, also called the energy-time curve or ETC, is useful to identify reflections and see the overall shape of the impulse response. The plot below shows the envelope, the spikes after the initial peak are due to reflections from room surfaces, the first spike occurs 3.25ms after the initial peak indicating that the sound travelled an additional 1.11m or 3.7 feet to reach the microphone.
The step response shows the output which would result if the input signal jumped to a fixed level and stayed there. It is the integral of the impulse response. If there is an offset in the measurement input chain the step response will show an overall rise or fall as time progresses, rather than tending back to zero.
A property of the log sweep analysis method is that the various harmonic distortion components appear as additional impulses at negative time, with decreasing spacing as the distortion order increases. For example, this plot shows spikes from distortion components up to the 8th harmonic on a laptop soundcard loopback measurement:
Here is a similar measurement for an external USB soundcard, it is a 44.1k card rather than 48k, which limits us to the 6th harmonic in the 1s pre-impulse period - however, only the 2nd, 3rd and 5th harmonic peaks are evident, the 4th harmonic peak is barely visible above the noise floor (which is about 10dB lower than the laptop card). The extended lobes after the impulse are due to the card's much lower -3dB frequency, 1.0Hz versus 22.1Hz (note that the right side of the time axis is 2.0s in this plot compared to 0.5s in the previous plot):
The control panel for the Impulse graph has these controls:
The impulse response may be plotted with or without normalisation to its peak value according to the setting of the Plot Normalised control. When normalised plotting is selected the peak will be at 100% or 0dBFS.
If Show points when zoomed in is selected the individual points that make up the response are shown on the graph when the zoom level is high enough for them to be distinguished.
The response may be plotted inverted according to the setting of the Invert impulse control. Note that this has no effect when the Y axis is set to dBFS. If the soundcard you are using inverts its inputs that can be corrected using the Invert checkbox in the Soundcard Preferences Input Channel controls.
Generate Minimum Phase will produce a minimum phase version of the measurement using the current IR window settings. The minimum phase impulse then shows the response of a system having the same frequency response as the measurement but with the lowest phase shift such a system could have. This control also activates minimum and excess phase and group delay traces on the SPL & Phase and GD graphs respectively.
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 Hanning 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.
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.
The t=0 offset controls can be used to shift the zero time position by either a specified number of samples, a specified time or a specified distance. Distances are converted to times using a speed of sound of 343 m/s. These controls can be used to manually remove measurement time delays or determine the correct delay to align measurements of different speakers or drive units. Note that shifting the impulse response will clear any spectrogram which had been generated as the plot would no longer be valid. 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 Scale FR Peak control re-scales the impulse response to achieve a desired maximum SPL figure in the corresponding frequency response. This may be useful to rescale an imported impulse response.
ETC Smoothing is used to smooth the envelope (ETC) trace using a moving average filter of the duration specified in the spinner.