DEM AudioLinFIR Documentation
LinFIR

Sweep Spectrogram License

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License Required: The Sweep Spectrogram window requires a valid LinFIR license.

The Sweep Spectrogram shows a driver's recorded sweep as a time-frequency colour map: the fundamental climbing from the start to the end frequency, with the harmonic distortion running above it as parallel tracks at 2×, 3×, 4× and 5× its frequency. It shows at a glance where in the sweep the distortion appears - a rub, a buzz or a resonance lights up a harmonic track over a narrow stretch of the sweep - and how it compares to the fundamental.

Unlike the Spectrogram, which analyses the processed impulse response of the system, this window looks at a single raw driver measurement, before any filter or time window.

Accessing the Sweep Spectrogram

Menu: View → Sweep Spectrogram

The window opens in its own viewport. Cmd/Ctrl+W closes it.

How the Sweep Is Rebuilt

LinFIR does not keep the recording of a sweep measurement - only what the ESS (Farina) deconvolution extracted from it: the raw impulse response of the fundamental and, ahead of it, the harmonic impulse responses (H2 to H5). The window rebuilds the sweep from them:

  1. The sweep used for the measurement is generated again from the parameters stored with it (start and end frequency, duration, sample rate).
  2. It is convolved with the raw impulse response including its harmonic part. Convolving the sweep with the fundamental gives back the linear response to the sweep; the harmonic impulse responses, which sit before it in time, add each harmonic's own track.

The impulse response already carries the sweep's band shaping - the fade-in below the start frequency, and the fade-out of a limited end frequency - so the sweep used here is the measurement's own reference sweep with that shaping undone. The fade-in octaves therefore read at the level they were recorded at, and with a limited end frequency the harmonics above the end of the sweep, which the capture keeps, are rebuilt too.

The result is the sweep as the microphone heard it, without what the capture left out: harmonics above H5, the noise away from the sweep and the reverberation beyond the kept impulse response. It is then analysed with the same Morlet wavelet transform as the Spectrogram, but with a resolution matched to the sweep in each frequency band: the analysis window lasts as long as the sweep takes to cross the band. This draws the fundamental and its harmonic tracks as thin as the sweep allows - a few percent of their frequency in the midrange, about one percent in the treble of a long sweep - so the harmonics stay apart up to H5. A longer sweep gives thinner tracks. Each band is also corrected so that the sweep and its harmonic tracks read at their true level.

Controls

  • Driver - the driver to look at (in Room Calibration, the measurement).
  • Angle - there is no angle selector: like the Distortion Impulse Responses window, the map shows the measurement at the angle selected in the main window (the on-axis one if the driver has none at that angle), and the window title shows that angle.
  • Min dB - bottom of the colour scale, relative to the loudest point (-140 to -20 dB, -80 dB by default).
  • Harmonic guides - draws the theoretical tracks of the fundamental (H1) and of H2 to H5 as dashed lines, to tell the harmonics apart from noise or reflections.

0 s is the start of the sweep; the map begins 0.1 s earlier, to show the noise and pre-ringing before it. Hovering the map shows the time, frequency and level under the cursor (below the bottom of the colour scale, just "< Min dB"). A line above the map recalls the stored sweep parameters: these are the limits of the sweep actually played, which extends below the start frequency you set (down to a quarter of it) and above the end frequency (up to twice it, or the Nyquist frequency) so that the requested band is fully covered.

Requirements

  • The measurement must have been captured with LinFIR's sweep (Sweep Measurements), or imported from a WAV or TXT file exported by LinFIR with its distortion data: it needs the harmonic impulse responses and the stored sweep parameters. Measurements imported from other software have neither, and are not listed.
  • A valid LinFIR license is required to open the window.