Tool to measure loudspeaker frequency and step responses and distortions
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QLoud is a tool to measure a loudspeaker frequency response and distortions. This is just an attempt of porting it to QT5. The original app page is here:

Writing this app was inspired by excellent applications written by Fons Adriaensen:

Theoretical background belongs to Angelo Farina:

In particular, this method was used:

Target use

  • loudspeakers DIY-ing (xover tuning).

Measurement chain

  • the app, generating excitation signal,
  • sound card line out,
  • power amplifier,
  • loudspeaker under testing,
  • measurement microphone (with mic preamp),
  • sound card line in,
  • the app, capturing loudspeaker response.


After audio system response capturing is done, IR (Impulse Response) is calculated. Currently these IR-aware analysis outcomes are available:

  • plotting of SPL (Sound Pressure Level) as function of frequency (dB/Hz scale),
  • plotting of IR-power as function of time (dB/s),
  • plotting of Step Response as function of time (amp/s),
  • plotting of harmonics as function of frequency (dB/Hz scale):
    • 2nd – green
    • 3rd – red
    • 4th – light green
    • 5th – light red

IR-power plot is useful to see a room reflections behaviour. Step Response plot is useful for speakers time alignment in multiway loudspeakers. All plots support zooming with history and panning.

A few hints

  • move mouse above “?” sign at plot window and wait,
  • to change a measurement description double-click it,
  • to delete a measurement, use context menu on measurements table,
  • to see what the app do, just connect app’s JACK ports directly and try,
  • to see what your sound card do, use loopback for line in/out,
  • take into account your measurement mic slope at high frequencies when observing harmonics plot.