Contrast

Contrast Analysis analyzes a selected, single non-stereo audio track to determine the average rms difference in volume (contrast) between foreground (the speech) and background (music, audience noise or similar). The purpose is to determine if the speech will be intelligible to the hard of hearing.

Accessed by: Analyze > Contrast...

To use Contrast Analysis, select in either a single mono speech track, or in the left or right channel only of a stereo speech track.

If you have an unsplit stereo track (meaning that it states "Stereo" to left of the waveform), use the Track Drop-Down Menu, choose Split Stereo Track, then select in either the resulting left or right channel.

With the single track selected, choose Analyze > Contrast... then:

Step One:

• Select a region containing the speech (this is the "foreground" selection)
• Click the Measure selection button in the "Parameters" box in the "Foreground:" row; the start and end times of the selection will be displayed
• The average rms of the selected foreground audio will be displayed in the "Volume" box at the end of the row.

Step Two:

• Select a region containing only the background sound (this is the "background" selection)
• Click on the Measure selection button in the "Parameters" box in the "Background:" row ; the start and end times of the selection will be displayed
• The average rms of the selected background audio will be displayed in the "Volume" box at the end of the row.

Result:

• Contrast Result: Shows if the difference between foreground and background gives a pass or fail against the "WCAG 2.0, Success Criteria 1.4.7" guideline for accessible audio. A pass is given if the difference is 20 dB or more.
• Difference: The actual average rms difference value found.
• Reset: Clears all the parameters and result values.
• Export: Saves the foreground/background values and the result to a text file for proof that you checked your audio file. If you export again to the same file, the new result will be appended underneath the previous one.
• WCAG2 Help: Links to an external illustrated tutorial for using Contrast Analysis with the mouse.
• Close: Closes the Contrast Analysis window, retaining foreground/background and result values for the duration of the Audacity session.

To ensure the track is fully compliant with WCAG2, you should check the contrast between foreground and background in several places.

Links

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