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WCAG 2.0 Audio Audio Contrast Tool Help
For Success Criteria 1.4.7.

WCAG 2.0, Success Criteria 1.4.7 is a guideline for accessible audio files on the internet. The goal is to help people who are hard of hearing understand the speech (foreground). This is done by making sure that any background music or noise is very quiet. Background music (or sound) should be at least 20dB lower than the foreground speech.

Preparing to test the audio

Before beginning, download and install the Audacity program (free of charge) from http://audacity.sourceforge.net/download/

If the audio track is not currently on your local machine, you will need to save it locally. Right Click>Save As ...

Directions for mouse users (keyboard instructions follow)

  1. Open the Audacity program and load the sound file that you would like to evaluate
  2. Listen to the file and choose a region of the speech that is characteristic of the rest of the speech.

    Audio tool play stop and pause buttons

  3. Listen to foreground and background

    Audio file

  4. Select this speech (foreground) by dragging the mouse over the section you would like to measure. (Naturally the foreground selction also has background sound included in it.)

    Selected region

  5. Start the Audio Contrast tool. On the main menu select Analyze>Contrast
    (or press CTL+SHIFT+T)

    COntrast tool on the Audacity menu

  6. A message box will say "Now select background" - Press OK


    Measuring Foreground

  7. Select background music or sound next to the speech that was measured above. Make sure it contains only background music or sound. (To select it, drag mouse over the selection as described above)

    Selection of background

  8. Open the Audio Contrast tool Analyze>Contrast (or press CTL+SHIFT+T). In the results section there will be will be the Pass or Fail Evaluation. It will also display difference in volume between the foreground speech and the background music or sound. If the difference is less than 20dB the result will be a fail, if it is greater than 20dB it will be a pass.

laout of results

Note: You may need to check the contrast in several sections sections of the file. If it fails anywhere then the success criteria has not been met.

There is a reset button in the "results" section to begin again.

Saving and Exporting the Results

The Results can be exported as a *.txt file. The *.txt file includes:

  • File name and date of evaluation
  • Start and end times of the foreground and background
  • Foreground and background measurements in average RMS dB
  • Results: pass or fail notification and the difference in average RMS dB

The export file will also include any previous comparisons that were done while the tool has been open. Below is an image of the export file.

export file of tool

The measure(d) buttons are useful if you are entering the audio selection in the time boxes instead of selecting the section with a mouse. The "Use Selection" button will measure whatever is selection (indicated by the darker gray section in the audio file).

The "Use Selection" is for measuring whatever is selected by the mouse. The selected area is the darker part of the audio file.

Note: the measurements are recorded in average RMS so occasional drops and increases in volume are averaged over the selection made. This is to minimize artifacts which are allowed in the Success Criteria.

Directions for non-mouse, keyboard users

  1. Open the Audacity program and load the sound file that you would like evaluate
  2. Listen to the audio file by hitting the "Spacebar" to start and stop the audio
  3. Note the beginning and the end times of the section of the foreground speech and the background audio

Audio file

  1. Start the Audio Contrast tool. (CTL+SHIFT+T)
  2. A dialogue box will come up saying "Now select background." Ignore this box by selecting "OK"

Measuring Foreground

  1. Press CTL+SHIFT+T again and this will open the tool
  2. Tab to the "reset" button and hit it.

Main Dialogue box

  1. Tab to the Foreground Start time box
  2. Enter the "Start" time of the foreground
    (arrow keys will move from number to number in the time box)
  3. Tab to the "End" time of the Foreground and enter that.

Keyboard time entry

  1. Type in the start and end times of the foreground
  2. Tab to the "Measure" button and activate it
  3. Tab to , and enter the Background start and end times
  4. Tab to the "Measure" button and activate it. Once it's been measured it will "grey out" until the time changes again.
Measure button

In the results section there will be will provide the Pass or Fail Evaluation, with the specific difference between the foreground speech and the background music or sound.

laout of results

More about Success Criteria 1.4.7

Here is the text of the Success Criteria:

1.4.7 Low or No Background Audio: For prerecorded audio-only content that (1) contains primarily speech in the foreground, (2) is not an audio CAPTCHA or audio logo, and (3) is not vocalization intended to be primarily musical expression such as singing or rapping, at least one of the following is true: (Level AAA) How to Meet 1.4.7 Understanding 1.4.7

  • No Background: The audio does not contain background sounds.

  • Turn Off: The background sounds can be turned off.

  • 20 dB: The background sounds are at least 20 decibels lower than the foreground speech content, with the exception of occasional sounds that last for only one or two seconds.

    Note: Per the definition of "decibel," background sound that meets this requirement will be approximately four times quieter than the foreground speech content.

Credits

The Audio Contrast Tool for Audicty was developed by the Trace R&D Center University of Wisconsin in collaboration with David MacDonald of E-Ramp Inc., and programming by Martyn Shaw of the Audacity team, under funding from the National Institute on Disability and Rehabilitation Research, US Dept of Ed under grant # H133E080022