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.
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 ...
You will be selecting sections of foreground and background. The following images shows an example of what foreground speech and background sound look like.
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.
The Results can be exported as a *.txt file. The *.txt file includes:
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.
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.
Press CTL+SHIFT+T again and this will open the tool.
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.
This can be exported as per the instructions in the previous section.
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.
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