Techniques for WCAG 2.0

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G56: Mixing audio files so that non-speech sounds are at least 20 decibels lower than the speech audio content

Applicability

Any technology

This technique relates to:

Description

The objective of this technique is to allow authors to include sound behind speech without making it too hard for people with hearing problems to understand the speech. Making sure that the foreground speech is 20 db louder than the backgound sound makes the speech 4 times louder than the background audio. For information on Decibels (dB), refer to About Decibels.

Examples

Example 1: An announcer speaking over a riot scene

Example 2: Sufficient audio contrast between a narrator and background music

The following is a link to an mp3 file. It is an audio track that has been mixed so that there is sufficient contrast between the foreground and background sounds. When it is listened to, the foreground is heard clearly above the background.

Example of good audio contrast (MP3)

Here is a transcript of the speaking voice on this good contrast example:

"Usually the foreground refers to a voice that is speaking and should be understood. My speaking voice right now is 20 decibels above the background which is the music. This is an example of how it should be done."

Example 3: Audio track that has been mixed with sufficient contrast between the foreground and background as seen in a popular audio editing program.

After the foreground and the background sound tracks have been mixed to one single file, that single file can be opened in any popular audio editing software package and viewed in an editing window that renders a visual representation of the audio content. This can be used to help determine the contrast level between the foreground and background of an audio track.

The graphic below (figure 1) is a visual representation of the mp3 audio file above that has sufficient contrast between the foreground and the background. Both the foreground and the background are in the selected portion of the wave file.

Figure 1

Figure 1: visual representation of sufficient contrast both foreground and background are selected.

The selected region of the wave file in figure 1 above which contains both the foreground and background sound is a very large wave. The selected region of Figure 2 below which contains only background sound is a much smaller wave.

Figure 2

Figure 2: visual representation of sufficient contrast where only the background is selected.

Failure Example 4: Insufficient Audio Contrast between a narrator and background music

The following is a link to an MP3 audio example of insufficient contrast between a narrator and background music:

Example of bad audio contrast (MP3)

Here is the transcript of the voice on the insufficient contrast example:

"This is an example of a voice that is not loud enough against the background. The voice, which is the foreground sound, is only about 2 decibels above the background sound. Therefore is difficult to understand for a person who is hard of hearing. It is hard to discern one word from the next. This is an example of what not to do."

Resources

Resources are for information purposes only, no endorsement implied.

(none currently listed)

Tests

Procedure

  1. Locate loud values of background content between foreground speech

  2. Measure the volume in dB(A) SPL

  3. Measure the volume of the foreground speech in dB(A) SPL

  4. Subtract the values

  5. Check that the result is 20 or greater.

Expected Results