1.4 Audio Contrast Draft Example

I recorded these tracks and set the various levels by measuring the RMS power for the foreground and background of each example. RMS is the average volume level. In the first example the the voice (foreground) is recorded at -17.52 decibels and the music (background) is at -37.52 decibels, which makes the foreground 20 decibels louder than the background.

In the second example the voice (foreground) is at -18 decibels and the music (background) is at about -16 decibels making the foreground only 2 decibels louder than the background.

Background music by David MacDonald (c)2004

Level 3 Success Criteria for Guideline 1.4

  1. Audio content does not contain background sounds or the background sounds are at least 20 decibels lower than the foreground audio content, with the exception of occasional sound effects. [V]

Note:

A 20 decibel difference in sound level is roughly 4 times quieter (or louder). Background sound that meets this requirement will be approximately four times (4x) quieter than the foreground audio content.

Example 1 - Good audio contrast. Foreground is 20 decibels above the background. Link to MP3 sound file (requires a media player)

Transcript of voice on 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 2 - Bad audio contrast. Foreground is less than decibels above the background. Link to MP3 sound file (requires a media player)

Transcript of voice on bad contrast example:

"This is an example of a voice that is not loud enough against the background. The voice which is the foreground is only about 2 decibels above the background. 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."

I had a meeting with the forensic audio engineer for the Royal Canadian Mounted Police (RCMP) and discussed with him the results of my examples. Our WCAG 2.0 document says if the foreground is 20db above the background then it is about 4x the volume of the background.

The ABC’s of Audio perceived volume, decibels and power:

Power, volume and perceived volume are not the same.

Perception of loudness is logarithmic. All things being equal, when you double the amount of power that you feed to your speakers, you get a 3-decibel increase in the sound pressure level at the output: 3 dB is considered to be a noticeable difference in volume. A 10-dB increase is twice as loud. Therefore, the difference between 1 and 2 watts is equal to the difference between 10 and 20 watts—likewise for 100 and 200 watts.

“The human body perceives ten times the sound-power as being only twice as loud. And 100 times the sound power is only 4 times as loud to our ears.
Weird but true. And actually good for us, probably designed to that effect by evolution, so that we can hear and effectively use the extremely wide range of sound
powers that frequently exist in various times and places.” http://www.newton.dep.anl.gov/askasci/eng99/eng99325.htm 

The formula for the difference in decibels between two different sound sources is defined to be:

10 log (P2/P1) dB (where the log is to base 10)

If the foreground was 4x louder (perceived volume) than the background, it would need to be 100 times the power of the background.

The formula would be
10 (log (100/1)) db
= 10 *(log 100) db
= 10 * 2db
= 20db

20db is a huge drop in volume, and as can be heard on my example. It will be hard to get people who produce sound tracks to buy into our guidelines given this ratio, I think.

David MacDonald

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