Mix audio so foreground speech is louder than background sounds
no. 134
Make sure foreground speech is always four times louder than background sounds
Whenever pages embed audio files, where the information being conveyed comes over some background audio (music, traffic noise, crowd murmur, yelling, etc.), end users – especially those with hearing impairments – are at a risk of having difficulty clearly discerning the information presented in the foreground. As the audio file is consumed as a single object within the page, separating the foreground audio from the background noise is usually impossible. In order to help users clearly understand the information contained in the audio file, authors need to make sure the information in the foreground comes out much louder than the information in the background when they mix their audio tracks.
To do so, simply make sure than whenever foreground information such as speech is put against background audio, that the foreground sound information is at least four times louder than the background audio. A simple rule of thumb when it comes to audio is that for every 10 decibels increment, the sound becomes twice as loud. So by making sure the foreground audio track is 20 decibels higher than [...]