Toyota Prius C 2011 Owner's Manual page 252

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Adjusting the Automatic Sound Levelizer (ASL)
When ASL is selected, turning
amount of ASL in the order of "LOW", "MID" and "HIGH".
Turning to the left turns ASL off.
ASL automatically adjusts the volume and tone quality according to vehi-
cle speed.
3-2. Using the audio system
to the right changes the
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