Radio in my car is audible between about 20 (engine off) and 40 (top tuuuunes whilst hammering down the motorway). Not all volume levels are okay:
Acceptable: 20, 24, 25, 28, 30, 32, 35, 38, 40
Unacceptable: 21, 22, 23, 26, 27, 29, 31, 33, 34, 36, 37, 39