Surround sound and control rooms 563 The surround loudspeakers shown in Figure 21.6 (b) are 10 JBL 8340A, standard cinema surround loudspeakers. They are each driven by separate amplifiers, which allows not only their precise level settings but also the simple switching by the Dolby processor between 5.1 and 6.1 modes of operation. Obviously, where little reverberant integration of the loudspeaker outputs is occurring, any imbalances in the sounds arriving from each loud- speaker would be more noticeable than normal, so individual amplifiers help considerably, here. 21.12.1 Room-to-room compatibility In general, once reverberant colouration is added by the room to the output from the loudspeakers, many changes take place in the perceived sound. Although some degree of reverberation is necessary in public cinema theatres in order to give a more uniform sound character over the entire audience area, in the mixing rooms the reverberation can mask low level details and possible problems and noises on the recorded soundtrack. Room sizes, also ­ surprisingly to many people ­ affect perceived frequency responses. There- fore, if a film is mixed in a room of different size and reverberation/decay time to a room in which it is subsequently shown, the frequency response of the loudspeaker systems will need to be adjusted if the most similar percep- tion of frequency balance is to be achieved. This is a totally separate and additional issue to the one of reverberant colouration changing the sound character.