From: JBL Cinema Sound System Manual
"JBL pioneered the concept of flat power response in the cinema (2,3). It has become the
guiding principle in much of JBL’s product design, and it has been adopted by the industry at large."
And from the Lenard Audio Institute: "Old JBL documentation of sound system application and alignment, states a sound level difference between the front to the rear of a cinema of less than 6dB. Considering that the direct sound energy would decrease approx 24dB for the inverse square law, for an average large cinema, and the directivity of the hi-frequency horn would give an approx 6dB improvement, that means approx 12dB of reverberant energy is accepted. It is difficult to understand how any intelligibility is heard with this level of reverberation at the rear of a cinema."
And an article on the x-curve and home use: