Note that the theoretical Q or Directivity index of a loudspeaker is very theoretical. Real live loudspeakers tend to vary significantly from theory, and one manufacturer's 90x40 loudspeaker is not often the same as another's. Loudspeakers also vary widely in directivity with frequency. Where a speaker may actually have a Q of 12.8 at 2kHz, it is likely that that the directivity value would drop to a Q of 4 at 500Hz, and as low as a Q of 1 at 125Hz. A thorough acoustical and sound system design would include the prediction of expected sound system behaviour across the entire audio bandwidth, taking into consideration the variations in loudspeaker coverage and variations in reverberation time. This is where computer modelling is very handy.