The volume of the rear chamber should change if another compression ratio is used. In fact this volume varies lineair with this ratio.For some horns it really makes a difference if the volume is chosen wrong. Might cause 5-6dB bumps! Use McBean to model a rearloaded hyberbolic horn with:
l=2.8m
ah=75cm2
am=850cm2
In this case the volume should be kept as small as possible!
Best regards,
Walt