Most horn designs are maximum-efficiency, meaning that their impedance versus the amplifier may rise up to the point where maximum sensivity is not obtained. This criteria addresses that issue, allowing a compromise between impedance and efficiency.
My criteria for designing horn loudspeaker throats:
1. Maximum sensivity (so as the horn extracts maximum power from the amplifier)
2. Maximum efficiency
St - horn throat area
ws - driver free air resonant frequency
Vas - Volume of air having the same compliance as the driver suspension
Qms - Driver mechanical quality factor
Qts - Total driver quality factor
c - speed of sound, 345m/s
If Qts(roughly)=Qms 1 will correspond to 2
ws, Vas, Qts, Qms are part of the driver Thiele-Small parameters.