Ah = cross sectional area at the beginning of the horn
Am = cross sectional area at the end of the horn
l = horn lengthThe numbers I gave correspond to a hyberbolic horn as it was descriped in a previous edition of the german magazine "Hobbyhifi" as you will model this horn you will see that in this case a too large volume will result in a non flat frequnecy response. In this case the volume should be kept as small as possible:
http://www.hobbyhifi.de/Archiv/02/04_02/04_02.html
Best regards,
Walt