A horn with a path less than 1.67 meters works as both a horn with a
high cutoff point and as a bandpass type enclosure. The driver has a
rear chamber, a front expansion chamber and a horn, all these functions
are those of a 4th order bandpass enclosure as well, the horn acting as
a tapered port in this case. The enclosure can be designed as a horn
with the ¼ wl fo point well above the desired fo, or as a bandpass
enclosure using a tapered port. Both routes will yield results that are
very similar and altering the operating conditions in one enclosure type
will show very similar changes to the response in the other enclosure
type. Any reflex port can be tapered just as long as its overall length
is not changed and the internal volume remains the same. Below are some
observations about tapered ports and there use
1) A tapered reflex port may be viewed as two back-to-back horns
operated well below their [fo]. Here, phase velocity is infinite and the
air inside the horns moves back and forth as a unit. In this setting, it
is treated as an acoustic mass [kg/m^4].
2) The effective volume of air extends beyond the ends of a port. If the
ends are flared, the end corrections will be larger as they are
determined by aperture area and the absence/presence of an adjoining
surface.
3) If the volume of air is surrounded by a minimal surface, that by
definition is required to be smooth, then airflow resistance will also
be at a minimum and radiating efficiency (from both port apertures) will
be maximized. Bottom line is that the port should be circular in section
and flared at both ends in a manner as smooth as possible. The geometric
object that accomplishes this is a full catenoid, i.e. two horns
(cantenoidal) joined at the throat.
|
|