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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.
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