Tapered/fluted ports vs. non

[doyall]

How does one compare, if at all, fluted ports to non-fluted ports? Is there a difference in their relative effects on some sonic parameter(s)? Can they be reasonable correlated to one another? For instance, if I have a fluted port 4.75" dia. on each end tapering to 3" dia. in the middle and 9.5" long (as in an E250P sub), is there any effective method to determine if the effects of it are comparable to a non-fluted port with a dia. of "X" inches and a length of "Y" inches? Or is this beyond the purview of non-engineers?

[grumpy]

if you pickup the free WinISD (at least the "pro" version), it has options
for port shape (circle or rectangular) as well as whether the ends of the
port are flared (tapered?) or not on either/both ends. You might play with
that to see how the software authors modeled that effect in terms of
changes in port length. port noise is purportedly reduced with flared port
ends ("chuffing"). -grumpy

[whizzer]

The flared duct acts as if it has a larger diameter; it is generally somewhat longer than a duct with one flush end; if the duct is flared on both ends, it acts larger still; its end correction factor will be somewhat greater, thus its length is greater as well, assuming the same tuning frequency. Why is this important? For subwoofer tuning, it's not; the reduction of port noise is a distinct advantage, but in the case of wide-bandwidth bass drivers that operate up into the lower midrange, flared ports, because of their greater length, will produce more resonance effects, resulting in some rather nasty peaks and dips in the driver's response curve, generally in the 400-1000 Hz region, and at periodic intervals dependent on the resonance of the duct itself. As always in audio, for everything you gain, there is a compensatory loss. There is a freeware program called UniBox that will allow you to compare the effects of various sorts of ducts, one flush end, two flush ends, one flared end, and two flared ends, for a given tuning frequency.

[grumpy]

cool (re Unibox)... WinISD has a single "port resonance" value that it
reports (simpler modeling?)... I've just assumed that a user might want to
adjust port size/length/number in an attempt to stay away from that resonance
being in the bass driver's passband (lower crossover frequency). Thanks!

-grumpy

[Ian Mackenzie]

whizzer,

Great post.

Is this the same thing as pipe organ resonances as discussed in The Loudspeaker Cook Book?

[remusr]

Bass reflex tuning is tied to the mass of air in the port - the mass of air in a flared port is larger than that of a port having a constant diameter equal to the minimum dia of the flared port. Friction and coupling in & out would also vary from that of a straight-bore port. The effective bore calculated using dimensions of representative cones should result in a comparable straight bore duct that would give the same resonance.
....from Wikipedia:
a bass reflex system has an opening called a port or vent which consists of an opening, generally backed with a pipe or duct of circular or rectangular cross section. The air mass in this opening resonates with the "springyness" of the air inside the enclosure in exactly the same fashion as the air in a bottle resonates when a current of air is directed across the opening. The frequency at which the box/port system resonates, known as the Helmholtz resonance (http://en.wikipedia.org/wiki/Helmholtz_resonance), is determined by the cross sectional area and length of the duct and the volume of air inside the enclosure.