I'm building a subwoofer that fits into a space at the rear of my minivan. Because of design constraints my port starts at 1.75" wide and 8.5" high for the first 26.5"
It then goes from 8.5" to 9.5" (x 1.75") for a 1" stretch
and the last 2.5" is 9.5" Total port length = 30"
I'm using a width of 1.75" and a weighted average height of 8.7"
I have no clue how It will tune, I know it won't be the same as if it's one constant size, but will it be close? I haven't found anything designed this way, normally that's because it's a bad idea. Should I find another Solution?
It then goes from 8.5" to 9.5" (x 1.75") for a 1" stretch
and the last 2.5" is 9.5" Total port length = 30"
I'm using a width of 1.75" and a weighted average height of 8.7"
I have no clue how It will tune, I know it won't be the same as if it's one constant size, but will it be close? I haven't found anything designed this way, normally that's because it's a bad idea. Should I find another Solution?
Any discontinuity in a reflex tuning port will cause turbulence and non-linearity in the port's airflow. Whether the result is listenable/acceptable will only be determined by building a prototype.
Is it possible to use a passive radiator? A reflex port resonates because the mass of air in the port is 'bouncing' on the springiness of the compliance of the air in the enclosure and the springiness of the bass driver. This creates a mass on a spring with it's natural resonance. The passive radiator replaces the mass of air in a port, with a diaphragm's mass.
If there is no room for a passive radiator on the baffle, a passive radiator coupled to a short port will be better than the convoluted layout described above. In this case the effective tuning mass is the mass of air in the port added to the mass of the passive radiator acting together on the compliance of the system.
It is a bit convoluted to design passive radiator alignments with software designed for ported alignments. But because a passive radiator is tuned by simply adding mass to the diaphragm (usually by bolting on washers) it is easy enough to tune the alignment by looking at the impedance curve and noting the position and size of the two bass peaks.
Is it possible to use a passive radiator? A reflex port resonates because the mass of air in the port is 'bouncing' on the springiness of the compliance of the air in the enclosure and the springiness of the bass driver. This creates a mass on a spring with it's natural resonance. The passive radiator replaces the mass of air in a port, with a diaphragm's mass.
If there is no room for a passive radiator on the baffle, a passive radiator coupled to a short port will be better than the convoluted layout described above. In this case the effective tuning mass is the mass of air in the port added to the mass of the passive radiator acting together on the compliance of the system.
It is a bit convoluted to design passive radiator alignments with software designed for ported alignments. But because a passive radiator is tuned by simply adding mass to the diaphragm (usually by bolting on washers) it is easy enough to tune the alignment by looking at the impedance curve and noting the position and size of the two bass peaks.
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if it's a smooth expansion it will probably have acceptably smooth airflow.
but it will be hard to predict the actual tuning frequency.
can you build, and then make an impedence sweep measurement?
you can then just trim the length to get the resonance you want.
the Dayton DATS unit has been very helpful for me in building cabinets.
no more guesswork due to inaccurate sims or manufacturer data.
but it will be hard to predict the actual tuning frequency.
can you build, and then make an impedence sweep measurement?
you can then just trim the length to get the resonance you want.
the Dayton DATS unit has been very helpful for me in building cabinets.
no more guesswork due to inaccurate sims or manufacturer data.
Thanks to you both I now realize that I can fill in the areas in question after the fact. I'll try the design and let you know how it goes.
The Dayton unit is $130, I'm building just one unit, is there a free (or inexpensive) impedance measuring app that might suffice?
yeah, the price seems steep for what it is.
but the "old-fashioned" way is pretty cumbersome:
https://audiojudgement.com/loudspeaker-impedance-measurement/
but the "old-fashioned" way is pretty cumbersome:
https://audiojudgement.com/loudspeaker-impedance-measurement/
REW (donorware) and a jig made from a few bits of cable and one, accurately measured resistor can do impedance measurements: https://www.roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html#top
Rather than measuring impedance, you can look at cone excursion while doing a sine wave frequency sweep (don't use too much voltage..), which, like impedance, will be at minimum at Fb (box tuning). Putting a white dot on the cone makes the movement easy to see, the excursion will increase sharply above and below Fb.
With a high aspect ratio port like you plan, the Fb will be below what a square or circular port of the same cross sectional area would be.