Dual tuning of ports

[sparky77]

Has anyone had experience with dual tuning of ports? Such as one port tuned to 30hz and the second one tuned to 15hz, and is there any software currently that could model such a thing. I know bassbox 6 pro can't do it.

[Mike P.]

The only dual tuning I'm familair with is a 6th order bandpass system, which WinISD will model. If you mean dual tuning within the same cavity, there's an 8th order bandpass where two different tunings are in one side of the enclosure. Subwoofer Simulator is the only program I'm aware of that will model it.

http://www.geocities.com/f4ier/newsubsim.htm

Click on "8th Order Bandpass" on the left hand side.

[mrstampe]

I wonder if you mean plugging one or two of three ports as SVS recommends for some of their subs in order to achieve differently tuned configurations?? In that case, you would just run different simulations for one, two or three ports operating at the same time. I know WinISD Pro can do this.

[sparky77]

actually I was considering a low tuned sub design using both ports at the same time to create a smoother transistion from the peak to the rolloff of the low end shelf. This technique is commonly used in compact bookshelf speakers to extend response. Has anyone tried this with a subwoofer?

[Bent]

are you meaning a dual chamber bass reflex with both chambers ported - ala weems, or a dual chamber bass reflex with one chamber ported into a second ala Fostex?

[sparky77]

ok, people seem to be making this even more complicated than it really is. A single chamber with a singer woofer and two ports tuned to different frequencies.....

The object is to create a slower taper from the peak response down to the lower shelf rolloff.....

Has anybody here actually tried this before, I would like to hear from someone that has tried it or at least has some knowledge on the subject.

[Bent]

the idea would be to make all ports the same length and plug one for lower tunes.
If they were different lengths, then the box wouldn't "see" the lower tuned one, as the driver will have unloaded at the higher port's tuning frequency.

[tued]

Bent wrote:

the idea would be to make all ports the same length and plug one for lower tunes.
If they were different lengths, then the box wouldn't "see" the lower tuned one, as the driver will have unloaded at the higher port's tuning frequency.

I have also though about doing this but can you explain why this is the case?

[Mike P.]

sparky77 wrote:

actually I was considering a low tuned sub design using both ports at the same time to create a smoother transistion from the peak to the rolloff of the low end shelf. This technique is commonly used in compact bookshelf speakers to extend response.....

Can you give us an example of a bookshelf speaker that uses this technique?

[Ricci]

I do believe that Polk has patented this as ARC (Acoustic Resonance Control) and use it in their bookshelf speakers. They claim to use it to cancel out resonances though not add additional low end.
I've toyed with this idea myself. Someone should get a cheap 8" driver build a cheap throw away cab and try this out + get some measurements.

[collo]

Sorry dude, it won't work.

The tuning frequency of a ported box is determined by the "springiness" of the air in the box and the load presented by the air in the ports.

To use an electrical analogy: Since the two ports are basically in parallel, the load presented can be visualised in the same fashion as two resistors in parallel

With ports of different sizes, each will flow different amounts of air, but the system sees a single load.

Multiport systems like SVS only run a single tune at a time, depending on how many ports are plugged.