Okay, I really do need help this time (as opposed to promoting discussion). Don't ask how but I've got a 28 litre cabinet with two ports. One is 45 x 125 and the other is 65 x 150. How can find the overall tuning frequency of the cabinet. I'm guessing it around 60 to 65hz but . . .?
I just plug everything into the sim and start iterating.
If you start witht he forumuas it is likely easily solvable.
dave
If you start witht he forumuas it is likely easily solvable.
dave
REW is free, and can generate all sorts of test tones.
Regardless of using two different port size and length, there still will be one Fb (frequency of box tuning), though you won't find it in a simulation.
Run a sine wave signal into the cabinet, shift up and down from your guess, the cone will move the least at Fb , and will increase above and below.
Putting a white dot on the cone makes it easy to see the movement- keep the tests short to avoid roasting the voice coil if it requires more than a few volts to see movement.
Cheers,
Art
I'd use the sum of the port areas and the average of the two lengths to work out an equivalent single port
Ah, but a very short port would “short out” (for lack of a better term) a long port.
Perhaps RMS averaging would be better..?
Chris
Perhaps RMS averaging would be better..?
Chris
As Planet10 says - use the right formulae, and away you go.
The basic model is a Helmholtz resonator with multiple ports, with each port representing a volume of air with mass, whose area is supported by the compliance of the shared spring represented by the air contained in the enclosure. That needs to be modelled properly - and not be the subject of stabs in the dark...
The basic model is a Helmholtz resonator with multiple ports, with each port representing a volume of air with mass, whose area is supported by the compliance of the shared spring represented by the air contained in the enclosure. That needs to be modelled properly - and not be the subject of stabs in the dark...
Done. With interesting results. The two ports give me an Fb of 60Hz and result in a pair of 'boomers'. The plastic top from a roll-on deodorant bottle makes a perfect bung for the smaller port, who knew? Using Using a single port results in an Fb of 42Hz.
In a traditional, perfect world the single port produces the smoothest response but the tandem ports only start losing out below 52Hz, and that's really sub territory . . . and we all have subs.
Before any suggests improvements that will cost corn: A had some old empty Sony floor-standing that were very well made. I threw in a pair of seven inch woofers and Eltax dome tweeters that I had lying around. The original system was three-way, so I was left with a hole in the baffle. The hole turned out to be a perfect fit for pre-moulded ports.
If I've learned anything from this exercise it's why big box manufacturers tune their ports so high.
Thanks, all.
In a traditional, perfect world the single port produces the smoothest response but the tandem ports only start losing out below 52Hz, and that's really sub territory . . . and we all have subs.
Before any suggests improvements that will cost corn: A had some old empty Sony floor-standing that were very well made. I threw in a pair of seven inch woofers and Eltax dome tweeters that I had lying around. The original system was three-way, so I was left with a hole in the baffle. The hole turned out to be a perfect fit for pre-moulded ports.
If I've learned anything from this exercise it's why big box manufacturers tune their ports so high.
Thanks, all.
Increasing your port area is a good thing.. you just need to make them longer. Obviously there are limits. One of them is the port length resonance and another is fitting it in the box.
The most interesting aspect of the exercise for me is the subject of 'port tuning'. I learned from internal combustion engines, what you gain in power you generally lose in torque. The question in diy speaker design is: what do you want the cabinet to do? In the case of the tandem port configuration it offers +5dB @80Hz if relieved of sub 50Hz responsibilities. As intimated, we all have subs on sub-50hz duty, why wouldn't we take the 'free' volume offered at musical bass frequencies?
I guess the reason they say that, is the same reason that an applied pipe diameter has a bandwidth. The same reason that a 4V head is only better than a 2V head if you're expecting to hit the revs.
The speaker port isn't quite the same, it is expected that it will perform the same at all levels. However at some high level the speed of the air will become a problem. Hence the points I made in my last post.
You can also round them over.
The speaker port isn't quite the same, it is expected that it will perform the same at all levels. However at some high level the speed of the air will become a problem. Hence the points I made in my last post.
You can also round them over.
It's all done. They can go to the goodwill now. The results are quite shocking. They are the bassiest of all the floor-standers I've listened to, somewhat reminiscent of the old Wharfedale Lintons but even boomier.