Port length in WinISD?

[Marsh]

Just a quick question, In winISD when using more than 1 vent/port does the length displayed refer to the length of each port or the total length of them?

I'm re-designing my box as there is too much port noise below 25Hz, so I'm adding another port to 1/2 the air flow and thus reduce noise (hopefully)

Thanks in advance.

Rich.

 

[AndrewT]

Hi,
I got confused when using WINisd for selecting ports diameters and lengths. The algorithm is wrong.
Use WINisd pro. It gets the calculations right.

The port length is dependant on the box (internal) volume and the port areas. The length shown is the length of each port.
Add a second port of the same diameter and the length of both ports goes up markedly. This reduces the box volume which you will have to change and then the programme gives an even longer port length reducing the volume even more.

 

[Marsh]

Hi,
I got confused when using WINisd for selecting ports diameters and lengths. The algorithm is wrong.
Use WINisd pro. It gets the calculations right.

The port length is dependant on the box (internal) volume and the port areas. The length shown is the length of each port.
Add a second port of the same diameter and the length of both ports goes up markedly. This reduces the box volume which you will have to change and then the programme gives an even longer port length reducing the volume even more.

Thanks, that answers my question, it's a bit of a catch 22.
I am using Pro, I should have said.

Thanks.

 

[mpmarino]

Andrew,

Could you exemplify what the issue is with winisd? Does it actually get the area / length wrong?!

Thanks,
Marc

 

[AndrewT]

Hi,
it's a few months since I stopped using WINisd and moved over to alpha release of WINisd pro.
I seem to recall that when dual or multiple vents were called up the results became nonsense. The predicted mach number was wrong as well as the length. I think it indicated that four 2inch vents could handle MUCH higher power than a single 4inch vent.

 

[mpmarino]

Thanks, I'll keep an eye on it.

 

[AndrewT]

Hi,
port length agrees in both versions.
It's the mach no. that varies badly.
Sorry to mislead.

 

[Svante]

Just a quick question, In winISD when using more than 1 vent/port does the length displayed refer to the length of each port or the total length of them?

Don't know, but you can test it rather easily; Adding another vent should approximately double the length of the vents.

Actually, I see no reason that it should show half or double the required length...

A recommendation though, is to measure the resonance frequency on the finished design, it is not uncommon that the tuning becomes slightly too low due to the isothermalizing effects of box stuffing (which makes the box appear larger). OT, sorry.

 

[Buzzy]

Does winisd pro account for the port volume ? i.e I have a box volume of 3.5ft^3 modelled in winisd, do i need to add the volume of port to this 3.5 ft^3 when making the final box ?

 

[AndrewT]

Hi,
the volume of the port (walls and tube) must be added to the required internal volume.

 

[Andy Westcott]

I've just seen this thread, and was interested to hear that winisd seems to create errors when calculating ports.

I had just this experience when designing a bass cabinet with 3 ports, and discovered that the dimensions suggested for a particular tuning frequency were incorrect, the program suggesting that the ports needed to be longer than was actually the case, by, if my memory serves, about 4 - 5 centimetres.

I finally got the ports sort-of correct (for my purpose) by adjusting the port lengths to produce the desired tuning frequency as determined by the impedance dip.

Still a very useful program, and free, so I'm not complaining really!