Bass reflex tuning frequency, port length correlation

[Daphnary]

Hi Dear DiyAudio community,

Need help for newbie here, regarding tuning frequency for DIY two way build, :
I found formula on Troels Gravessen web site for tuning bass reflex box:
Lv =(23562.5*Dv^2*Np/(Fb^2*Vb))-(k*Dv)
where:
Dv = port diameter (cm)
Fb = tuning frequency (Hz)
Vb = net volume (litres)
Lv = length of each port (cm)
Np = number of ports
k = end correction (normally 0.732)
Neither of above parameters is related to drivers parameters... is this correct? Does not it matter what driver I will put in the box? And problem is if I would like to increase port diameter, the port length also increases considerably, is there any way sort this problem out (beside putting passive radiator instead of port).

 

[indianajo]

Some people use PVC sewer pipe as a port, with a 90 degree elbow to lengthen the port up the long dimension of the box.

 

[brig001]

Does not it matter what driver I will put in the box?

Not for the tuning frequency of the box, no. Wether any given driver will work well with that cabinet volume and tuning frequency is a different matter. Try WinISD of HornResp to model your drivers in the cabinet

Brian

 

[Davor D]

Vb and Fb are related to driver parameters. They have to be calculated first. Then you calculate port dimensions.
Port diameter Dv also depends on driver cone displacement. It has to be big enough to avoid excessive air velocity inside port.

 

[TBTL]

And problem is if I would like to increase port diameter, the port length also increases considerably, is there any way sort this problem out (beside putting passive radiator instead of port).

Unfortunately, no. You can however fold the port to make it fit. The fold should not have sharp edges.

Image taken from https://blog.jseaber.com/2009/08/30/logitech-z-2300-subwoofer-disassembly/

 

[giralfino]

As others have said before, port length depends only on box volume and tuning, once you fix the port diameter. The port diameter depends more or less on the driver diameter, and should be calculated in order to avoid chuffing.
However, the box volume and tuning depend totally on the driver parameters, in other words you can't choose the box volume and tuning frequency by chance. Choosing those parameters is not really easy as it seems, and is the reason of bad bass behaviour in many DIY projects.
I suggest to use a proper bass simulator instead of the simple formula. I use Unibox (requires excel), or WinISD.

Ralf