Tuning frequency of dual ported system

[croseiv]

I have a bookshelf speaker (bass reflex) that has two ports which are 1.5 inches in diameter, but are different lengths. One is 3 inches long and the other one is 8 inches long. The box volume is 23 L.

What is the tuning frequnecy of the ports? Would you calculate the tune of each individually and then average it?

 

[weltersys]

The pair of ports will average out to one tuning frequency, Fb.

You can determine Fb by running sine waves through the speaker, at Fb, the speaker will have the least movement.
Putting a contrasting color dot on the cone makes it easier to see the movement.
If testing at low levels where cone movement is not visible, a light touch with your finger will make it possible to feel when movement is least.

 

[head_unit]

LEAP and maybe some others could simulate the one frequency the combined ports tune to. But with possible end corrections, better to measure as suggested.

We're presuming the ports vent a common enclosure. If there's a sub-enclosure inside, thats another story.