Vented box tuning frequency

[manek]

Hi,

In my DIY speaker project with the Vifa P17WJ-00-08, I'm using a 22L, The F3 is 44.3hz. I have currently decided to put the box port tuning frequency to 42hz. Is this OK or is there a better alignment ?

Manek.

 

[kelticwizard]

The F3 is 44.3hz. I have currently decided to put the box port tuning frequency to 42hz.

This is a non sequitor.

F3, the 3 dB down point, is not usually the same as Fb, the box tuning frequency.

What is Fb, the box tuning frequency now?

 

[kelticwizard]

WinISD shows that with a box volume of 22 liters and a port tuning of 43 Hz, the F3 is 45 Hz.

You can move the port tuning frequency up or down 10% without much real change in output, if you like.

 

[manek]

Yeah, I've modeled it in WINISD.
the driver Fs=37hz(resonance frequency), What you mentioned kelticwizard was for a maximally flat response. I've kind of stretched the curve a bit to my numbers.

what I want to know is there any fixed formula I have to use to arrive at a port tuning frequency vis-a-vis the F3 of the loaded driver ?

As the driver frequency starts to get closer to the port frequency, the driver begins to get loaded and damped and that restricts driver excursion. That is the reason i wanted the F3 to be a little away from the Fb. What should that gap be ideally ?

 

[BillFitzmaurice]

You misunderstand. The driver is heavily damped at fb but that does not cause a reduction in output despite the reduction in excursion, as the port output is dominant. What you propose to do is the opposite of what you should do.

 

[Ron E]

Originally posted by manek
what I want to know is there any fixed formula I have to use to arrive at a port tuning frequency vis-a-vis the F3 of the loaded driver ?

There is no such relationship.

Originally posted by manek
As the driver frequency starts to get closer to the port frequency, the driver begins to get loaded and damped and that restricts driver excursion. That is the reason i wanted the F3 to be a little away from the Fb. What should that gap be ideally ?

Fb is the tuning frequency of the box. F3 is the -3dB frequency of the sum of the driver and box output. Where the driver contribution is minimum is at Fb. Fb can be anywhere in relationship to F3 and Fs. It depends on Qts.

Fb ~=0.42*Fs*Qts^(-0.96)
Vb ~= 20*Vas*Qts^3.3

F3 is a consequence of the two equations above, not an independent quantity.

Make sense?

 

[manek]

yes, thanks guys....I get it now.
Since this is my first project....I need to get a few things clear and any pearls of widom you guys have to share are most welcome

Was also wondering whether a terraced baffle or a sloped baffle would be the right way to align the acoustic centres of the two drivers ?

I am making a pair of 2 way speakers.

Manek.