Help with tuning a speaker enclosure

[retroman]

Hi all,

After years of building nothing but sealed enclosures I am ready to try a ported enclosure. A few questions... What specs of a woofer tells me the size of the box and what frequency it should be tuned to? Is there a "rule of thumb" for this or can I make it anything I want? Your advice is appreciated.

Thanks,
retroman

[Ron E]

Rule of thumb:
Don't vent drivers with a Qts much greater than 0.4
Box volume , Vb = 20*Vas*Qts^3.3
Tune Frequency, Fb = Fs*0.42*Qts^-0.96

You can go larger than this recommendation, but you usually have to tune lower or you will get a peak in response. You can also go smaller than this recommendation, but tuning should basically stay the same or you may get peaking. If you go too small, the vent won't provide much improvement in bass extension over a sealed box. Basically with a vented box you want to select a Vb and Fb that maximize bandwidth and have a smoothly increasing response with frequency. Any peaking is to be avoided for the most accurate sound. IF you like "boom", a little peaking may be preferred.

One often runs into the situation that the port doesn't fit in the box, In this case a folded port or a passive radiator can be an option.

There is a lot of software out there that can help with design.

If you prefer to roll your own to understand the process, look at www.diysubwoofers.org - while it may seem like it is oriented toward subwoofers, the enclosure design principles there are applicable to any enclosure.

[Ty_Bower]

Good tips, thank you!

After the ported system is built, are there any recommended techniques for fine tuning the length of the port?

[69stingray]

For tuning the port, I prefer using impedance. Get your hands on a Woofer Tester 3 (or similar testing software). Start with the port longer then calculated and measure the impedance. Keep shorting the port until you reach the correct tuning frequency. The tuning freq. is the lowest point in the impedance (between the two humps). The phase also crosses zero at tuning.


http://www.parts-express.com/pe/show...number=390-803

[Ron E]

For port length, Use the formula:
Lv=K*Dv^2/(Fb^2*Vb)-0.73*Dv
for Lv, Dv in mm, Vb in Liters, K is 2650, IIRC.

Start with a longer port, like Zach says. You can measure or use your ears. IF it sounds too boomy, add length, if you want more oomph, take some off. IF you just can't get it right, stuff a sock in it

[R-Carpenter]

Parts Express has adjustable port tubes which can make the process easier and reversible.

[69stingray]

Here is a good calculator:

http://www.ajdesigner.com/phpvent/su...n_length_l.php

It includes the values of K . Typically, you will have one flanged end (0.732).

Also, don't forget the volume is the net volume, after you subtract the volume of internal braces, the actual port and the motor structure of the woofer.

[tinitus]

I have not yet tested various kinds of BR tuning

But when calculation drivers I try to achieve a rounded gradual smooth rolloff
Usually ends with low port tuning
But always look at port size, to be within sensible limits
But different drivers seem to behave VERY different

Forgot to say that it may count only fore BIG woofers
Funny to see a big pro woofer work ok in just 60liter
It looses 6db below 100hz, but who cares when its a 99db driver fore subs

[Helmuth]

Use a port 1/3 of woofer area. this can lead to a very long pipe in some cases.

A large port sound tighter.


Use Winisd-pro this is a free ware closed, ported and bandpass enclosure simulator.

[PeteMcK]

I second the use of WINISD - this will let you see the effects of different box size and tunings very easily