Finally porting the Tang Band 6.5's...need help/ideas

[BrianChia 10+ year member]

Here you go, I modeled 3 different alignment with predicted transfer function. To do that I added a Linkwitz-Transform filter. The corner frequency F0 of 80hz was just a guess. The actual corner frequency is determined by the speed of sound / 2 * (the longest internal length of the cabin) or =c/2l. The filter Q of 0.707 creates an effect like an inverted 2nd order Butterworth filter.

As you can see the ported box produces a nasty peak and has severely reduced low end extension. This is typical of most ported boxes used in cars. The 4th order and sealed give the flattest response because the roll off of sealed enclosures is roughly equivalent to the inverted slope of the transfer function, thus summing equally to a flat response.

All 3 curves show predicted SPL at 300 Watts of power.

 

[ItalynStylion 10+ year member]

^wow, thanks for doing that!

The 4th order response curve looks MUUUCH better than the others. I have my Focal's crossed over at 80hz though; would there be any way to get that nearly flat response from 80hz down?

PS: I don't even know what a 4th order band pass is but I can look at those response curves and know what's what i want. Can you explain this to me? I can build anything so construction is not a problem //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

 

[BrianChia 10+ year member]

A 4th order bandpass is basically a sealed enclosure that fires the subwoofers into a ported chamber. The ported chamber acts as a low pass filter cutting off frequencies above tuning the same way that a ported box acts as a high pass filter.

See this page for more details:

http://www.diysubwoofers.org/bnd/4thorder.htm

Of course everything comes with a price. The 4th order box I modeled was 2.5 cf in the rear chamber and 1.0 cf in the front chamber, with the port tuned at 65hz. We can compromise some if you fill the rear with heavy poly-fill, and get by with about 1.5-2.0 cf. The larger the sealed chamber is the lower it will extend with flat response. The front chamber can be ported around 68-70 hz to increase the passband up to around 75hz. If you are running active high passes for the mid-woofers, then it should be easy to drop the high pass down to 70 hz or so to match.

 

[ItalynStylion 10+ year member]

****, it sounds cool but I don't know if I'm ready to give up that much trunk space. I was hoping to get out around 2 cubes.

If I did do the ported box that I had listed before and tune to 35; what will do you think that the cabin gain will compensate for the sharp drop off that happens around 36hz?

Oh, and a build log will follow all of this indecision for those that care...

 

[ngsm13 5,000+ posts]

Why not face the port the same direction as the subs?

You going to make your enclosure that large?

nG

 

[BrianChia 10+ year member]

You going to make your enclosure that large?
nG

He could use an elbow.

 

[ItalynStylion 10+ year member]

You going to make your enclosure that large?
nG

The enclosure would be right about 1.94 cubes tuned to 34hz. With a 4" precision port it would only have to be 11.3 inches. That's not that long.

 

[its_bacon12 10+ year member]

A 4th order bandpass is basically a sealed enclosure that fires the subwoofers into a ported chamber. The ported chamber acts as a low pass filter cutting off frequencies above tuning the same way that a ported box acts as a high pass filter.
See this page for more details:

http://www.diysubwoofers.org/bnd/4thorder.htm

Of course everything comes with a price. The 4th order box I modeled was 2.5 cf in the rear chamber and 1.0 cf in the front chamber, with the port tuned at 65hz. We can compromise some if you fill the rear with heavy poly-fill, and get by with about 1.5-2.0 cf. The larger the sealed chamber is the lower it will extend with flat response. The front chamber can be ported around 68-70 hz to increase the passband up to around 75hz. If you are running active high passes for the mid-woofers, then it should be easy to drop the high pass down to 70 hz or so to match.

in theory they should do as you described, but in reality, 4th order bandpass boxes really are designed for output at a very narrow frequency band, but could gain up to 12 db at those frequencies..

if you're up to it OP and want to try a bandpass, look into 6th order. covers a much wider range.

ofcourse, i would suggest a ported box where you can get as close to a qtc of .707 to get the best response. mess around with winisd to see what kind of things you come up with

 

[its_bacon12 10+ year member]

^wow, thanks for doing that!
The 4th order response curve looks MUUUCH better than the others. I have my Focal's crossed over at 80hz though; would there be any way to get that nearly flat response from 80hz down?

PS: I don't even know what a 4th order band pass is but I can look at those response curves and know what's what i want. Can you explain this to me? I can build anything so construction is not a problem //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

i doubt you would notice a difference between ported and 4th order bandpass in that situation.. personally, i hate the sound of 4th order bandpass, even if done right.

if you want some more help on them though, talk to 80inches he was into them for a long time and recently decided ported was for him.

 

[BrianChia 10+ year member]

in theory they should do as you described, but in reality, 4th order bandpass boxes really are designed for output at a very narrow frequency band, but could gain up to 12 db at those frequencies..
if you're up to it OP and want to try a bandpass, look into 6th order. covers a much wider range.

ofcourse, i would suggest a ported box where you can get as close to a qtc of .707 to get the best response. mess around with winisd to see what kind of things you come up with

The trick to getting flat frequency response out of a 4th order bandpass is to utilize the "peakiness" of the design to match the transfer function of the vehicle. In other words, I'm well aware of the narrow frequency and it can be used as an advantage to balance out the cabin gain. The response curves I posted above are ALL with transfer function AREADY factored in. The anechoice response of each would be drastically different. You would see a large peak on the bandpass and a -12db/oct rolloff. The ported box would look flat. The sealed would rolloff by -12/db but without a peak, hence the similar profile to the 4th order but with less output. A 6th order would only complicate the peaking problems associated with the regular ported box.

Also, ported boxes don't have QTC //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif.

 

[ItalynStylion 10+ year member]

i doubt you would notice a difference between ported and 4th order bandpass in that situation.. personally, i hate the sound of 4th order bandpass, even if done right.
if you want some more help on them though, talk to 80inches he was into them for a long time and recently decided ported was for him.

Well I think in this situation a ported box will work best for me but I still need to learn about the other enclosure types. I'll probably end up reading a bunch of his threads.

 

[its_bacon12 10+ year member]

The trick to getting flat frequency response out of a 4th order bandpass is to utilize the "peakiness" of the design to match the transfer function of the vehicle. In other words, I'm well aware of the narrow frequency and it can be used as an advantage to balance out the cabin gain. The response curves I posted above are ALL with transfer function AREADY factored in. The anechoice response of each would be drastically different. You would see a large peak on the bandpass and a -12db/oct rolloff. The ported box would look flat. The sealed would rolloff by -12/db but without a peak, hence the similar profile to the 4th order but with less output. A 6th order would only complicate the peaking problems associated with the regular ported box.
Also, ported boxes don't have QTC //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif.

my bad.. i was taught by someone to get the qtc as close to .707 as possible in a sealed box, then make a ported box with the same internal volume after all displacements for best results..

yeah the 6th order would make life alot more complicated for him.. unless you plan on researching a lot and doing some test trials to your vehicle, just do something simple. i doubt you'd be able to pick the difference if both had that kind of response in car anyway

 

[its_bacon12 10+ year member]

and it was way too early for me and i was hungover when i typed that //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif

 

[Megalomaniac 5,000+ posts]

hmm 1.7 cubes.

Why not run a single 10 or a 12 and have it under a cube and a half....?(perhaps that new mag might be a good idea or the shallow BMs)

 

[TPABlazer 10+ year member]

hmm 1.7 cubes.
Why not run a single 10 or a 12 and have it under a cube and a half....?(perhaps that new mag might be a good idea or the shallow BMs)

This is the dillemma that I'm running into. 2 7 inch subs ported sounds pretty good, albeit a little peaky, but I'm around 1.5 cubes with it all said and done. I could have easily done a slim 10 or something of the sort and used up the same amount of space. Would I get the same output? Thats the question.