What should i use as Ql, Qa and Qp values.

[Troy Bakker]

I recently build a subwoofer I used the parameters
Ql: 15
Qa: 30
Qp: 30

I should have gotten this frequenty response from a 6,5 inch dd506d2 in a double tuned bass reflex

But for my ears and my phone (using the app spectroid) it sounds more like this:

It has a huge peak around 20hz (at low volumes (such as the volume i took these measurements with) it makes port noise here so that could be a factor. The port has a diameter of 68mm so 2.677 inches with a length of 80 cm so 31.5 inches for a 6.5 inch driver.

When I use these parameters
Ql: 100
Qa: 100
Qp: 100
I get this response:

Which looks more like the actual response then the ones with "normal" Ql. Qa and Qp parameters. So now I don't know what parameters to use for my next build. Suggestions would be appreciated 🙂

 

[GM]

Standard QL = 7 and audibly under damped is < ~3.

 

[Troy Bakker]

@GM I know but i used a higher value (so more bass output from the same size enclosure) and the program still said it had less bass then it has in reality.

I used 15, But got the response of 50-100

is this because I made it too air tight or did the software (Vituixcad) make a oopsie

 

[Troy Bakker]

I recently build a subwoofer I used the parameters
Ql: 15
Qa: 30
Qp: 30

View attachment 1148620

I should have gotten this frequenty response from a 6,5 inch dd506d2 in a double tuned bass reflex

But for my ears and my phone (using the app spectroid) it sounds more like this:
View attachment 1148628
It has a huge peak around 20hz (at low volumes (such as the volume i took these measurements with) it makes port noise here so that could be a factor. The port has a diameter of 68mm so 2.677 inches with a length of 80 cm so 31.5 inches for a 6.5 inch driver.

When I use these parameters
Ql: 100
Qa: 100
Qp: 100
I get this response:
View attachment 1148629
Which looks more like the actual response then the ones with "normal" Ql. Qa and Qp parameters. So now I don't know what parameters to use for my next build. Suggestions would be appreciated 🙂

Extra info:
Driver DD506D2 (6.5 inch)
Port 1 diameter 68mm
Port 1 length 30cm
Port 2 diameter 68mm
Port 2 length 80cm
Chamber 1 volume ~5 liters
Chamber 2 volume ~ 20 liters

 

[GM]

OK, misunderstood and have no experience with Vituixcad/Vcad, just that a (minimum) max flat BR =

(Vb) (L) = 20*4.395*0.501'^3.3 = ~9 L

(Fb) (Hz) = 0.42*43.74*Qts'^-0.96 = ~36 Hz

So tuning > octave below Fs in a total of vented 25 L it seems reasonable it would 'ring like a ten penny nail struck with a ball peen hammer', so wondering if Vcad is programmed for when this far out of accepted design range, ergo best answered by its 'brain trust'.

(Qts'): (Qts) + any added series resistance (Rs): http://www.mh-audio.nl/Calculators/newqts.html

 

[Troy Bakker]

@GM
I don't understand a lot of what you just said. The only thing i could understand was this sentance

"So tuning > octave below Fs in a total of vented 25 L it seems reasonable it would 'ring like a ten penny nail struck with a ball peen hammer', so wondering if Vcad is programmed for when this far out of accepted design range, ergo best answered by its 'brain trust'."

But the only thing wrong with the enclosure is a lot of port noise when at tuning frequency (starting between 6 and 13 watts of power) even tough the port isnt that small.
And it has a huge hump of sound output at the tuning frequency.

But at those power levels the air velocity is well under 17m/s (right bottom graph)

Does it behave this weird because it tuned it an octave lower then the fs of a driver or did i make the box too air tight so it now behaves like this:

 

[David Morison]

Have you accounted for room gain when deciding to shoot for flat to 20-ish Hz in your design?

 

[Troy Bakker]

@David Morison
I did not, but my room is relatively small so gain around 20hz would be weird.
my room has the following dimensions. and it has a slope in the ceiling on the left side. The rest of the ceiling is almost flat.