I'm designing a box for my friend, and i'm having a mental block...
When calculating PTF, do you use GROSS or NET internal volume? Thanx guys.
-Ian-
Replies (13)
swez on 10/17/2006 00:00:06
We use Vb. Vb is the net box volume of airspace inside the box, after we subtract sub and port displacement values.
Example:
1. A 16" x 16" x 16" cube box, 3/4" MDF = 4096 in^3/1728 = 2.37ft^3 externally... (that's 1.76 ft^3 internal net volume)
2. Subtract sub displacement
3. Subtract port/vent displacement
4. Internal net - Sd - Pd = Vb
Here's a handy tool for calculating all these factors:
http://www.the12volt.com/caraudio/boxcalcs.asp
Hope that helps,
swez
cybersailor420 on 10/17/2006 11:00:39
Thanks Swez. It ended up coming back to me after I took my mind off it for a while...
I was using that website last night, and I finally decided I should see what WinISD is all about... since i've never used it. It's a pretty neat program, but I was curious as to how much to trust it. Do you stick pretty closely to it's suggestions, or just use it as a guideline?
I am desiging a ported box for an Alpine SWR-1042D for my friend's extended cab Toyota truck.
On Alpine's website, they suggest a 1.3 cu. ft. box (gross internal), with a port tuned to 36 HZ.
On WinISD, they suggest a 1.7... cu. ft. box and a port tuned to 24ish hertz.
What considerations might Alpine be taking into effect that would cause such a difference in specs?
I mainly want to know this because I am using WinISD to determine the port length for a box tuned to 33hz (hopefully). I want to know if WinISD's calculations will be exact enough for me to use.
http://www.alpine-usa.com/images/products/documents/OM_SWR-1042D.pdf
Another question (while i'm avoiding work):
In all of these formulas to calculate port length, you're supposed to use the net internal volume. However, you don't know the port volume until you know the length... This seems to lead in a circle. I know I could just be content with guess-timating, but I assume the pros have a better way of doing this... Comments???
Thanx guys.
-Ian-
swez on 10/17/2006 12:09:38
If the Mfg gives specs on a given product, I tend to go in that direction and call a "fudge factor" of +/- 10%, good nuff for most enclosures.
WinISD numbers are accurate in most cases, but they also tend to spec a driver at a Qtc of 0.707. (Measured in a large room) In many of today's high efficiency, medium compliance woofers, that tends to be a bit boomy and not as smooth a plot when cabin gain factors are taken into account. A Qtc of 0.85 - 0.90, seems to work very well for mobile sub applications.
In this case, the Vb is 1.7 ft^3 and port tuning is very low at 24 Hz. That's great for HT apps and a large room to fill with deep, thundering bass. However, to get that, we also sacrifice power handling of the woofer. It takes somewhat less power in this alignment, to reach X-max. (Basically, not a good box alignment for mobile audio)
Have read some and done some experiements on my own and just find for car audio apps, a Qtc of 0.707 is just too much boom and not enough punch. A bit too muddy for my tastes and just not crisp enough in car installs. Hense, the smaller box and higher Qtc # is more desirable.
I agree with you about port calculations. It's a chicken and the egg thing here. One parameter (Port displacement or Pd ) does affect Vb and ultimately PTF. However, if you look at Pd, it's a rather small variable when compared to Vb. This is mostly true in round ports.
When using large, vented port designs, the Vb to Pd ratios are substantially more important. That's where some serious re-calculations come into play.
When doing advanced vented systems, we can either go directly to the sub maker for specs and details or hash them out on our own. If the sub maker gives a range of usable enclosure specs, I tend to shoot for the middle of that range. This often gives us good SQL performance, better power handling than larger boxes and keeps the sub from undue strains by exceeding X-max parameters of a given sub. We still have strong low bass, but also get the nice upper punch too.
Comments?
swez
cybersailor420 on 10/17/2006 12:32:56
Thanks Swez, as usual :)
Yea, I guess I'll get as close as I can and just call it good. I normally stay close to mfg. specs, but get more satisfactiong out of doing the fine-tuning calculations myself. I guess that's why I'm an engineer :p
cybersailor420 on 10/17/2006 15:38:37
So I was just double checking my work in WinISD with the website you listed above, and that website said my port should be 1.5" longer than what WinISD calculated..... Which one would you use?
I can give you all the parameters if you want to check it yourself, but i'm sure you don't. :p
swez on 10/17/2006 15:55:23
I'd use the port area recommended by Alpine's install manual:
http://www.alpine-usa.com/images/products/documents/OM_SWR-1042D.pdf
Just make sure you look under the correct sub column.
Swez
PS 25 years ago, I could not spell engginer. Now, I are one. hehe
cybersailor420 on 10/17/2006 16:41:06
I can't use the recommended port area due to the way this box is designed. I am using a smaller port area than suggested. Therefore, my port will also not need to be as long to achieve the same tuning frequency.
I'm basically just wondering, if you were designing a box, which port tuning frequency calculation would you trust? (i.e. which website/formula/program)
Thanx again.
-Ian-
swez on 10/17/2006 17:20:58
Ahhhh, got it.... I trust the 12volt.com site and it jives with other formulas used on other sites too.
Yes, if the port area is smaller, the length needed for proper port tuning will also be shorter. If you wish to try another site for verification purposes, go here and drop in your numbers:
http://www.bcae1.com/spboxad2.htm
May I suggest using a Qtc value of 0.80. 0.85 or even 0.90 here? The default is 0.707. For car sub apps, a 0.707 Qtc is a bit muddy.
Swez
cybersailor420 on 10/17/2006 17:36:49
Glad we're on the same page now :)
Do you know of any way to keep volume constant while playing around with the Qtc?
I already have my box specs, but I would like to see how different Qtc values change with reference to port size and tuning frequency.
Thanx again Sensei!
-Ian-
swez on 10/18/2006 07:08:48
Funny you should ask... there is a way to change Qtc of a box and monitor the effects.
Here, we start with a larger Qtc enclosure, (say 0.707) and add material (2 x 4, 2 x 6 or 2 x 8 blocks) inside the box to change Qtc. We may not detect a difference between 0.7 and 0.8 as Qtc. However, as more materials are added to the box, we should note changes for the good or bad.
A common block of dimensional lumber, 2" x 6'' x 12" L, will add 181.5 in^3 of displacement inside a given chamber. (1.5" x 5.5" x 12" = 181.5 in^3) That nets 0.1 ft^3 per block. If the chamber need say 0.3 ft^3 of adjustment, we use 3 blocks per chamber.
These blocks can be screwed into the box for testing purposes and removed as needed. Once we find the exact displacement desired, screw and glue the blocks in as needed. Good to go!
How's that sound to you "grasshoppa"?
Domo arigato
swez
Sensei... Teacher, Guide, Mentor, Master of a given profession, art form or skills? Hehe... Oh brother, do I really deserve such reverence? Maybe, maybe not... but thank you just the same.
Uhmmmm, It does feel good to grow older and help others as they master their own passions and become future mentors to others... That's a legacy we all can offer to others that come into the journey of life. Do your part when the time comes. It's "very rewarding" to watch others climb out of the nest and take to flight, on thier own accord.
cybersailor420 on 10/18/2006 12:59:02
I agree. The reason I try to learn to do everything the right way is so that I may pass it on to all the people who are always asking me for help. I live on the smallest island in the U.S. Virgin Islands (near Puerto Rico). I am considered to have superior car audio knowledge to almost anyone else that is involved in it down here. That's why I am always working to keep my facts straight, and my calculations spot-on. Your teachings are contributing to more than you know :)
I finally found a website that I like and works well enough for me. It's the one you mentioned in the "box question" post:
http://www.bcae1.com/spboxnew2.htm
Thanx again for the help!
-Ian-
PS - My next project is to start working on a 6th/8th order setup for a purely SPL system... This will be "interesting"
swez on 10/18/2006 14:33:32
Yes, this is a very good reference site. It has a lot of details and since you're an engineer, much of it will make good sense too. I have used this site many times over the years and never fail to learn something interesting as I read old and new topics in detail.
That next SPL box should be interesting Ian... ;-)
swez
cybersailor420 on 10/18/2006 15:23:12
haha! thanx Swez, I'll most definitely keep you guys posted on that project.
-Ian-
When calculating PTF, do you use GROSS or NET internal volume? Thanx guys.
-Ian-
Replies (13)
swez on 10/17/2006 00:00:06
We use Vb. Vb is the net box volume of airspace inside the box, after we subtract sub and port displacement values.
Example:
1. A 16" x 16" x 16" cube box, 3/4" MDF = 4096 in^3/1728 = 2.37ft^3 externally... (that's 1.76 ft^3 internal net volume)
2. Subtract sub displacement
3. Subtract port/vent displacement
4. Internal net - Sd - Pd = Vb
Here's a handy tool for calculating all these factors:
http://www.the12volt.com/caraudio/boxcalcs.asp
Hope that helps,
swez
cybersailor420 on 10/17/2006 11:00:39
Thanks Swez. It ended up coming back to me after I took my mind off it for a while...
I was using that website last night, and I finally decided I should see what WinISD is all about... since i've never used it. It's a pretty neat program, but I was curious as to how much to trust it. Do you stick pretty closely to it's suggestions, or just use it as a guideline?
I am desiging a ported box for an Alpine SWR-1042D for my friend's extended cab Toyota truck.
On Alpine's website, they suggest a 1.3 cu. ft. box (gross internal), with a port tuned to 36 HZ.
On WinISD, they suggest a 1.7... cu. ft. box and a port tuned to 24ish hertz.
What considerations might Alpine be taking into effect that would cause such a difference in specs?
I mainly want to know this because I am using WinISD to determine the port length for a box tuned to 33hz (hopefully). I want to know if WinISD's calculations will be exact enough for me to use.
http://www.alpine-usa.com/images/products/documents/OM_SWR-1042D.pdf
Another question (while i'm avoiding work):
In all of these formulas to calculate port length, you're supposed to use the net internal volume. However, you don't know the port volume until you know the length... This seems to lead in a circle. I know I could just be content with guess-timating, but I assume the pros have a better way of doing this... Comments???
Thanx guys.
-Ian-
swez on 10/17/2006 12:09:38
If the Mfg gives specs on a given product, I tend to go in that direction and call a "fudge factor" of +/- 10%, good nuff for most enclosures.
WinISD numbers are accurate in most cases, but they also tend to spec a driver at a Qtc of 0.707. (Measured in a large room) In many of today's high efficiency, medium compliance woofers, that tends to be a bit boomy and not as smooth a plot when cabin gain factors are taken into account. A Qtc of 0.85 - 0.90, seems to work very well for mobile sub applications.
In this case, the Vb is 1.7 ft^3 and port tuning is very low at 24 Hz. That's great for HT apps and a large room to fill with deep, thundering bass. However, to get that, we also sacrifice power handling of the woofer. It takes somewhat less power in this alignment, to reach X-max. (Basically, not a good box alignment for mobile audio)
Have read some and done some experiements on my own and just find for car audio apps, a Qtc of 0.707 is just too much boom and not enough punch. A bit too muddy for my tastes and just not crisp enough in car installs. Hense, the smaller box and higher Qtc # is more desirable.
I agree with you about port calculations. It's a chicken and the egg thing here. One parameter (Port displacement or Pd ) does affect Vb and ultimately PTF. However, if you look at Pd, it's a rather small variable when compared to Vb. This is mostly true in round ports.
When using large, vented port designs, the Vb to Pd ratios are substantially more important. That's where some serious re-calculations come into play.
When doing advanced vented systems, we can either go directly to the sub maker for specs and details or hash them out on our own. If the sub maker gives a range of usable enclosure specs, I tend to shoot for the middle of that range. This often gives us good SQL performance, better power handling than larger boxes and keeps the sub from undue strains by exceeding X-max parameters of a given sub. We still have strong low bass, but also get the nice upper punch too.
Comments?
swez
cybersailor420 on 10/17/2006 12:32:56
Thanks Swez, as usual :)
Yea, I guess I'll get as close as I can and just call it good. I normally stay close to mfg. specs, but get more satisfactiong out of doing the fine-tuning calculations myself. I guess that's why I'm an engineer :p
cybersailor420 on 10/17/2006 15:38:37
So I was just double checking my work in WinISD with the website you listed above, and that website said my port should be 1.5" longer than what WinISD calculated..... Which one would you use?
I can give you all the parameters if you want to check it yourself, but i'm sure you don't. :p
swez on 10/17/2006 15:55:23
I'd use the port area recommended by Alpine's install manual:
http://www.alpine-usa.com/images/products/documents/OM_SWR-1042D.pdf
Just make sure you look under the correct sub column.
Swez
PS 25 years ago, I could not spell engginer. Now, I are one. hehe
cybersailor420 on 10/17/2006 16:41:06
I can't use the recommended port area due to the way this box is designed. I am using a smaller port area than suggested. Therefore, my port will also not need to be as long to achieve the same tuning frequency.
I'm basically just wondering, if you were designing a box, which port tuning frequency calculation would you trust? (i.e. which website/formula/program)
Thanx again.
-Ian-
swez on 10/17/2006 17:20:58
Ahhhh, got it.... I trust the 12volt.com site and it jives with other formulas used on other sites too.
Yes, if the port area is smaller, the length needed for proper port tuning will also be shorter. If you wish to try another site for verification purposes, go here and drop in your numbers:
http://www.bcae1.com/spboxad2.htm
May I suggest using a Qtc value of 0.80. 0.85 or even 0.90 here? The default is 0.707. For car sub apps, a 0.707 Qtc is a bit muddy.
Swez
cybersailor420 on 10/17/2006 17:36:49
Glad we're on the same page now :)
Do you know of any way to keep volume constant while playing around with the Qtc?
I already have my box specs, but I would like to see how different Qtc values change with reference to port size and tuning frequency.
Thanx again Sensei!
-Ian-
swez on 10/18/2006 07:08:48
Funny you should ask... there is a way to change Qtc of a box and monitor the effects.
Here, we start with a larger Qtc enclosure, (say 0.707) and add material (2 x 4, 2 x 6 or 2 x 8 blocks) inside the box to change Qtc. We may not detect a difference between 0.7 and 0.8 as Qtc. However, as more materials are added to the box, we should note changes for the good or bad.
A common block of dimensional lumber, 2" x 6'' x 12" L, will add 181.5 in^3 of displacement inside a given chamber. (1.5" x 5.5" x 12" = 181.5 in^3) That nets 0.1 ft^3 per block. If the chamber need say 0.3 ft^3 of adjustment, we use 3 blocks per chamber.
These blocks can be screwed into the box for testing purposes and removed as needed. Once we find the exact displacement desired, screw and glue the blocks in as needed. Good to go!
How's that sound to you "grasshoppa"?
Domo arigato
swez
Sensei... Teacher, Guide, Mentor, Master of a given profession, art form or skills? Hehe... Oh brother, do I really deserve such reverence? Maybe, maybe not... but thank you just the same.
Uhmmmm, It does feel good to grow older and help others as they master their own passions and become future mentors to others... That's a legacy we all can offer to others that come into the journey of life. Do your part when the time comes. It's "very rewarding" to watch others climb out of the nest and take to flight, on thier own accord.
cybersailor420 on 10/18/2006 12:59:02
I agree. The reason I try to learn to do everything the right way is so that I may pass it on to all the people who are always asking me for help. I live on the smallest island in the U.S. Virgin Islands (near Puerto Rico). I am considered to have superior car audio knowledge to almost anyone else that is involved in it down here. That's why I am always working to keep my facts straight, and my calculations spot-on. Your teachings are contributing to more than you know :)
I finally found a website that I like and works well enough for me. It's the one you mentioned in the "box question" post:
http://www.bcae1.com/spboxnew2.htm
Thanx again for the help!
-Ian-
PS - My next project is to start working on a 6th/8th order setup for a purely SPL system... This will be "interesting"
swez on 10/18/2006 14:33:32
Yes, this is a very good reference site. It has a lot of details and since you're an engineer, much of it will make good sense too. I have used this site many times over the years and never fail to learn something interesting as I read old and new topics in detail.
That next SPL box should be interesting Ian... ;-)
swez
cybersailor420 on 10/18/2006 15:23:12
haha! thanx Swez, I'll most definitely keep you guys posted on that project.
-Ian-
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