Jump to content
osiris10012

Slot port length difference between Torres and Carstereo.com calculato

Recommended Posts

Hello everyone. I am redesigning my enclosure for my 15" ethos for different mounting options. When I use the Torres calculator I am using the total net feature with 3 cu ft and also 3 common walls since it is a slot port. Using a port that is 4x18.5 at 56" long gives me a tuning of 31.94 hz and 74 in2 of port. Then when I use the same size port in the carstereo.com calculator it tells me 48.5" of length is required to give me 32hz tuning. So my question is which one should I use. In sketch up I am designing the port by measuring down the center. Thank you in advance for the help.

Share this post


Link to post
Share on other sites

There's a relationship between port area, port tuning, port length, and enclosure volume. If port tuning and port area are constant and the volume of the enclosure is reduced then port length must increase to maintain the same tuning.

 

I'm going to speculate that the car stereo calculator doesn't account for the displacement of the port itself, which means the actual box volume is less and thus the port length given by the calculator is too short (so the real tuning is higher), and perhaps the Torres calculator does account for such and that could explain why it's port length given is longer--or perhaps you're calculating the net volume differently for the online calculator than what Torres is doing. Either way I'm not certain only thinking of one possible explanation, this is a prime example of why many people do the math by hand and don't rely on such calculators.

Edited by ssh

Share this post


Link to post
Share on other sites

And it is double damned.  By default it is uber unscientific to report two digits after the decimal which makes me generically want to ignore that calculator; however, it may be just too dumb to report anything but real math in which case it may be right.  Or, the possibility exists they are both wrong.

 

Hard to tell without remodeling or calculating it out yourself.  Equation is rather easy if you want to double check the calc.

Share this post


Link to post
Share on other sites

 Equation is rather easy if you want to double check the calc.

 

do u have the equation on hand? id love to have it to double check my work aswell

Share this post


Link to post
Share on other sites

 

 Equation is rather easy if you want to double check the calc.

 

do u have the equation on hand? id love to have it to double check my work aswell

 

you can google it but...

 

damn near anything you need to know about loudspeakers for under 30 bucks

Share this post


Link to post
Share on other sites

port.gif

 

R would be in inches.

 

From the above book.

Edited by ssh

Share this post


Link to post
Share on other sites

Thanks for all the input. I didn't realize it was so simple to just do it by hand. I will do the math when I get a chance and post the results.

Share this post


Link to post
Share on other sites

I would put my money on the Torres calculator being correct for the exact reasons ssh had mentioned.  I've done the math by hand over and over and over again awhile back while working on my own calculator.  The Torres calc does a fantastic job of maintaining accurate numbers while doing all this math.  It's quite accurate and while I've had some hand calculations be off by a few hundredths it's easily attributable to either rounding going on in the math or possibly the conversion of measurements to/from imperial and metric that makes that tiny little difference.  Either way being off that little tiny bit would not be noticeable and I do believe the carstereo.com calculator does indeed have some accuracy issues.

Share this post


Link to post
Share on other sites

Thats why I like the 12 volt calculator, just enter port size, box size and tuning desire and tells you how long it needs to be. I can figure all the other shit out. The ones with how many common walls etc just confuse my ass, I just want to know how long the SOB needs to be, lol

Share this post


Link to post
Share on other sites

Thanks for all the input. I didn't realize it was so simple to just do it by hand. I will do the math when I get a chance and post the results.

Note that the above equation for port length is not taking port displacement into account.

 

E.g. arbitrary values but say you have box volume 2 ft^3, port area 20 in^2, tuning 30hz, port length 25"

The above values for the port and tuning are based upon a box volume of 2 ft^3, but the port itself is displacing (20 in^2 x 25in)*1ft^3/1728in^3 is 0.29 ft^3 so the net volume is now 1.7 ft^3

since the volume is actually less than what was used for calculation the actual tuning will be higher than 30hz. So you have to either play around with the numbers until you end up with the volume, tuning, and port length needed, or you simply add the port volume to the box volume and come up with new box dimensions (if you have the space), you can also calculate it here: http://box.bitpusher.in/enclosure/?page=portlength

 

Also don't forget to account for the thickness of the port depending on the enclosure it may or may not be negligible.

Share this post


Link to post
Share on other sites

Thanks for all the input. I didn't realize it was so simple to just do it by hand. I will do the math when I get a chance and post the results.

Note that the above equation for port length is not taking port displacement into account.

E.g. arbitrary values but say you have box volume 2 ft^3, port area 20 in^2, tuning 30hz, port length 25"

The above values for the port and tuning are based upon a box volume of 2 ft^3, but the port itself is displacing (20 in^2 x 25in)*1ft^3/1728in^3 is 0.29 ft^3 so the net volume is now 1.7 ft^3

since the volume is actually less than what was used for calculation the actual tuning will be higher than 30hz. So you have to either play around with the numbers until you end up with the volume, tuning, and port length needed, or you simply add the port volume to the box volume and come up with new box dimensions (if you have the space), you can also calculate it here: http://box.bitpusher.in/enclosure/?page=portlength

Also don't forget to account for the thickness of the port depending on the enclosure it may or may not be negligible.

Alright well now I am a bit confused. I used the given formula for using port area and got 57.8" of port length when using the 3 cu ft and 74 in2 of port area. The part I am confused about is where in the formula am I looking to see the reduction in volume. I see where it is converting it to cubic inch but not changing the overall volume. I was initially under the impression that I would need to use the net volume. Sorry for the newb question but up until now I was relying on the Torres calculator. I just want to make sure I get the tuning length right. Edited by osiris10012

Share this post


Link to post
Share on other sites

 

 

Thanks for all the input. I didn't realize it was so simple to just do it by hand. I will do the math when I get a chance and post the results.

Note that the above equation for port length is not taking port displacement into account.

E.g. arbitrary values but say you have box volume 2 ft^3, port area 20 in^2, tuning 30hz, port length 25"

The above values for the port and tuning are based upon a box volume of 2 ft^3, but the port itself is displacing (20 in^2 x 25in)*1ft^3/1728in^3 is 0.29 ft^3 so the net volume is now 1.7 ft^3

since the volume is actually less than what was used for calculation the actual tuning will be higher than 30hz. So you have to either play around with the numbers until you end up with the volume, tuning, and port length needed, or you simply add the port volume to the box volume and come up with new box dimensions (if you have the space), you can also calculate it here: http://box.bitpusher.in/enclosure/?page=portlength

Also don't forget to account for the thickness of the port depending on the enclosure it may or may not be negligible.

Alright well now I am a bit confused. I used the given formula for using port area and got 57.8" of port length when using the 3 cu ft and 74 in2 of port area. The part I am confused about is where in the formula am I looking to see the reduction in volume. I see where it is converting it to cubic inch but not changing the overall volume. I was initially under the impression that I would need to use the net volume. Sorry for the newb question but up until now I was relying on the Torres calculator. I just want to make sure I get the tuning length right.

 

 

I'm a little confused by your question so I'll start from the very beginning.

 

If you have a box 10" x 20" x 15" = 3000 in^3 or 3000 in^3 * (1 ft^3 / 1728 in^3) = 1.74 ft^3 your enclosure volume isn't 1.74 ft^3 because you need to account for the wood thickness. So say you used 3/4" then 10-0.75 x 20-0.75 x 15-0.75 = 2537 in^3 = 1.47 ft^3. Anything inside the enclosure displaces volume, so if you put a 2 liter soda bottle inside the enclosure then you need to subtract that volume (2 liters) from the enclosure volume. Obviously you wouldn't put a soda bottle in, but you get the idea? Anything inside displaces volume and that volume must be accounted for. When you mount the subs they displace volume, something around 0.15 ft^3 depending on the size and make, etc.

 

So you have your measurements of the enclosure and account for the wood thickness, double baffle, speaker displacements, bracing displacement, and than you finally end up with a "net" volume. This volume is what you use with the equation posted above, the result of the equation is a port area of x, port length of y, and a port tuning of z for that amount of enclosure volume. So if you used an external port outside of the enclosure than you would simply make a port with area x, length y, you'd end up with z tuning, and you're done--that's it.

 

The problem is if you use the equation to calculate your port length, area, and tuning based upon 3 ft^3 volume AND then you stick that port inside the enclosure, now the enclosure volume isn't 3 ft^3, it's 3 ft^3 - the volume of the port. The true and final net volume of the enclosure is less and thus the port tuning ends up being higher if you do the calculation again.

 

If you use the above equation you either need to:

design the enclosure in two parts so to speak where one is the net volume you're striving for and the rest is the port then put the two together so it fits the space in your vehicle.

OR

use the equation, find the port length, area, tuning, etc. Calculate the port volume, subtract it from the original volume you used to do the port calculations, do the port calculations again with the new net volume and use the same port length and you'll find out what the actual tuning is (it'll be higher). Then incrementally increase the port length, recalculate the port volume, subtract it from the enclosure volume, and input this into the equation again, now the tuning will be decreasing, and you can repeat this step until you end up with the correct port length that gives you tuning you originally wanted. I know this might sound complicating, but it's actually very simple. If you still need help post your math and I'll explain it. Alternatively you can use this calculator http://box.bitpusher.in/enclosure/?page=portlength which uses the exact same equation posted above but it will automatically do the latter steps to account for the port volume as long as you checkmark "Internal Port."

Share this post


Link to post
Share on other sites

I apologize for the confusion. You answered my question. I understand about having to account for displacement when calculating net volume. I just wanted to make sure I was correct using the net volume in the equation. From reading your last post I now know I was correct in doing so. Thank you for all your patience and help.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×