box for 15" sub math check

[barkingspider]

I am building a box for a single L7 15 sub and want to tune it to around 28 hertz 

The external  dimensions are 31"x  32" x 12"/ The port is  5 5/8" x 10.5" x  34"

using 3/4" mdf

If someone could please help check my calculations it would be greatly appreciated

here is a photo of the box 

 

 

[Joshdashef]

So one thing you need to be careful with is how you measure your port. What you have there is 2' 1 5/8" or 25.625 and another 8.75 inches which does equal 34.375. However, that is not the correct way to measure port length. Firstly, I recommend switching to standard units, ditch the feet and use inches solely. Feet will only confuse you when you go to cut this out imho. With those dimensions I come to a net volume of 4.15 cubic feet without considering sub displacements with a port volume of 1.31 cubic feet.

 

When you measure a port you need to measure from the middle most point at all times, making sure the port remains the same width the entire length. Therefore, that sketchups actual port length is split up into two section because of the port turn.

 

1. the length from the end of the port to the middle of the port turn is going to be the entire depth of the box less (minus) the thickness of the back panel (0.75) less (minus) half the port width (5.625 / 2 = 2.8125). Your final calculation is Depth - Back panel - half port width or 32" - 0.75" - 2.8125" = 28.4375" long

 

2. the length from the bend of the port to the inside of the enclosure, where your subwoofer goes, will need to be the rest of the distance to reach your tuning. In this case you wanted to get to 34" long so subtract the previous length of the port we just calculated to get 34 - 28.4375 = 5.5625" of port length required in the bend. Now to achieve this we can use the method of jointing you have in your sketchup so we can actually subtract the thickness of the inner port wall since it will count when we calculate the length of the port bend wall. Therefore, we can again use half the port width to orientate ourselves to the middle of the port, add in the thickness of the inner port wall (not the turn but the main wall). This will be as follows: 2.8125 + 0.75 = 3.5625" of port length already achieved, so we need to have the inner port wall turn piece to be the remaining 5.5625 - 3.5626 long or 2 inches long.....

 

 

I get this can be a little hard to follow on text so I will make a diagram here in a minute. Actually found a perfect video that describes how to measure the port length, so I'll link that instead.

 

 

 

 

[barkingspider]

2 hours ago, Joshdashef said:

So one thing you need to be careful with is how you measure your port. What you have there is 2' 1 5/8" or 25.625 and another 8.75 inches which does equal 34.375. However, that is not the correct way to measure port length. Firstly, I recommend switching to standard units, ditch the feet and use inches solely. Feet will only confuse you when you go to cut this out imho. With those dimensions I come to a net volume of 4.15 cubic feet without considering sub displacements with a port volume of 1.31 cubic feet.

 

When you measure a port you need to measure from the middle most point at all times, making sure the port remains the same width the entire length. Therefore, that sketchups actual port length is split up into two section because of the port turn.

 

1. the length from the end of the port to the middle of the port turn is going to be the entire depth of the box less (minus) the thickness of the back panel (0.75) less (minus) half the port width (5.625 / 2 = 2.8125). Your final calculation is Depth - Back panel - half port width or 32" - 0.75" - 2.8125" = 28.4375" long

 

2. the length from the bend of the port to the inside of the enclosure, where your subwoofer goes, will need to be the rest of the distance to reach your tuning. In this case you wanted to get to 34" long so subtract the previous length of the port we just calculated to get 34 - 28.4375 = 5.5625" of port length required in the bend. Now to achieve this we can use the method of jointing you have in your sketchup so we can actually subtract the thickness of the inner port wall since it will count when we calculate the length of the port bend wall. Therefore, we can again use half the port width to orientate ourselves to the middle of the port, add in the thickness of the inner port wall (not the turn but the main wall). This will be as follows: 2.8125 + 0.75 = 3.5625" of port length already achieved, so we need to have the inner port wall turn piece to be the remaining 5.5625 - 3.5626 long or 2 inches long.....

 

 

I get this can be a little hard to follow on text so I will make a diagram here in a minute. Actually found a perfect video that describes how to measure the port length, so I'll link that instead.

 

 

 

 

Ok i definitely forgot about considering the turn of the port thank you so much for reminding me.  So as the box sits considering the turn in the port and measuring from the middle would the port length be 40.12" ? 

[barkingspider]

2 minutes ago, barkingspider said:

Ok i definitely forgot about considering the turn of the port thank you so much for reminding me.  So as the box sits considering the turn in the port and measuring from the middle would the port length be 40.12" ? 

 

[Joshdashef]

3 hours ago, barkingspider said:

 

I get exactly 40" according to your first picture. Your second one has numbers overlapping some, but you've got the gist by just measuring the port length according to the middle of the port you will get the actual port length. The only reason port length matters is it will decide the tuning of said port. Generally the longer the port the lower the tuning, but box volume can affect this. The smaller the volume the longer the port needs to be to achieve the same tuning frequency.

 

What kind of power are you running on that sub? Anything over 1k-2k and you might want to consider a double baffle for the sub. 

 

   Note  *I got 40" from the following: 2' 1 5/8" = 25.625 + 2.8125 (half the width of port) = 28.4375. Then for the turn I got 8 3/4" + 2.8125 (half port width) = 11.5625. Add them together and its 40"

 

Looking back at it I think your only mistake was converting your 3/8. Seems like you made it into a .75 instead of a .625, leading to you being .125 beyond. Could be off from where you put center line of port in the program even. 

[barkingspider]

10 hours ago, Joshdashef said:

I get exactly 40" according to your first picture. Your second one has numbers overlapping some, but you've got the gist by just measuring the port length according to the middle of the port you will get the actual port length. The only reason port length matters is it will decide the tuning of said port. Generally the longer the port the lower the tuning, but box volume can affect this. The smaller the volume the longer the port needs to be to achieve the same tuning frequency.

 

What kind of power are you running on that sub? Anything over 1k-2k and you might want to consider a double baffle for the sub. 

 

   Note  *I got 40" from the following: 2' 1 5/8" = 25.625 + 2.8125 (half the width of port) = 28.4375. Then for the turn I got 8 3/4" + 2.8125 (half port width) = 11.5625. Add them together and its 40"

 

Looking back at it I think your only mistake was converting your 3/8. Seems like you made it into a .75 instead of a .625, leading to you being .125 beyond. Could be off from where you put center line of port in the program even. 

Im running a rockford fosgate t2500 but only gona let the sub see around a 1000 watts. 

   So with a 40" port length would the box be tuned to about 28 hz? Also do i have enough port area to keep the port velocity low enough? Thank you for all your help its been years since ive built a box and your doing me wonders

[Joshdashef]

1 hour ago, barkingspider said:

Im running a rockford fosgate t2500 but only gona let the sub see around a 1000 watts. 

   So with a 40" port length would the box be tuned to about 28 hz? Also do i have enough port area to keep the port velocity low enough? Thank you for all your help its been years since ive built a box and your doing me wonders

I’ll throw it into winisd in a bit and let you know. Np man I’m glad to help 

 

What are you going to be wiring to? 1 ohm or 2?

[barkingspider]

3 hours ago, Joshdashef said:

I’ll throw it into winisd in a bit and let you know. Np man I’m glad to help 

 

What are you going to be wiring to? 1 ohm or 2?

I will be wiring to 2 ohm why do you ask?

I was fiddling with WinIsd yesterday but wasn't sure on how to read the graphs

[Joshdashef]

@barkingspider So I threw it into winisd and found a 4 cube box with a 40" long port (10.5 by 5.625) will give you a tuning of 29.57 hz. If you are looking for more output you won't lose too much bandwidth below tuning if you tune to 32 hz instead. Here is some graphs to look at.

 

Blue is the box tuned to 32 hz (33.37" long port) and red is the box tuned to 29.57 hz (40" long port).

 

Either way you will need a infrasonic/subsonic filter at either 23 hz or 24 hz depending on your tuning to avoid over excursion. 

[Joshdashef]

1 minute ago, barkingspider said:

I will be wiring to 2 ohm why do you ask?

I was fiddling with WinIsd yesterday but wasn't sure on how to read the graphs

Is the sub your going to use a dual 2 ohm or dual 4 ohm? they have minor differences in t/s parameters