The Correct Way To Find Slot Displacement?

[bcarpenterfhl 10+ year member]

Jmac, i could really use your help on this one.

what i've always done is (width + 0.75) x (heigth) x (length - 0.75) / 1728.

i add the 0.75 to the width do account for the volume the MDF displaces, and subtracted 0.75 from the length to account for the 0.75 part of the port that's next to the baffle. this seems most logical to me, but there's been much confusion on how to properly calculate it. anyone?

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[Moe Lester 10+ year member]

if theres a bend you need to calculate it in sections. If its a straight port, youve got it right.

 

[bcarpenterfhl 10+ year member]

you mean if its an "L" shape port you calculate it sections?

 

[tommyk90 5,000+ posts]

if theres a bend you need to calculate it in sections. If its a straight port, youve got it right.

Can you give me an example of this? I figured since the port length is the same that it displaces the same amount of air.

 

[Moe Lester 10+ year member]

you mean if its an "L" shape port you calculate it sections?

yeah

edit: in the 2nd pic, the one port is straight, the 2nd is bent. when you bend it, there is an open area that takes up more room. the 2nd part doesnt need to be quite as long though so the difference is pretty small, most of the time i would say its not really important, but if you want to tune your box to 32.238947298 hz, then you may want to factor it in like that.

 

[tommyk90 5,000+ posts]

yeah

Those are two different port lengths though.

I'm asking if there is a difference between a 40" straight port and a 40" L-port displacement wise.

 

[bcarpenterfhl 10+ year member]

Can you give me an example of this? I figured since the port length is the same that it displaces the same amount of air.

yea so did I.

 

[InhumanAcura 5,000+ posts]

^ why is the shaded part end on an angle like that?

 

[tommyk90 5,000+ posts]

^ why is the shaded part end on an angle like that?

I wondered that too //content.invisioncic.com/y282845/emoticons/confused.gif.e820e0216602db4765798ac39d28caa9.gif

 

[Moe Lester 10+ year member]

i edited my post, read it again

port end correction.. when using the side of the box as a port, the ports length extends 1/2 of the ports width out of the port, thats why the shaded is angled. its 1/2 the area of a square the width of the port

 

[tommyk90 5,000+ posts]

i edited my post, read it again
port end correction.. when using the side of the box as a port, the ports length extends 1/2 of the ports width out of the port, thats why the shaded is angled. its 1/2 the area of a square the width of the port

While i see what you did (you chopped the port, then turned it), the only problem with that is you effectively increased the port length. While the purple lines are the same length (thats how you made it an L-port), you added an inch to the port length.

 

[Moe Lester 10+ year member]

originally posted by me in that post blah blah "the 2nd part doesnt need to be quite as long though so the difference is pretty small"

what i meant was that the 2nd port piece needs to be shorter to get the correct port length.

 

[tommyk90 5,000+ posts]

originally posted by me in that post blah blah "the 2nd part doesnt need to be quite as long though so the difference is pretty small"
what i meant was that the 2nd port piece needs to be shorter to get the correct port length.

That can add up quick though depending on the length and port area.

Then the displacement should be the same then if you calculate the port length correctly. Although i will say for the average person there really isnt a noticeable difference, but if you're doing a one-note-wonder box it can make a difference.

 

[Moe Lester 10+ year member]

When you bend the port, where the red line is, the port's width is larger than it is through the rest of the port. the slightly wider port in that area is going to add more displacement. Its not very much most of the time, but if you were to draw the same port out as a straight port, itd look something like the 2nd picture. The port is one width, then in the middle it gets a bit wider, then it narrows back down as you go through the curve.

 

[tommyk90 5,000+ posts]

When you bend the port, where the red line is, the port's width is larger than it is through the rest of the port. the slightly wider port in that area is going to add more displacement. Its not very much most of the time, but if you were to draw the same port out as a straight port, itd look something like the 2nd picture. The port is one width, then in the middle it gets a bit wider, then it narrows back down as you go through the curve.

Hmm, yeah. Never really thought about that. Probably because i put 45 degree pieces in there. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif