# Slot port information

Discussion in 'Home Theater Projects' started by JulianS, May 7, 2004.

1. ### JulianS Auditioning

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i've done quite a lot of searching on this forum about slot ports,
and I've found alot of people talking about them,
but none who have actually build them in their enclosures,
i'd like to know from anyone who actually build one :

1 - what formula did you use to calculate the slot length/bends
2 - how accurate this formula turned out to be
3 - whether it was worth it, building a slot port
4 - any problems with organ pipe resonance from the long ports
5 - whether having bends in the slot reduced this resonances, etc.

Any help will be greatly appreciated

2. ### Chris Brock Second Unit

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Hey! I have built dozens of slot ports actually. here is one of my favorits!

http://bigchris.caraudioplanet.com/I...olio/Mikesbox/

I have never noticed any problems with resonance, expecially when a bend is involved.

You can calculate the length the port needs to be using WinISD. when you go to the port tab just change the shape from a circle to a square.

Also if there is a bend involved in the slot port you will want to make the center measuremnet of the slot port match the actual vent length that WinISD tells you. you can do this by measuring the inside length of the port and then measureing the out side lenght of the port, adding those 2 numbers togather and then divide by 2. this will give you the effective length of the port.

Im sure there are things that I am missing but I hope this helps!

3. ### JulianS Auditioning

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Thanks! strange i never found this, I see you're working on a al1804 project with a slot port though, that should be quite awesome once its complete! i want to build a vented av12 (want to save on pr's ), and to tune it to 18hz in 3-5 cubic ft with anything more than a single 3 inch port will probably require either a very long pvc port, or preferably a slot port

4. ### Rob Formica Stunt Coordinator

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Although somewhat unrelated to the "slot port" question...

When I built my sub, I initially designed my port length based on winISD and I finalized my tuning using test tones and a multimeter. I was surprised by how different the actual tuning was versus the theoretical one... as it required shortening the port about 10% to 15% (I don't remember the exact amount, I'd have to pull the files out and measure what I cut it at).

To make sure I made no mistakes, I crosschecked it on BassBox Pro, which gave a different length very close to that of winISD, but almost as far from actual measured one.

Just trying to say that there are many factors which these formulae do not account for... so it'll give you a pretty good idea if what you need for a box and port, but it's not a precise science.

Later..
Rob

5. ### JulianS Auditioning

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Do you need to compensate for the volume occupied by the port?
I'm working out a slot port for an 80l enclosure tuned to 18hz,
and with a 35x5cm port the length required = 191 cm, if I work out the volume displaced (35x5x191) i get ~ 30 litres,
and thats not including the volume displaced by the wood required to create the slot, which will be ~ 10 litres, so I would end up with a extra 43 or so litres to compensate for with the driver !

6. ### Chris Brock Second Unit

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yes you do need to componsate for the displacmnt of the port. you also must compensate for the displacment of the driver, the bracing and anything else that could take up space on the inside of the box. Fo instance I am building a large enclosure right now that is going to have a total interal volume of around 20 cubes, but after I take way the sub, slot port and bracing I will only net around 16 cubes. so I am loosing 4 cubes.

7. ### Geoff L Screenwriter

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Yes it bites, but it beats the stove pipe look.

The last 10" I did, still finishing it, the slot port area was nearly as big as the starting box needed for the driver.
So my .750cf^³ box in the end, was up to a little over 1.25cf^³ when I finished adding it all up, driver and port area.

Small box, low tune= 26-28Hz in this case, and boom the box size grew big time..

But short of a 12" or bigger good long throw PR, or a couple telephone poles {3" PP-ports) sticken out, the slot is really the only thing left.

Needless to say I just went sealed (to stay small & stuffed with .800-lbs of Acousta fill) and it still should hit very nice according to sims. I just lost some db's especialy down deep.

8. ### JulianS Auditioning

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2 more questions

1 - How do you calculate the length of your last bend?
the way I have it is dividing the total length by
the width of the port and taking the decimal part,
and taking the product of that decimal part and
the port length, and using that as the last bend
length, is this right?

2 - Should you round off the edges of the slot, inside
the enclosure on the end of each bend, and outside
on the termination of the port?

9. ### Geoff L Screenwriter

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=====>
Didn't your program tell you the length of the slot needed from entry to exit???
=====>

Rounding Slot Edges:
Depending on what driver your using, and the power, rounding over all the egdes (may or may not be necessary)...!

BUT, it always a good idea to do it anyways, as edge drag will be reduced and can only help keep the slot from making any noise. Turbulance from the sharp edges can cause whisteling and a kinda of chuffing simalar to regular ports if tuned relitively deep 20's to 30Hz and enough power is being applied.

This is assuming a long throw driver, deeper tune, & high power application..

If you have the means I would round over the slot edges before gluing them in place regardless....
Even if you do not have a router and the proper round over bit, just some sanding of the edges will help. The slot entry inside, only needs the one side done, and also the exit.
The rest you will want to do both sides.
====>
What driver are you using and what are you planning to run for power?
====>

Cheers
Geoff

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