Slot ported version of The Tango

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#82
scottjoplin said:
Yes, the volume of the port would be the same if you doubled the area and halved the length. The velocity would be less with a larger port area. Yes if it was 7" deep the velocity would be the same
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Jim Shearer said:
Just noticed above, someone suggested a higher slot dimension, but wanted to keep the volume the same by shortening the port.

Noooooo!

If you increase the cross-sectional area of the port, then it needs to be LONGER to maintain the same tuning. If you enlarge the area of the port, you will need to re-calc the length--and be sure you can accommodate the greater length in the cabinet.

Cheers, Jim
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Good catch, Jim.

Looks like I've missed some fun.
Plastic tube ports take up basically the volume of the air inside - the thickness of the port's wall is trivial.
The slot port we're using will have the same volume of air, but we've added an extra bit of wood to make it (the volume of which is non-trivial). All you'll need to do is find the volume of the bit of wood you're using and find a way of increasing the cabinet volume by the same amount.
I'd tend to make the cabinets deeper, since baffle diffraction effects will often be compensated for in the crossover. Changing the baffle dimensions would mean the crossover would be out. Simply making the cabinet deeper avoids all that.

Chris
 
#87
Having it in the front would allow the box to be up against the wall if necessary. I think if the slot is near the bottom which I presume is what you are planning to do, it won't effect the function of the baffle. I can help you with the maths but when it comes to port theory I had better leave that to others...😉🙂
 
#88
Don' t know the in depth technical reasons why but i find slot type ports less problematic with regards to near rear wall placement allowing more scope for mounting, i also prefer slot loading at the front ( bottom ) to as ' chuffing ' seems less of an issue .
 
#90
Yep. So, we are not changing the length or the area of the port, but we do need to work out the volume taken up by the timber used to make the port and make the cabinet slightly bigger to account for it? I'll have a go at that, unless you've already done it? It's going to be 7 inches wide * 7 inches deep * 0.75 inches thick = 36.75 cubic inches need adding to the cabinet volume. That's presuming one piece of timber creating a port at the bottom of the cabinet. Have you done any drawings?

The internal area of the cabinet base is 7 * 11 = 77 sq in.

To add the extra volume we can divide 36.75 by 77 which gives 0.477 inches, which is how much you need to increase the height of the cabinet by.

I'm pretty sure I'm right but I would like someone else to check my sums, PLEASE......
 
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#91
Ah, just realised this has worked out quite well, Chris was concerned that changing the baffle dimensions was probably not a good idea because of the baffle step compensation. But, we now have a slot 0.45" high and have added 0.477" to the height.......so the baffle is near as damn it the same aha!
 
#92
I can't see the small change in height of the baffle affecting the baffle step compensation in the cross over. And it isn't worth worrying about a 1mm difference in the port length. I would be willing to wager a significant sum of money that shortening a 7.75" port by 1mm will not be audible.

It is a good idea to keep to the original dimensions of Bagby's design, but don't drive yourself crazy worrying about a millimeter!

And do keep us posted as you progress!

Cheers, Jim
 
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