n0catchyname

Just thinking, and thought I'd ask some opinions or answers.

When making an L-port I would guess that having the corner at a 45 degree would be optimal, similar to this?

| |
| |
| |
\ \____
.\___________

Now to calculate proper port length, using this manner, it would be the same as a regular slot port because the port doesn't widen in the corner, correct?
Or is there a more proper way of doing this?

Secondly, I google searched kerf ports and came up with mixed results on tuning. Some say measure from the beginning of the opening, some say half way from where it begins to widen, and others say from where the port is at a constant width. I'd think the easiest (if you can call it that) way, if there was no concrete way to determine the length, would be to calculate it at the longest length and test, shorten if needed, test again, rinse and repeat, until one had the desired frequency. Does anyone here have any experiance with this type of port, and if so, is there a correct way of tuning?

Kerf port building is at least a year out for me - only testing equipment I have are my ears and if I'm going to put in the effort to build something like that, I'd like to actually test it better than that, and won't be spending money on equip like that until my Celica is officially on the road next summer - but information is always useful.

Thanks

Lord Huggington

iTrader: (1)

If the 45 is in the corner than it acts as nothing, it only helps as like a wave guide. A square corner is dead weight.

Measure from the center line, and remember that if the port runs along an outside wall than the port acts like it continues for half of the width of the port past the physical inside-wall port length, that's an end-correction. Example:

1" MDF
port width 3"
outside box depth 20"
Inside port wall 16"
Actual tuning would be calculated as 17.5"

That's a square shape. View it as an angle - the calculated port length ends at the physical inside wall, and continues on the outside wall for the port width past the inside wall. It's the same thing, just angled.

The air in the port doesn't move squarely so it's better to view it as moving around that angle, and with the 45 in place the port length is still the same. It's the same if the distance between the middle of the 45 and the end of the inner port wall is the same length as the port width. Obviously angles provide more length than the other two dimensions so your 45 shouldn't be the corner squared off on an angle.

Example: the red & blue lengths should be the same, which is determined by the size of the 45. The red is what a calculator will give, and so you make the inner port wall shorter by half of the port width.



All that for a damn corner.

Dukk

Right - always measure the length of the vent down the 'middle' of the vent. I used to cut port tubes on the 45 as well. Bonus points if someone can figure out why

For a kerf it depends on the flare rate. If you are just using a roundover bit on the MDF, really it's nothing. If you are doing a kerfed flare it could be up to 1/3 of the flare depth. It is always best to design the box to be able to adjust the inner leg of the vent so you can fine tune it later. (Reality is it will make just a few hz difference in the tuning so you could not worry about it at all)

n0catchyname

Great info as per usual. Very helpful. Thanks

Lord Huggington

iTrader: (1)

How would you adjust a kerf after it's built?

If using an aero pointing at a 45 than wouldn't the tuning end up being a little lower? I mean, besides the obvious improved air flow. Not what you meant?

I can tell the difference between 30, 35, and 40 Hz easily, mostly because of how early it drops off, so "a few hz" is important to me, it just bugs me knowing it's off at all lol.

Dukk

Oh yeah? You can hear the difference in tuning, on music, between 30 and 35hz? Impressive. How about 30 and 32? 32 and 33?

You don't. You adjust the length of the straight leg.

n0catchyname

I may be relatively new to box building, but the more I learn, that's sort of what I was thinking, no disrespect to Lord Huggington, obviously. I can see being able to tell between 10 hz difference in tuning, perhaps even 5hz. But I mean under normal circumstances, even if you are 1-2in off on your port length, you will only be 1-2hz difference. I could see the importance if you were doing spl specific boxes, but for everyday music listening, I doubt anyone would notice that miniscule of a difference. I guess doing your best to know the in's and out's of tuning to get it as close as possible is optimal (hence my questioning and threads on such topics). But the more I read, it's pretty clear that having it perfect spot-on to what you are going for is more personal preference of perfection than actually being important to overall performance. For instance, if I said my box was tuned to 33hz and it actually ended up being 31 or 35 because I messed up the port length, would you really be able to tell the difference by ear? I'd assume it'd be doubtful.

Lord Huggington

iTrader: (1)

Now I'm not so sure, I think I could if I listened to the same song (music that drops to 25) with two boxes at 30 & 35, everything else equal. You mean 30 doesn't sound lower than 35? I tuned to 36-37 and it fell on its face below it, I think if I tuned to 30 it would have made a big difference. I said 5 hz incriments, and it depends on if I know the music and sub. I don't listen to a stereo for the first time and guess the tuned frequency.

n0catchyname

I think the point he was trying to make was what I was referring to in my previous post. Take, for example, my current box. It's supposed to be tuned to 27 hz with a slot port, but I didn't realize that you have to factor in the wall as the other side of the port when figuring length and measure down the center to the cross-section, so because of this my port length is probably ~1.5" longer than what it's supposed to be for a 27hz tuning. This making it tuned slightly off, but not enough to make a noticeable difference. That's just how I read it though.

Dukk

^ That's what I was getting at. Don't get me wrong, I like to design everything to be as precise as possible but I know full well that a few hz here and there isn't going to make or break a design and I try to not obsess about it.

Playing below tuning is cheating! lol.. That's a mechanical thing. Without resorting to extreme examples, the F3 of the enclosure is still largely determined by the volume of the box rather than the tuning and that will drive the overall sound of the box. The vent just helps out.