Quarter Wave TL's and various Questions.

[demon2091tb]

I've been researching more on the idea of using QLTL's and using the drivers FS for tuning, and thinking about a driver specifically to use in a simple TL just to do it. The Seas CA18RNX has an FS of 36hz which equals a 1/4 length of 93", this is very long, is it really needed in a TL to go down to the FS of the driver like mentioned above, or even below the tuning.......I could go with 40hz or 45hz tuning and have a Length of 84" and 75" respectively and probobly be more than happy with the low end response, as getting near 36hz tuning with 6mm xmax at decent volumes seems to be playing on my mind, i fear for overexcursion under driver FS or tuning. How much does the FS of the driver react to the tuning of the line. I just feel as though i don't need a tuning of 36hz, which seams oddly very low and likewise long.

I've also heard the idea of anywhere from FS (36hz) - 2x FS (72hz) to be workable for tunings, but which is proper and which will do me well. I have plenty of substage for the low end, just looking for a smooth response and something not so ordinary to try out.

Also thinking about line taper, since a 1:1 closed to open end SD ratio is considered "classical". MJKing suggests a higher success rate using a tapered line <1:1 which spaces the harmonics out better than a =1:1 design. King suggests using his mathcad worksheets to simulate the upper harmonics of the designs but i have no mathcad to figure this out. If i should be using a tapered design as far as i know the length will be longer than 1:1 sd ratio design. When they are talking about a <1:1 ratio though are they suggesting a 10:1 sd or 1/.1 sd, or what kind of relationship......what should the actual opening area be, equal to or less than the origional sd?

I've read that .5-.7lbs/cf is ideal for stuffing, how do i figure up where my harmonics will be present in my TL lenth to add wool/foam to soak up these upper harmonics?

I pretty much have a hand drawing worked out to be 90" in a 45" tall tower with a single fold, if i could use 40hz or even 45hz for my tuning, respectively 84"/75", etc this would be much more ideal for my length situation, but does it defy rules to shorten the length and change tuning away from driver FS? Also how will the driver react below tuning if i tune at 40hz or 45hz and FS is 36hz.....will there be an added bass shelf below tuning to the driver FS.

Also wondering really if tuning as low as suggested FS will cause problems with excursion the lower you get when volumes are normal, From graphs that i've seen there is excursion increas below 100hz as it gets near tuning, lack of excursion at tuning where driver FS=FL (FL being pipe tune), then an exaggerate increase blow tuning. Will this caues problems with overexcursion and 6mm of xmax.

Sorry just a few random questions i've been wondering about and getting the design layed out.

[cheesehead]

I've had some thoughts of using the CA18 as well...

I'm going to take a stab at replying, but I'm a relative noob & perpetual lurker.

MJK's TL tables are more for the "classic" TL. They give really big cabs & not always the best results. If you want to do it right, you'll need to get the Mathcad sheets - I believe that there is a free program (Mathcad Explorer) which you should be able to pull down from somewhere, you just won't be able to save changes.

Let me outline two options that come to mind looking at your post:

1) Going with something really big. I'd suggest looking at the BIB ("Bigger-is-better") thread on the fullrange forum. That's a great implementation of a classic TL design. I don't know if the CA18 would be a good choice... and I don't know if there's any advantage to doing it for a two-way. You'd have to ask (I'm interested too!). The BIB gives great bass extension, using corner-loading. Problems - it's big, and it needs a corner!

2) You could look at a MLTL design. This would be much smaller.

I'm not an expert, but the CA18 might require too small a volume to get a good MLTL. I've started some sims with your dimensions (45" high), and using a 50 in^2 cross-section (a little narrow!). Put the CA18 14" from the top and the port near the bottom. Problem is the port is pretty long (3" wide, 8" long) The response:


A resistor in series with the driver would help to level out the response (but decrease dB).

I might come back to this later, got some stuff to do... hope this helps.

[cheesehead]

Or if you wanted a large floor-stander, you could try a 70" high tapered pipe, closed at the top end (S0 = 0 in^2) with 100 in^2 at the bottom. Put the CA18 40 in down or so and a 3" dia 6" vent at the bottom. Stuff about the middle third.

This would give a tapered front, which might help with time-alignment of the tweeter & woofer.

In either design, I'm not sure if there's enough "air" behind the driver. More of an expert than I will have to answer.

[demon2091tb]

Thanks Big cheese for your help, i'm too wondering if any of the so called experts are gonna chime in.......Its onl been a few days so mabey in the next few i can get some true reassurance.

I'm only looking for a good 45" tall tower. You mentioned a 70" with 0 SD on closed end, and 100sq in on the open end. What effectively would this be tuned to, at a 75" length is 45hz. The room i have right now for listening has a tapered ceiling going from 4.5' to 7' at the peak, its a very low ceiling room, so a 70" tall tower isnt workable, i would need to fold it at least once, but its still possible, A base of 10x10 up 45" or so to the top of the enclosure speakers would be a good 30-35" from the floor or like you mentioned , then the rest of the lenght would fold around the top and down the rear meeting somewhere down the length of the bottom section.

J/w though why the small amount of area on the port opening, 3" dia x 6" long is quite a bit smaller than i had thought. But it looks to be a nice design.

BTW that response graph you have looks very nice, How could i flatten out the response up to 1k though and beyond, it looks like it might have a fairly ragged response up there, and this is traditionally a very smooth responsed driver.

Again thanks Cheese for your effort and help, i really appreciate it.

[cheesehead]

Originally posted by demon2091tb
I'm only looking for a good 45" tall tower. You mentioned a 70" with 0 SD on closed end, and 100sq in on the open end. What effectively would this be tuned to, at a 75" length is 45hz.

The port changes the tuning frequency - this is what MJK refers to as "mass loading". I believe that the origin of the term is that the equations are similar to the addition of a mass to the end of a vibrating spring - it lowers the fundamental frequency. That particular design is tuned to about 38 Hz if I remember.

The room i have right now for listening has a tapered ceiling going from 4.5' to 7' at the peak, its a very low ceiling room, so a 70" tall tower isnt workable,

Maybe the BIB is an option - this is essentially a folded TQWT open at the top. Will your speakers be in the corners? If so, you can essentially vent the speaker into the upper corner of the room and you get more bass. (Don't take this into account and put the speaker there and it will be bass heavy.)

BTW that response graph you have looks very nice, How could i flatten out the response up to 1k though and beyond, it looks like it might have a fairly ragged response up there, and this is traditionally a very smooth responsed driver.

The curve I published was with no stuffing in the line. If you stuff the line strategically those spikes will disappear. The peaks in the 500 Hz - 1 kHz range are not likely to show, anyways... the worksheets make a number of approximations and these aren't usually a problem.
(Selectively stuffed.)

Good luck. Hope we get a little more interest in this - I am worried that there might not be enough room behind the driver for this design. (I've only built two sets of speakers, both TL, and kinda have wooden ears. Comes with age.)