Can anyone help with a TL issue

[JC]

Merry Christmas all.

I was wondering if someone familiar with MJK’s worksheets would be willing to help with a problem I have. My request is kind of reversed from the usual. Instead of help designing a box to fit certain drivers, I need help finding drivers to complement a box.

I built some Thor cabinets and wasn’t satisfied, so I built some curved small Thor MLTL cabinets to get better bass. Now I want to do something with the old cabinets and need to figure out what new, less expensive, drivers might work best. It might also be interesting to see how varying TS parameters makes a difference; e.g. is a higher or lower QTS better or what is the tradeoff between low FS and high VAS versus high FS and low VAS?

The cabinet is an MTM with the center point of the two woofers 10.75 inches from the top inside of the cabinet. The first part of the line is 38 long. It starts at 8.25 deep and 7.5 wide and tapers to 5.5 deep. It then turns and runs 5.5 long, 5.5 deep and 7.5 wide. It then turns up and runs 37 inches starting at 5.5 deep, tapering to 2.75 deep where it exits at a port 2.75 by 7.5. Cabinet drawings are here:

http://www.seas.no/images/stories/diykits/pdfdataheet/thor_cab_inch.pdf

Some of the drivers I am considering are:

Exodus Anarchy: QTS .55, FS 49.6, VAS 7.8 L. Complete specs here:

http://zaphaudio.com/temp/EX-Anarchy-TS.gif

Usher 8945A: QTS .39, FS 37.3, VAS 24.6 L. Complete specs here:

Zaph|Audio

Dayton RS 180: QTS .392, FS 40.4, VAS 20 L. Complete specs here:

Zaph|Audio

Usher 8948 A: QTS .296, FS 29.6, VAS 37.19 L. Remaining specs:

Piston Diameter = 140.0 mm
* R(e)= 5.90 Ohms
* Z(max)= 34.76 Ohms
* Q(ms)= 1.745
* Q(es)= 0.357
* L(e)= 0.79 mH 10K
* n(0)= 0.26 %
* SPL= 86.22 1W/1m
* M(ms)= 25.84 grams
* C(ms)= 1.12 mm/N
* BL= 8.92
If anyone has some other driver suggestions that are not too expensive I am willing to consider them as well (the 8948A’s are pushing the cost limit).
Thanks for any help you can give.

 

[pkitt]

I set up a reasonably accurate model for the Thor's tapered cabinet, then sequentially plugged in the T/S values for the drivers you listed. The results for all of the drivers were very similar with F3 ending up around 40 Hz based on a stuffing density of 0.75 lb/ft3 in ~the first 2/3 of the line length. There was a lot of ripple in the response, however, primarily caused by the taper ratio being so low at 3:1. Also, in general, this line's 1/4-wave resonant frequency of ~30 Hz is a bit too low IMO for all but the Anarchy driver. And, the design center isn't very ideal based on the line's total length of ~76". I've attached the modeled system bass response for the 8948A, because it was the last one I modeled, for reference; it's pretty typical. Hope this helps.
Paul

 

[JC]

Thanks and a couple of questions

Thanks very much. That ripple doesn't look too good especially considering that is quite a bit of stuffing.

I could possibly make some modifications to the cabinet if it would help. I was thinking that I could cut out the panel in the back with a flush trim bit and then glue in a piece of MDF at an angle attached to the partition in the center to make the line length shorter, although the last part of the line would then have a steeper taper. The port at the back would then be lower down if you can visualize what I am trying to describe.

I could also probably cut holes in the top part of the partition and then seal it from the back of the speaker, which would create a larger cavity at the beginning of the line.

Would anything like that help?

I am surprised that the Anarchy is the only one where the line resonant frequency wasn't too low. I expected the opposite since it has a fairly high FS. I thought the 8948 would work best in that respect.

 

[pkitt]

This Thor line really isn't ideal for any of the drivers you suggested, but the reason the Anarchy is better is because its Qts is rather high. When optimizing a TL for a driver with a Qts greater than 0.4, the line's 1/4-wave resonant frequency needs to be made lower than the driver's Fs.

Regarding your modification suggestions, shortening the line would, of course, raise its resonant frequency, but that, then will also increase the ripple magnitudes. You would then have to increase the taper ratio to reduce those ripples, but then that would again lower the resonant frequency. Unless you remove the angled divider entirely and convert this cabinet to a simple ML-TL, I don't think you'll be able to salvage it.
Paul

Thanks very much. That ripple doesn't look too good especially considering that is quite a bit of stuffing.

I could possibly make some modifications to the cabinet if it would help. I was thinking that I could cut out the panel in the back with a flush trim bit and then glue in a piece of MDF at an angle attached to the partition in the center to make the line length shorter, although the last part of the line would then have a steeper taper. The port at the back would then be lower down if you can visualize what I am trying to describe.

I could also probably cut holes in the top part of the partition and then seal it from the back of the speaker, which would create a larger cavity at the beginning of the line.

Would anything like that help?

I am surprised that the Anarchy is the only one where the line resonant frequency wasn't too low. I expected the opposite since it has a fairly high FS. I thought the 8948 would work best in that respect.

 

[JC]

Do you think all drivers will have that much ripple or are there some with TS parameters that might reduce it? If so, what TS parameters should I be looking for that are the best match?

For example, the Seas CA18RLY has a bit higher QTS than the Anarchy (.56) with a little lower FS (45.8) and a significantly higher VAS (23.3.L).

The Usher 8945P has a bit lower QTS than the Anarchy, but still above .4 (.412) with a significantly lower FS (38.2) and a significantly higher VAS (24.2L).

I hate to trash the cabinets, they were a lot of work and look good. I could possibly rout out the back panel of the speaker to I could get to the partition inside and cut it out with a jig saw (but it wouldn't be flush, it would be like a brace inside. Then I could put in a new partition.

Instead of tapering from 8.5 to 5.5 in the first part of the line it would taper from about 10.5 to 5.5 in the first part of the line. The taper in teh back part would be similarly increased. The line would also be a few inches shorter. Is this enough to make a significant difference?

 

[pkitt]

I hate to dash your hopes to reuse the Thor cabinets with different drivers, but the original design just wasn't very good from a TL standpoint. Increasing the taper would decrease the ripple, but even if you shorten the line several inches in the process, you'll end up pretty much back where you started as far as its resonant frequency is concerned, albeit with less ripple. There a number of variables in a TL's design that need to be optimized for a specific driver, the most important will be its resonant frequency. If the driver's Qts is close to 0.4, then the resonant frequency will need to be close to the driver's Fs. In a tapered TL that resonant frequency is determined by the line's effective length. For a taper of 10:1, the effective length will be ~62% longer than the actual length. Stuffing density and line length stuffed affect both ripple and, somewhat, resonant frequency. Then, the volume contained in the line determines the system F3. I'd like to help more. And, where the drivers are located along the line, the TL design center, also affects the response smoothness. If the line had been a good design and properly optimized for the drivers chosen, it would be a bit easier to substitute other drivers if they had similar T/S values.
Paul

Do you think all drivers will have that much ripple or are there some with TS parameters that might reduce it? If so, what TS parameters should I be looking for that are the best match?

For example, the Seas CA18RLY has a bit higher QTS than the Anarchy (.56) with a little lower FS (45.8) and a significantly higher VAS (23.3.L).

The Usher 8945P has a bit lower QTS than the Anarchy, but still above .4 (.412) with a significantly lower FS (38.2) and a significantly higher VAS (24.2L).

I hate to trash the cabinets, they were a lot of work and look good. I could possibly rout out the back panel of the speaker to I could get to the partition inside and cut it out with a jig saw (but it wouldn't be flush, it would be like a brace inside. Then I could put in a new partition.

Instead of tapering from 8.5 to 5.5 in the first part of the line it would taper from about 10.5 to 5.5 in the first part of the line. The taper in teh back part would be similarly increased. The line would also be a few inches shorter. Is this enough to make a significant difference?

 

[JC]

I broke down and got Martin's mathcad program and have been playing with some basic designs. With the 8945A woofers and a 74 inch line starting with So of 1.77 Sd held constant for the top 3 sections of the line and then starting to taper to an Sl of .25 Sd it didn't look too bad. I used .5 lb of stuffing per cu ft and put no stuffing in the last two sections of the line.

My model is pretty basic as it does not take in to account the bends in the line so I am not sure how accurate it is. I also tried an MLTL which was flatter until the low frequencies started to drop off at a steeper slope than the regular TL. (At 40 hz the MLTL had more bass; at 25 hz the regular TL had more.)

If I knew how to post a picture of the graph I would bee interested in your opinion of how these look.

 

[JC]

Here's the graph. 74 inch line.

 

[pkitt]

That's pretty good, but I would shoot for a different response shape. For me your line is tuned a bit too low and I would prefer its response to remain flatter, providing a lower F3 but at the expense of response below F3. What I usually shoot for is an F3 in the low- to mid-30s with the response as flat as possible down or close to 40 Hz. Personally the music I play, virtually all instrumental symphonic music, doesn't have much if any content below 30-40 Hz, and I'd rather maximize the performance there and give up stuff in the 20s. I've built two systems, one using a pair of 8945Ps in an MTM, and the other a single 8945A in a 3-way. Both were ML-TLs having an F3 in the low-30s and a fairly rapid drop off below that. Remember that we all have different preferences and there's no single "correct" design.
Paul

Here's the graph. 74 inch line.