Tl Basic Design Question.

[christophorus]

Hi all,
I'm going to build a fullrange TL. Since it's hard for beginner like me to use MJK TL MathCad directly, I would like to ask some basic question to guide me first.
1. Which is better, tune TL box slightly lower or higher then driver fs?
2. Which is better, tappered, straight or expanded TL?
3. Which is better, making TL length slightly shorter or longer?
4. Which is better, TL opening at the same side with driver or opposite (at the back)?
5. Which is better, TL opening close to driver or farther?
6. Is it correct to measure TL length as my dwg below (the red line)?

Thank you in advance for your help.

[Brendon]

Hi Christophorus,

1. Tune the TL at fs or higher. Perhaps as high as 1.5*fs

2. Don't do expanded. Tapering allows you to reduce the tuning of the line compared to a striaght line. If you taper from 4 * driver Sd to 0.5 * driver Sd, you can figure that the line will be tuned to about 65% of the equivalent straight line frequency. For example, if you take a 72" line: 13500 in/s (speed of sound) / 4 / 72 gives you about 47 Hz. If you taper 8:1, you'll reduce it by about 35%.

3. Shorter gives you higher tuning and more amplitude, but doesn't attenuate the higher harmonics as much with the same stuffing density. Of course you can increase the density to offset this.

4. Open on the back near the bottom if possible to make use of the floor boundary for additional gain.

5. Farther is fine. If you open on the back near the floor, you get a gain boost and also you allow the higher harmonics to radiate into the reverberant field rather than direct. They will be less noticable...

6. Yup.


Brendon

6.

[MJK]

Hi Christophorus,

"I'm going to build a fullrange TL. Since it's hard for beginner like me to use MJK TL MathCad directly, I would like to ask some basic question to guide me first."

If you are having trouble with the MathCad worksheets, you might try the alignment tables to get a design concept. Then you can decide if you want to put this design into the worksheets and see if it could be optimized.

"1. Which is better, tune TL box slightly lower or higher then driver fs?"

Depends on the driver Qts, I don't believe that there is a hard rule one way or other. I tend to tune a little below fs to try and extend the response just a bit lower. I like drivers with a Qts between 0.3 and 0.5 best for a classic TL design. If the Qts is higher then 0.5 then the bass will be a little less controlled and may ring.

"2. Which is better, tappered, straight or expanded TL?"

Any of the three can be optimized to give almost the same SPL response. For the smallest enclosure I would go with a tapered TL, I think that this is the safest design approach if you use the alignment tables.

"3. Which is better, making TL length slightly shorter or longer?"

Making it the correct length with the end correction accounted for is the best, again the alignment tables will help set the correct length. There is a lot of flexibility in a tapered TL and small mistakes will not be a disaster.

"4. Which is better, TL opening at the same side with driver or opposite (at the back)?"

Doesn't matter.

"5. Which is better, TL opening close to driver or farther?"

Doesn't matter.

"6. Is it correct to measure TL length as my dwg below (the red line)?"

Yes, your sketch is correct and the length is measured down the middle of the line.

Hope that helps,

[christophorus]

Hi Brendon and MJK,
Thanks for your quick answer, basically I get what I need to know,
however I still have a few unclear thing about it.

1. If I tune the TL below fs, how far it can go before something terrible happen?
any chart for this or just use MathCad to see what happen?

2. Brendon:"Open on the back near the bottom if possible to make use of the floor boundary for additional gain." is there any unwanted effect can happen for this setup?

3. is there any change in sound quality with placement of TL opening (far/near driver or at the same side/opposite driver) like if we seperate mid & tweeter to far? Or I can place it anywhere?

4. How big total volume of TL box compared with Vas of the driver as a rule of thumb?

Thankyou.

Chris.

[Brendon]

Hi again, Christophorus,

1. I think Martin's sheet could probably give an indication as such, but what I'd estimate is that you'll see the amplitude of the pipe drop dramatically as you go below fs. The driver can't support the resonance below.

2. You can get a result that's a 'boomy'. The solution, though, is additional stuffing to dampen the output to taste. You'll, of course, find that bass is very room dependant regardless of the speaker in question. Within a single room, different setups will also have different results. I see this as a positive since you have more freedom to tune to your taste.

For instance, I've tried a very short line of about 1.5 * fs, emptying directly down to the floor. The height of speaker above the floor influences the amount of gain. 1" above the floor gives more apparent amplitude than the same length line with a 3" gap between the floor and the terminus. Stuff unchanged.

3. No change, not like a tweet to mid x-over. But I would recommend against having the terminus on the front facing the listener. Top back like you showed earlier would be fine. Having it at the floor just gives you a bit more freedom to tweak / adjust to taste.

4. I think Martin's tables would be a good judge for this, but in my mind, I normally look for drivers in the Qts range Martin mentioned. Further, I mentally choose 4 Sd as a start for the line and go from there, depending on what I'm doing. For instance, a short line, heavily sfuffed with few bends (or none) is good if you're going to bi-amp with a separate sub. Why worry about getting low end support from resonance if you have a sub? I see the benefit in this case to be the open-ness of the TL's output, not the extension benefits.

If you start at 4 Sd and then taper to 0.5 Sd, you'll have as optimized a line as you can do in terms of total volume. You could reduce the starting area, but then you lose the benefit of some of the taper.

Hope it helps,


Brendon

[christophorus]

Hi Brendon,
thank's for your information, so now I have a better understanding for this thing,
may I know what driver are you using with 1,5*fs? Do you have a website to see it?
Do you use sub for your system and what kind of sub?

Happy New Year 2004, for all you guys!

[christophorus]

Hi MJK,
from your Mathcad, how do I know TL freq. resonance (Fr) of my calculation? (I use TL offset driver.mcb for my tapered TL calc.)
Is there any direct corelation between TL length and TL Fr?
I look mostly at freq. response graph (6th graph from top) for calculation, what else (graphs) should I pay attention to get the best result?

Thankyou,

Chris.

[christophorus]

Hi MJK,
Now I try using your TL section.mcd, but apparently I'm stuck!
My design is folded tapered TL, optimized with your TL offset driver.mcd and then I feed thee result into TL section.mcd and I got stuck at this section:

n_closed=4
n_open=4

what does it mean?

Since I use folded tapered TL (as attch. dwg in my first post), what should I put into Lc0 to Lc4 and Lo0 to Lo9?
Would you mind giving me a brief explanation for this whole section?

Thankyou,

Chris.

[MJK]

Hi Chris,

If you look at the TL Application Note and the worksheet TL_Sections I'll try and give a brief explanation.

n_closed - the number of sections used to model the closed end

n_open - the number of sections used to model the open end

These are basically the number of elements defined below where each line represents one element (note the counter starts at 0). So for n_closed = 4 requires 5 lines of input to model the closed end of the TL, the maximum subscript number should equal n_closed. For n_open = 9 means that 10 sections are used to model the open end of the TL and again the maximum subscript number should equal n_open. Looking at the application note, the closed end is not shown in detail but the open end shows 10 sections defined by the dashed lines. The dotted line traces the path and the dashed lines show the initial and final areas of each section. You should understand everything in this worksheet and Application note before you attempt to modify TL_Sections.

You can use any number of sections that you want as long as n_closed and n_open are greater than 1 (two lines of input) If you look at the back loaded horn worksheets you can see another example ot TL_Sections.

You can input step changes in area by setting the final area of a section not equal to the initial area of the next section. You can accurately model corners but at low frequencies this is really not critical or needed.

If you really are struggling understanding TL-Sections, I recommend sticking with the other preformatted worksheets. TL_Sections will solve any problem that you input, even if it is not the correct model for your enclosure.

Hope that helps,

[christophorus]

hi MJK,
thanks for your brief explanation, I missed your TL app. notes, then I downloaded it and combine with your post above, now your TL section.mcd more easy to understand!
However, I still have a question if you don't mind.
Why n_closed and n_open number not equal with total of sections? (always -1)
That question came from closed end section from your TL app. notes:
On the closed end section, you divided it into 5 sections, right?
Actually, that 5 sections have the same initial and final area, so why you divided it into 5 sections?
Because it is a rectangular form, can we make it just as 1 section with section length=6"? (measured from center-line of the driver to the top, right?), so it becomes:

n_closed=1

Section length...Initial Area... Final Area
Lc0=6in...........Sc0,0=3.Sd...Sc0,1=3.Sd

so: n_closed equal with total of section, or I must add 1 line more? -->

Lc1=6in...........Sc1,0=3.Sd...Sc1,1=3.Sd


I need one step more to understand it, with your great help, surely I can make it.

best regards,

Chris.