Some TL design questions
Hi all !
After the idea of making TL speakers has been boggling my mind for several years, the time has come for me to design and build my first TL speakers.
I've read a lot of material related to the TL, but few issues still bother me.
1. It is said that one should aim for most linear impedance curve in any speaker. Since TL has double-humped impedance curve, and stuffing doesn't really affect upper hump to big degree, increasing the amount of stuffing will eventually kill the lower hump making the impedance curve look similar to the one of a closed box. I'm not sure how it should look in the end, with double or single hump ? Or should it just be ignored as long as the response is looking good ?
2. Taper and sizing of the opening. People are throwing around with different taper ratios, and some are even climbing up to 20:1. Except for higher taper ratio killing upper harmonics/ripple and making the line effectively longer, will it have any other influences on sound ? I was suggested by one person that having high taper ratio combined with small opening will make my box sound like a bass reflex box.
That brings me to another question, how small can the open end be before it starts restricting bass output or making mechanical noise ? Is 0,5Sd too small ?
3. Vas determines what size the box should be. Instead of making S0/SL 3Sd/1Sd I can as well make it to be 6Sd/2Sd or 9Sd/3Sd for the same taper ratio, but at one moment I will be reaching point of diminishing returns. Is there any rule of thumb on how many times x Vas should I choose the box to be for a starting point ?
I'm playing with Martin's worksheets for a week now, but I feel I'm not getting any smarter on the issue of TL, just more confused.
What I mostly like in sound of a TL's (so far I listened only to several PMC models) is the effortless dynamic bass and "slam" they have. When the drum kicks, it sounds like on the live event and I want to relate that to some design topology/graphs so I know how close I'm getting to it when designing the speaker.
1. If you do, indeed, stuff a TL enough to eliminate the lower impedance peak, you will have literally stuffed the life out of it, meaning the terminus is no longer providing any bass support. Instead, do what you suggest; stuff it to get the best response for your application and ignore the fact there are two impedance peaks. TLs and BRs are both inherently 4th-order systems.
2. I've built very successful tapered TLs with taper ratios up to ~25:1. As long as the terminus isn't so small as to have a really high air velocity and cause audible "port" noise, you'll be fine. IME, there is no specific minimum terminus size based on some fraction of Sd. A large taper ratio does NOT cause the TL to sound like a BR.
3. If you can reasonably master Martin's TL worksheets, rather than trying to arbitrarily choose a specific starting line area based on a multiple of Sd, start out with a line width equal to or slightly larger than the driver's flange diameter and a line depth of at least an equal dimension. From that determine what taper ratio you want to first try, which will give you the ending and terminus dimensions. [I don't even consider a taper ratio less than 10:1 in order to gain the double benefit of reducing line length and smoothing out those response ripples.] Set up this model and play with the stuffing density and length. If the response shape suits you but you want a lower F3, assuming the driver is capable, simply increase the line's volume by increasing its depth or width at both ends, keeping the same taper ratio. If the response shape causes the output to be tipped up below 100 Hz, you need to lower the line's resonant frequency by either increasing the line's length or the taper ratio (or both). Conversely if the response starts rolling off too soon, you need to raise the line's resonant frequency by either decreasing its length or its taper ratio (I'd do the former rather than the latter).
Hope this helps,
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