Good driver for TL?

[Vivek]

Hi all,
I have been reading up a bit about transmission line speakers and felt I should try it out. But I keep reading stuff saying that a TL speaker needs a certain type of driver. This is a bit confusing.
I have with me two pairs of Vifa TC18-WG49-08 drivers. Can anyone tell me if they would be good enough for a TL?

Here are the parameters:
Electrical Data
Nominal Impedance Zn 8 ohm
DC Resistance Re 5.5 ohm
Voice Coil Inductance Le 0.75 mH
TS Parameters
Resonance Frequency fs 37 Hz
Mechanical Q factor QMS 2.62
Electrical Q factor QES 0.58
Total Q factor Qts 0.47
Force Factor Bl 5 Tm
Moving Mass Mms 11.2 g
Effective Piston Area sd 138 cm^2
Equivalent Volume Vas 44 ltrs
Sensitivity 2.83V/1m 86 dB
Long-term Max Power IEC 18.3 180 W
Short Term Max power IEC 18.2 225 ohm
Voice Coil and Magnet Parameters
Voice Coil Diameter 25 mm
Voice Coil Height 12 mm
Height of the Gap 4 mm
Weight of Magnet 0.24 Kg

Thanks,
Vivek

[pkitt]

Using your supplied T/S values, I quickly modeled a 10:1 tapered TL and an ML-TL both having the same line length (66") and line volume (~1.9 cubic feet). I modeled for just a single driver with it located 20% of the line's length from the closed end. The ML-TL had an F3 of ~33 Hz, whereas the tapered TL had an F3 of ~37 Hz, very typical in comparison. The ML-TL had a smoother overall response, also typical, but the tapered TL would be considered by many as a "true" TL. Are you wanting to use your two pairs of these drivers in an MTM? Do you have any cabinet size or shape preferences? How low were you hoping F3 would be? I've attached the system response curves; the first one is for the tapered TL and the second for the ML-TL. Please remember these were quick modelings, not necessarily anywhere near optimized.
Paul

[Vivek]

Hi Paul,
Very kind of you to model it for me. I was planning to use them in an MTM configuration. I do not have any particular box preference. And something in the region of 35 or 40 Hz should be good enough for music I think.
Actually I had them in an mtm before i bought other drivers and tried the mtm with them. I was wondering if I could use the same box and modify it for a TL. The back is not fastened permanently.
Cheers

[pkitt]

Tell me about your existing box. What are its internal dimensions? What is the distance from the tweeter's center to the inside of the top? Is there an existing port, and if so, where is it located vertically relative to either the inside of the top or bottom and is the port on the baffle or rear panel? What are the port's length and inside diameter?
Paul

Originally Posted by Vivek

Hi Paul,
Very kind of you to model it for me. I was planning to use them in an MTM configuration. I do not have any particular box preference. And something in the region of 35 or 40 Hz should be good enough for music I think.
Actually I had them in an mtm before i bought other drivers and tried the mtm with them. I was wondering if I could use the same box and modify it for a TL. The back is not fastened permanently.
Cheers

[Vivek]

Okay, here goes (all in inches):
Inside height: 39.75, width: 8.75, depth: 12.5.
distance from centre of tweeter to inside of top: 11.75
port located at the back and on top half. distance from centre of port to inside top: 12.5
inside diameter of port: 1.8
length of port: 5

Vivek

[pkitt]

Your cabinet is a bit too small in volume to allow a really good-performing TL to be implemented in it for two midwoofers, IMO. It would need to be about twice as deep, and therefore twice its current volume. There are numerous other TL guys that frequent this and other forums; they may different opinions and I hope they chime in. I would prefer to start with a clean slate to create a specific cabinet and would be happy to do so; however, I won't be able to work on it until the middle of next week.
Paul

Originally Posted by Vivek

Okay, here goes (all in inches):
Inside height: 39.75, width: 8.75, depth: 12.5.
distance from centre of tweeter to inside of top: 11.75
port located at the back and on top half. distance from centre of port to inside top: 12.5
inside diameter of port: 1.8
length of port: 5

Vivek

[GM]

Originally Posted by pkitt

Your cabinet is a bit too small in volume to allow a really good-performing TL to be implemented in it for two midwoofers, IMO.

Agreed. The way I do a default TL design it will ideally take a few more liters than this box has just for one driver based on the published specs.

That said, if near/at a wall, or better, a corner, then this cab's Vb tuned fairly low in a TQWT might work out, but as you note, better to design/build from scratch.

GM

[pkitt]

The current box would not have to be doubled in volume. I actually meant it would need to have a depth a couple of inches or so more than it currently has. Thanks for chiming in, GM. Maybe you can come up with one of your typically good designs since I can't spend any time on it until next week.
Paul

Originally Posted by GM

Agreed. The way I do a default TL design it will ideally take a few more liters than this box has just for one driver based on the published specs.

That said, if near/at a wall, or better, a corner, then this cab's Vb tuned fairly low in a TQWT might work out, but as you note, better to design/build from scratch.

GM

[Vivek]

Thanks guys for your valuable inputs because this TL thing is very new to me. I am also doing some reading.
I do not mind building a new cabinet if it is going to be worth the effort. I heard a TL a friend has built and loved the tight low end response. Anyway, I am in no hurry so anytime you are free I look forward to your help.
Thanks,
Vivek

[pkitt]

Yesterday evening my wife and I returned from a 6-day driving trip and I can now get back to designing TLs.

I just finished modeling an MTM ML-TL for your TC18 drivers. The cabinet and component location dimensions I'll state are all internal or internally referenced, which is how the TL is modeled and also allows you to use whatever cabinet panel thicknesses you want. Since this is an MTM, the cabinet is fairly tall in order to get the tweeter to a reasonable height. The cabinet dimensions I used are 8.5"W x 14"D x 44"H. The tweeter's center, which is the TL's design center, is 10" below the top, with the two TC18s located above and below the tweeter, of course (I assumed the tweeter would have a flange diameter of ~4"). The mass-loading port's center is 2" above the bottom of the cabinet, with the port being 3" in diameter and 4" long. I used a stuffing density of 0.75 lb/cu.ft. which requires a total amount of stuffing of 18-19 ounces uniformly distributed by density in the top half (top 22") of the cabinet. You will need bracing, of course, and if you pursue this design, I'll make suggestions if you want. I've attached the graph showing the system response (the solid red line) which indicates an F3 of ~35 Hz. I will proceed with modeling a tapered TL in the very same size and shape cabinet, but it will take me awhile because I have to make a sketch and use a different, somewhat more complicated worksheet from Martin.
Paul

Edit: I forgot to mention I arbitrarily modeled with 0.3 ohms assumed to be in series with the paralleled TC18s to represent crossover inductor(s) resistance that would be in series with the drivers. Also, the response graph represents the output with a 2.83-volt input and, of course, does not include the effects of any BSC.
Paul

Originally Posted by Vivek

Thanks guys for your valuable inputs because this TL thing is very new to me. I am also doing some reading.
I do not mind building a new cabinet if it is going to be worth the effort. I heard a TL a friend has built and loved the tight low end response. Anyway, I am in no hurry so anytime you are free I look forward to your help.
Thanks,
Vivek