Critique this Tline

[chrusion]

Guess you can call me a 47 yr old newbie at DIY speaker building (as I've never built one... ahem), but not a technical or craftsmanship idiot (I'm a 3D animation and graphic design artist). So, after much research and gleanings from all the relevant sites on the topic of transmission line sub design, I calculated my own enclosure and wonder what those with experience (including those with software) think about it.

This will not see heavy duty use nor be driven to max wattage. It's just to replace a 15" crab driver (accordion-style surround) in a small sealed box car sub in my small living room. Just want to get a cleaner, deeper low end to music and family tv watching. I'm not really a purist either, but do care about quality sound.

From the Sketchup drawing, let me know what you think as I'm sure there's something amiss or not right or compatible with the driver listed. Or, I could've gotten lucky. I think I've given enough information in the illustration, but let me know if more needed.

I plan on using that South American 1/2" Sande (Cow Tree) plywood unless advised to use MDF or something different. Again, this sub won't be driven to window shattering levels, so I'm not sure I should be worried about cabinet resonances from the thin material, not unless advised to add some dowel bracing.

Amp will be a Bash digital 300 W plate. Home system is a 1997 Onkyo recvr/amp, no DTS or THX, just fake surround (R-L with 25mS max delay to rears).

Thanks for your help and assistance.

[bjorno]

Hi,

Your design looks OK, but IME, can be troublesome to place in room for to secure a flat sub pass-band, See the pictures

b

[chrusion]

Agreed.

The room is carpeted about 14' x 13' with faulted (single 12/12 pitch) ceiling starting at 9' sloping up to open balcony of second story, so about 18 feet. It is a very hard room for acoustic placement of primary drivers (Infinity 4" woofer + 1" dome tweeter satellites) let alone a sub. The room really only has 2 full walls adjacent to each other, the other two have floor to ceiling openings to kitchen and foyer, plus two closet doors.

Thanks for posting the Mathcad plots for this driver.

Any one else have input re. cabinet construction?

[Cal Weldon]

Yes continue with the thought of plywood but increase it to a minimum of 3/4" and allow for bracing. The cabinet resonance is relative to the level it's being played so it doesn't matter what volume you plan to have it at. I think this is at least as important as the corner deflectors you are showing.

[mondogenerator]

for a sub i would say either 1" mdf (cheaper) or 3/4" birch ply. Some have preference for one or the other. IME in terms of breaking strength and ultimate rigidity birch ply wins for an equal thickness, but somehow MDF just seems far less bendy, and....SPRINGY.

I feel like ive had some kind of epiphany.

im a 6 foot bloke, and i can easily bend a 3/4x8x40" length of ply, but i damnwell cant do it with MDF.

ply had huge tensile strength due to the LONG wood fibres, and the grain they form. MDF has its strength from the adhesive and friction bonds between the SHORT fibres.

the long fibres in ply have alot more stretch before the bonds weaken and the material breaks. in MDF it is reversed. the adhesive bonds are comparatively weak, the material has a lower ultimate breaking strength, and much of the energy used in bending the material is lost.

at the lower end of the scale, at relevant magnitudes of force, i believe that the springy nature of ply is its downfall, hence the need for bracing. i have made a sub box in 15mm MDF for 2 12" subs (for a car, hence about 2.5 cu.ft) and minimally braced. it was just fine, even at silly volumes.

BUT (to be fair to the ply advocates, and coz im not here to start a war, just share my opinion) :


i would never use a MDF box, even for a sub, WITHOUT lining all the walls with a good relatively heavy absorbent. roofing felt, then thick felt/cotton felt/wool underlay/even carpet.

it just seems to be more transparent to sound, rather than it actually re-radiating as much as others believe. try listening to music with an earplug in one ear. get a thin piece of MDF and ply(2mm?), and see which you can hear more thru, when you cover your free ear with it.

ply or MDF? if you like the cow tree(?) ply you mentioned, use it. im sure it will be great. just either brace it alot, or back each panel with 1/2" MDF or ply. I just think 1/2" is too thin for anything over about 1/3cu.ft in volume...

doesnt really matter, as long as you design and build suitably for either material. plys more costly, and more bracing needed due to springiness, but MDF 1" to make up for its lesser ultimate strength, and a good liner to compensate for the paper mache box effect. swings and roundabouts really in the end.

[chrusion]

Thanks, Cal... I'll redraw with 3/4" and adjust so line volume and length stay the same. Now, what about 1/2" of felt on the first half of the line aft of the driver? Better than none?

Re. bracing... what would be good, but not overkill? 1" dowel joining all points that are mid-span (X marks the spot on all panels) or 3/4" ply "dividers" with suitably large holes as though mounting a driver and placed on the vertical center-line of the box?

[planet10]

I never use MDF, particularily in a woofer, but 1/2" ply is too thin for this ap.

On 1st blush, and backed up somewhat by bjorno's sims, the TL is far to small to be anything but an aperiodic box. I'd suggest running the woofer thru Martin King's alignment tables at a minimum.

Transmission Line Theory

Dowels are not very effective as braces, and the corner deflectors should be eliminated (with no deflectors, the expansion of the line at the folds act to aid the low pass function of the line)

dave

[Cal Weldon]

I think if you wish to raise the resonant point of the pieces you might consider running stiffening battens longitudinally throughout the TL, mounted direct to the panels rather than any kind of cross braces which may interfere with the sound traveling down the line. Probably best if you offset them from centre a little bit too. I would consider using 3/4" X 1" material on edge.

The design part of the TL I will bow to Dave and others as it's all a bit boggling to me.

[chrusion]

Originally Posted by planet10

the TL is far too small to be anything but an aperiodic box.

How so? Math says the 103" long line is resonant at 33 Hz. Are you saying it needs to be tuned lower than that? 25? 20? Won't stuffing make it appear longer? I read that an unstuffed straight round test line tuned to 30 Hz (same as driver) actually lowered the driver Fs to 25. Does a folded, square-cornered tube not behave the same?

Or are you saying the 7.25 cu ft volume is insufficient and that I need a larger closed end area?

I wish I had access to King's sheets, but don't.

.

[PeteMcK]

I'd go with your 'alternate driver location' - this accords better with Martin's findings where placement of the driver at ~ 30% of line length eliminates a major resonance