Dual 416-8b MLTL design

[GM]

Dual 416-8b MLTL design

Hey everyone I just signed up to ask a few questions. I don't really know what I want to do so I'm still contemplating options. Please bare with me and I'll explain where I'm coming from.

I'm getting old now but remember way back in the olden days (1981 actually) I bought my first pair of really nice and new speakers - Altec Model 19's. They were nice but as many of you who has this audio bug in our blood know eventually we want to move on to try new and other things. So I sold them and went on over the years to own many other nice speaker designs some bought and some built.

I've always missed the big and effortless sound that my old model 19s had. They weren't without issues though - I'd bi-amped mine at 800hz with an 18db electronic crossover which did improve the sound. That large 416 struggled to do a good job up to 1200 hz. Likewise it struggled to go low. I bottomed the woofers out once (ya learn your lesson after once) trying to get them to go lower but they just couldn't do it. Regardless I still consider those to have been excellent speakers that did a lot right.

So I've wanted to build something again and have spent a few days reading as much as I could find on Altec's with special interest in the Transmission line design. Others have mentioned how the 416 generally cannot go low but I gather it can in a properly designed transmission line cabinet. Excursion might be an issue which is why my thought would be to use two 416s per channel. From what I've read 24 cubic feet. That's tough but doable.

If I do proceed with such a design I'd probably need to run them up to 600 or 700hz to some 511 horns.

One question I have - I'm old school and remember the old transmission lines were just open at the end of the line. Maybe tapered but open. The newer designs I've read about recently (I think a few I saw designed by GM) have a port towards the end of the line rather and being open. Would this port be tuned similar as we would tune a bass reflex port? I'm guessing it would have the necessary quarter wave length delay due to the port being towards the end of the line and the tuned port would provide better back pressure for the woofer (limit excessive cone motion at cutoff) hence raising power handling a bit. Better control over the woofer. Again I'm just guessing.

Another question would be I could make the cabinets wide but feel it would look strange having the two woofers mounted horizontally hence I would prefer to mount them vertically. However the first thing that comes to mind with them vertical is the two woofers would be at different points along the transmission line. I'm not sure how much that matters.

3rd question is it looks like the GPA 416-b models a little better than the Altec 416-b. I wish I could but I can't afford 5k for four woofers. I had no idea the GPA 416's were so expensive and wish I'd had the fore-site to have bought a whole bunch of them 15 years ago when they were less expensive.

That's all my thoughts so I appreciate incite from others.

Thanks,
Mark

Greets!


Yeah, from a technical POV, ~1 WL across is the limit so ~900 Hz for a 15".


FWIW all of the Altec [ML = vented] TLs I've designed/built are tuned to Fs.


You don't necessarily need duals to go low, just depends on how loud you want, room and max speaker size and whether or not you have two good corners, otherwise best go dual or a < 100 Hz sub system for a GPA 416-8C at rated power.


Based on some measured specs, ~10.92-15.73 ft^3/driver depending on whether there's a separate sub system, so 2x for duals. Of course they can be smaller, it just means less TL loading of the vent and even then we can get some of the 'lost' gain back by inverse tapering it [ML-TQWT] like I did with my dual 515Bs.



Vents are tuned the same, just can't be calculated like a reflex, i.e. must either be calculated with a horn designer program or with measuring equipment.


You guessed right.


With multiple drivers along a TL line, this driver 'line array's midpoint is where a single driver would normally go since acoustically it's just a single oval driver until long enough to be an 'infinite' one that mimics a long ribbon speaker [~71% of room height].


Technically, whether drivers are vertical or horizontal is a function of desired polar response, how far away the listening distance [Lp] is and XO point/slope, so make a scaled floor plan of where the LP, speakers, other furniture, windows, doors [swing] will be and Lp width, i.e. the more info the more optimized we can make it.


In general though, we want vertical for widest polar response and vice versa with horizontal allowing closest horn/woofer spacing, so combined with narrowest 'sweet spot' is best overall for short Lp distances and or narrow sound walls.


Re height, [ML] TLs are normally designed around average seated ear height [~38" default] aligned to the [super] tweeter, so in general around 60" high, but with large horns this makes the cab too short to have any useful 1/4 WL TL vent damping, so have to put the horn centerline somewhat higher, angled down as required.


FWIW me and some others prefer a more prosound woofer placement centered somewhere in the face/chest area, allowing more vent loading, sense of 'live' performance.


Also, unless the cabs are hard in corners, they need to be ~26-30" W depending on its depth to offset baffle step loss and ~40" wide for horizontally opposed 15".


Well, if I'm remembering the timing right, then GPA's products 15 yrs ago were made from worn out tooling and configured ['voiced'] to what EV was pumping out at the end, so not the low Fs, Qts, high Vas drivers of the 'glory' days, but more prosound centric for EQ'd, high continuous power, low dynamic headroom apps. The upside was they handled a minor amount of more power and performed better with high DF amps, i.e. more [mid] bass at the expense of lower system efficiency.


Nowadays for USA Altec/GPA type apps I recommend AE drivers, preferably with the Apollo upgrade; not cheap, but ~what ferrite GPA prices use to be IIRC: AE Speakers by Acoustic Elegance


For a pretty comprehensive listing of similar products around the world, recommend spending some quality time researching these threads:


Beyond the Ariel - diyAudio


Is it possible to cover the whole spectrum, high spl, low distortion with a 2-way? - diyAudio

GM