Giving Old JBL Drivers New Life - Need Enclosure Design Help

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I'm still not in a big hurry on simulations. I mainly want to make sure that some ideas I have are on the right track and not going wildly off the rails.

This latest idea ends up being pretty big. Inside dimensions of 47.25"x15"x15". That's just over 6 cu-ft. Woofer 1/3 from the top at 15.75". Tweeter center 10" up from the woofer center. Port size, shape and location TBD.
 
That does seem ballpark. I can run it. One thing I would suggest is not having the interior perfectly square in cross section. Gives you a resonance of ~450hz, which is on the cusp of not easy to damp out. Ideally, it would be a golden ratio, or one of the acoustic ratios. But frankly even 15x16 is going to be better.
 
Sure, there's likely some advantage to trapezoids, but sticking to the acoustic and golden ratios is easier and effective. I've made them before and it seemed like more effort than it was worth. I'm not sure I've ever seen a study quantifying any reduced internal resonances, so can't really say.

Trapezoids really only makes sense in my mind if the enclosures are designed for corner placement, or you simply like the look.
 
The consumer speaker world sure makes a big deal out of non-rectangular shapes. My interest was to keep the footprint smaller. If 15"x15" excited a resonance, then maybe something trapezoidal that fit in the same footprint would cancel it out. I'm not sure how I'd get it done, though. It would be interesting to see some research and not just a bunch of advertising on the subject.
 
Ignoring for the moment the 'getting the best effect' out of these drivers since 'best' means different performance criteria to many [knowledgeable] folks and why anyone will normally get a fairly wide range of loading suggestions to such a broad request, especially if posted on a variety of forums................
My original suggestion isn’t necessarily meant to be 'best' since I don't know all the details of the intended app, so based on what limitations you did give, I gave you enough info to design a cab that allows you the flexibility to set the design trade-offs while still overall 'besting' a typical vented alignment [bass reflex].

First and foremost, cab net Vb sets the maximum bass gain bandwidth [BW], so less is always less, though with an appropriately low tuning of a too small cab, room/boundary gain can boost the LF to compensate if said location’s construction is rigid/massive enough.

With a 10 x 10 ft room, there’s normally going to be significant room gain, so a ~ 5.8 ft^3/32 Hz [F6] alignment will probablly suffice and with a little overdrive and 20-25 W combined with even only a small amount of room gain, its 108+ dB transients [stereo] down to 32 Hz is likely louder than most folks would tolerate in such a small room.

Bottom line, make the cabs as big as practical or at least ~5.8 ft^3 net.

As already noted, the wider the cab, the less potential the need for BSC.

Up to a point of dimensioning returns based on the driver’s specs Vs woofer location Vs the cab alignment [Fb], there’s no path-length [height] limit to a [ML]TL, with the driver’s Fs TL alignment [Fp] being the normal max.

WRT tweeter/woofer orientation, historically, this has been a function of a passive XO’s slope order combined with the tweeter’s acoustic offset to the woofer’s. The centerline of this combined polar response of baffle mounted drivers is ~ at the mean between the two driver centerlines with odd orders tilting down the tweeter’s polar response and even orders [most common] pointing it straight ahead.

For these polar responses to combine ‘close enough’ to sum as one requires that they are < 1/3 WL apart [1/pi actually] at the XO’s -12 dB point, so this pretty much limits it to same size drivers XO’d fairly low. Fortunately, between our hearing, losses over distance, reflections and XO ‘steering’, up to several WLs can be tolerable [think auditorium or larger rooms], though 1 WL of the XO point is considered the limit for HIFI/HT apps.

Since the drivers are too large to meet the 1 WL rule, most folks just mount them as close as practical, though ideally they would be offset based on the HF XO’s ‘steering’ angle to overlap them at the listening position [Lp].

Once the woofer location is determined, then this ‘steering’ would dictate whether the tweeter is above or below it.

Different cab shapes yield different eigenmode patterns same as a room, so certain ‘room’ ratios or odd shapes can average them out, reducing the amount of internal damping required, ergo their impact on driver performance along with keeping cab acoustic efficiency as high as practical.

With your 46.5” i.d. height and 15” i.d. width limit though, depth needs to be ~15” i.d. also for the ~5.8 ft^3 net recommended minimum, so you’ll need more damping in the woofer area than normal to quell a dual ~452 Hz eigenmode.

The Sonas Faber resistive vent concept dates back to at least the late ‘40s and its vertical slit layout is from about the same time, so ‘what was once old is yet new again’.

Regardless, it’s for critically damping an otherwise high Q/’boomy’ [too small] cab alignment, so not suitable for your app or any MLTL alignment since it would turn your high aspect ratio cab into an over-damped reflex with probably a too high F3/6 for good [mid]bass output without some [added] BSC to tonally balance it out.

Vent wise then, round or = area square or mildly rectangular [1.0:1.273 max] is desirable, though higher aspect ratio slots up to 9:1 can be used, but need to be somewhat larger and found empirically by measuring Fb.

GM
 
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I haven't seen a good explanation of how the Zu/Tekton guys get away with woofers at the top of the box in their 2-way designs, rather than 1/3 of the way down.

What's to explain? I mean they chose to place the driver to maximize TL loading of a bottom firing vent for a given height/acoustic path-length and driver response, with the trade-off that more internal damping is ideally required around the driver.

After all, as I previously noted, the driver should be at an odd harmonic, which at the extreme top is the 1st/fundamental ;).

GM
 
I read somewhere that the D123 was related to a guitar amplifier speaker,


but somehow modified for HiFi.

these are a fullrange.

in a 6ft box,

they will get to 30 hz easily.


agree with the MLTL idea.


D123

Improving the sound of the JBL D123.

sansui -

SANSUI SP-505J






also -

perhaps the most overlooked bit of 50's 'art deco' .



KQIdvau.png
 
Wow, GM! :worship: That was a lot of info! Between you and Greg B providing some much needed guidance, I might finally hear these old JBL drivers come to life.

tomtt - I wish the cones and surrounds on mine looked that clean! The D123, at least the back of it, is a neat looking speaker.

I'm working on clearing some space in my garage for the upcoming woodworking project. :D
 
these are a fullrange.

Yeah, plenty good enough for mid '60s Muzak/stereo FM anyway. The first Eng. firm I worked at had a pair mounted in the front lath and plaster stud wall to keep us entertained. The lone FM station only had a one hour tape loop with just the various stock reports to break it up [pork bellies, anyone?], so to this day I still can't stand to listen to any of the songs played since we typically worked a ten hour day [theme song from Mondo Cane, anyone?].

GM
 
I did a little revisit to my earlier design. It works out pretty well if the cabinet is ~48" high. I can just flip the arrangement of the D123 and 075 and everything falls into place nicely.

Now, I may need to revisit the crossover again. I made my own N2400 crossover, similar to one in a thread on the Lansing Heritage Site. I thought I was doing myself a favor and 'upgraded' to an air core inductor. I know iron and air cores have different behaviors, so I either need to get the correct iron core, or figure out if there is some way to use the air core coil I bought already.
 
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