I've gone back into JBL Pro catalogues and other sources to find out the real story, and put this issue to rest.

As far back as 1962 ! In that year's catalogue the C 55 (ancestor of the 4520) is described as "This large rear-loading horn ... the 8-foot exponentially flared horn..." . The 4520 image fellow member Lee posted on March 3, 2017 is in fact one of the C 55 taken from an article written by George Augspurger, Importance of Speaker Efficiency, Electronics World, January 1962, page 3. Even if this image is missing dimensions making it uncertain to calculate exactly the horn length, it would seem, based on above-mentioned 8 ft, that fellow member Lee deserves an apology on this horn's length (though I don't recall seeing him say where he took his 8 ft figure), which I would be glad to do, since I'm a believer in truthful science. In the JBL catalogue, the exterior dimensions of the C 55 cabinet are given as 50 1/2" H, 35 3/4" W and 29 3/4" D. The C 43 (ancestor of the 4530) is also there with no mention of horn length. No weight is given for C 55/43. In the 1967 catalogue there are no mentions about the C 55's horn length.

HOWEVER, starting with the 1971 JBL Pro catalogue things turn around on horn length, even though the 4520 outside cabinet dimensions are virtually the same as those of the C 55 : 50 1/4" H, 35 3/4" W and 29 3/4" D, weight 215 lb without drivers (JBL, Professional Series, Low Frequency Enclosures, May, 1980, page 2 - See frequency response curve here (minus 15 db at 32HZ !) in conjunction with "usable to 30 HZ" below. The uniform response to 50HZ (-3 to -4 DB) appears to be a more meaningful number in practice, unless mutual coupling and/or corner placement are used to boost very low level, but the issue of cone excursion remains at high power/very low frequencies, see next para.). The 1971 Pro catalogue describes the 4520 as "Thirteen-foot folded horn with maximum loading to 42 HZ...Uniform response to 50 HZ, usable to 30 HZ." As for the 4530, "Seven-foot horn with maximum loading to 50 HZ ... Uniform response to 60 HZ, useable to 50 HZ ." The 1974 JBL Pro catalogue adds "A short throw, 13-foot folded horn, the 4520 ... high level sound projection up to 75 feet". "The 4530 is a short throw (to 75 feet) 7-foot folded horn...". The above facts/numbers re 4520 have remained the same in the following JBL literature (1976 Pro, 1978 Pro) up to and including the above-mentioned May 1980 Data Sheet. In the 1980 Pro catalogue horn lengths are the same (13 ft and 7 ft), but it adds an additional driver the E-140 to the recommended 2205 for 4520/4530, and two other drivers for the 4550A/4560A : the E-140 and E-145 in addition to the 2220 and 2205. The 4520/4530 have disappeared since the 1982 Pro catalogue, leaving space for the more recent 46 series, though the 4550A/4560A, both described as long throw directional horns, remained in the book for a few more years.

By the way, an interesting note appears in the 74, 76 and 78 Pro catalogues re 4550/4560 if used with the 2205 driver instead of the usual 2220: "When the 2205 is used in a 4550 or 4560 low frequency horn, some unloading of the driver cone will be experienced at very low frequencies. Power usage, therefore, should be somewhat more conservative than normally specified for the 2205." A similar note related to cone excursion striking the horn throat exists in the above May 1980 Data Sheet when using 2205, E-140 or E-145 in a 4550 or 4560 cabinet instead of the usual 2220. Moreover, a somewhat similar note also exists for both 4520/4530 using 2205 or E-140 drivers (since essential T/S parameters for both are quite similar), when "... the excursion limiting load presented by the horn is reduced," this could lead to "excessive cone excursion and possible damage," therefore "power input at very low frequencies should be restricted." Better be safe than sorry. Because of 4520/4530 abuse, the 2205/E-140/2225 are probably the most reconed drivers...

Back to our question. Outside dimensions of the C 55 and 4520 are almost the same, therefore pointing in fellow member Lee's argument of an 8 ft horn. What about inside these specific JBL cabinets one might ask ? Any weight difference cannot be singled-out since we don't have a number for the C 55 (a heavier 4520 could possibly have been explained by additionnal horn folds inside the cabinet to create a longer horn). It may also be caused by different material used over the years such as regular plywood, Baltic Birch or MDF...

JBL bass horn cabinets are certainly the most copied low frequency enclosures I've ever seen. There's so many of them out there, you have the full quality spectrum: the good, the bad and the ugly, plus everything in between... Many imitations are modified (e.g. for a larger 18" driver; for a smaller 12" driver without success), some get discouraged over build complexity and cut corners, etc. I'm interested in the real thing here, ORIGINALS. I don't think JBL ever published an official plan with ALL dimensions for these since it would have been an invitation to copy (a patented product ?). Likewise, the 4530 plan posted from Helmut Lengefeld is another home-made thing (could be perfect, could be bad) we don't really know, it's not an official JBL document. I tend to rely on proven facts from known sources, instead of hearsay for example:

"It is common knowledge among 4520 afficonados that JBL published a typo that time. ;-)"

Maybe, that time (re 1980 Data Sheet). However, how could such an important typo error on horn length be repeated systematically for close to 10 years (as shown above) in official JBL documentation without ever being noticed and corrected ? Either someone at JBL has been sleeping pretty deeply on the red flag for about 10 years!, or the horn's length is 13 ft as stated by JBL, who knows... Another fellow member may be of some help here. SUBWOOF already addressed this 4520/4530 horn length question here in 2007:

View Full Version : 4530/4520 horn lengths
Jonathan W
01-04-2007, 08:21 AM
Is anyone able to tell me how come the 4520 twin 15" scoops are described as having a 13' horn length, whilst the 4530 single 15" scoops are only 7' long. They don't appear to be that different in cross-section. Is width a factor? Am I just missing something?

Also, while I'm here: Wouldn't a horn length of approx. 11'6" be more suitable, so as to bring the reverse sound-wave from the scoop back in phase with the direct sound-wave at the 150Hz crossover point between the two (150Hz wavelength at room temp. = approx. 7'8")?

Many thanks for any help that may be offered,
Jonathan Weatherill
Hoerninger
01-05-2007, 04:04 AM
Some thoughts can be find here:
http://audioheritage.org/vbulletin/s...ad.php?t=12773
___________
Peter
subwoof
01-05-2007, 06:27 AM
I have built hundreds of each....the difference is the depth and height.

Lugging a loaded particleboard 4520 up a flight of stairs is a real joy so we were happy to see higher power amps + cones come along in the 80's so the SR buisness could switch to vented boxes.

But they sure pound out the lows for a dance club and are still preferred by installers if they have the room.

The 4530 is 24" deep and about 39" tall.

The 4520 is 30" deep and about 50" tall.

Both have added bases to allow casters, crossovers, etc to be mounted since they were popular as roll-around systems for theaters.

That added length really makes a difference below 50hz but neither goes below 30 or so without risking cone damge from unloading.

sub

Surprisingly, Subwoof who claims having built hundreds of each (4520/4530) didn't contradict the 13 ft horn length mentioned in the question ( I suppose he would/should have done so if exagerated), but rather goes on to justify the differences in length, and concludes "That added length really makes a difference... ". He appears to believe in the 13 ft and 7 ft lengths (his height measurement on the 4530 should be 47-8"). At first sight, the only numbers he gives don't seem to add-up to 6 more ft, unless there would be more horn folds inside the 4520 cabinet than meets the eye to increase horn length...

It looks like the "13 ft controversy" just won't go away now... In the mean time, even if it doesn't seem to be conclusive one way or another about the 8 or 13 ft horn, it appears the probabilities are slightly in favour of an 8 ft horn for the 4520, based solely on the above data/comparison between JBL's C 55 and 4520 cabinets, not because Tom, Dick or Harry thinks so, because this wouldn't be scientifically sound.

I hope some of the "horn specialists" here will jump in with regards to the method possibly used by JBL to state this 13 ft length. I guess the "horn guys" could do some quick "reverse engineering" to clarify the feasibility issue. Also, some of the "Tech Bible" guys here such as Giskard, Techbot, 4313B and others may have some old records or inside documents ( as they often do) from JBL on this issue. This would bury this issue for good ...

Richard