Speaker Asylum: To set the record straight on this antique JBL stuff... by villastrangiato

(eyeroll)

For those not familiar with the 075 JBL "bullet" tweeter - it is an antique that dates back to 1957!

No doubt, you're probably thinking - "what putz is bringing up a 55 year old loudspeaker transducer in the context of THE BEST MIDBASS EVER thread?"

Well that would be self proclaimed JBL /constant directivity expert - Thorsten...

If you've never heard a JBL D130 or 075, I can tell you from personal experience, they're not for high performance home sound. They aren't even considered high fidelity in today's pro market when compared to the 2200 series of woofers and 2400 series of compression drivers. The D130 worked fine in venues like the Grateful Dead's "wall of sound". In that application, directivity and controlled off axis response were not major concerns. Similarly, the 075 tweeter functioned reasonably well for the intended purpose.

But everything Thorsten has said in this thread about these drivers has been way off the mark. The 075 bullet was intended to be crossed down around 2.5khz - no where near the 6khz upper limit of the D130.

http://www.lansingheritage.org/images/jbl/specs/home-comp/075/page1.jpg

For those interested in taking a trip down memory lane, Audioheritage.org has a few threads dedicated to these drivers. For the most part, the knowledgeable experienced users of these drivers have commented that they were good for their day in a "Victrola" kinda way. If a Victrola is high fidelity to you - God Bless You. I suspect it isn't for most of the people that visit this website - except Thorsten, of course...

Edit:

In looking above at some of the rambling stupidity posted by Thorsten, I noticed a glaring error that could probably confuse some of the less experienced people who might come upon this wacky thread. In the interest of avoiding widespread misinformation, I'll add another correction here.

Constant directivity in the context of high performance loudspeakers implies that the flat frequency response expected on axis is largely duplicated as one steps further and further off axis horizontally from the speaker. Textbook constant directivity loudspeakers will show a gradual attenuation of response with increasing off axis angle. So for example, at 30 degrees off axis, the response will be essentially flat from the lowest frequency to around 10 or 12khz - but it will be consistently down 2 or 3 db from the on axis response. As one goes further off axis to 60 degrees, it might approach 5 or 6 db of attenuation up to around 10 or 12 khz. The Revel Ultima Salon 2 and Klipsch Palladium P-17B are a couple of decent examples of speakers exhibiting constant directivity.

Look at figure 5 here:

http://www.stereophile.com/content/klipsch-palladium-p-17b-loudspeaker-measurements

and figure 6 here:

http://www.stereophile.com/content/revel-ultima-salon2-loudspeaker-measurements

Notice the slight downward tilt in response as you traverse the frequency spectrum. Also notice the roughly parallel nature of each line as you compare the successive off axis incremental plots.

Now compare those responses to figure 5 of the Tannoy TD12 which crosses its 12 inch driver over at only 1.3khz:

http://www.stereophile.com/content/tannoy-dimension-td12-loudspeaker-measurements

Notice the degree of parallelism that exists between 300hz and 1300hz between the on axis response and the 60 degree response. Compare that - over such a limited range - to the same degree of parallelism exhibited by the Revel Salon. The major departure from parallelism for the Tannoy is only happening in a narrow frequency span. If this driver were operated up to 5khz, the difference in slopes would be dramatic.

Thorsten's statement here:

" the off axis response will be significantly attenuated above a few 100Hz, ultimately forming in the midrange and up to ultrasonic frequencies a CONSTANT DIRECTIVITY system."

is where the train runs completely off the tracks. If you look at the last two figures that describe the behavior of the later version of the 075 bullet tweeter here:

http://www.jblpro.com/pub/obsolete/2402.pdf

you'll see a nearly 15 db drop off from 2.5khz to 5khz in response at 60 degrees off axis but at 30 degrees off axis, the 2.5khz response value is virtually indistinguishable from the 5 khz value. This is a text book example of a driver that is NOT exhibiting constant directivity.

Constant directivity means the speaker's response level will be consistent at a given off axis angle as the frequency spectrum is traversed. Directiviy or the directivity index itself is defined as the portion of a total acoustic radiation that is being radiated in a particular direction. If a speaker possesses constant directivity, the portion of total acoustic power radiated in a particular direction needs to remain essentially constant with frequency. It doesn't mean that off axis response stays constant regardless of off axis angle. To have constant directivity, you have to see response lines from 10 degrees to 60 degrees that are essentially parallel to one another. All the response lines can can tilt downward together somewhat - a sign that output overall may be dropping with frequency - but if they aren't parallel, it signifies that the response is not exhibiting constant directivity. Clearly, from the polar plot of the 2402 (later 075 tweeter), the response lines cannot be remotely parallel and thus it cannot exhibit constant directivity. The slope of the response line from 2.5khz to 5khz at 20 degrees will be essentially flat while the line from 2.5khz to 5khz at 60 degrees will have a severe slope starting at the same point at 2.5khz but ending up 15 db down at 5khz. Hence Thorsten's characterization of the system as constant directivity is way off the mark.