'Looks like you're correct on the model number. But JBL's T/S parameters show RE at 3.1 ohms, so this is technically still quite close to the W15GTI's 2.95. All the other T/S specs are very close, but not exactly the same.Originally Posted by matsj
John
All readings were obtained using a professional ($2,500) Quest Technologies 2900 Type II SLM that is re-certified on a yearly basis and has never been over 0.14 dBL off during it's yearly certification. The meter was set in linear mode (no weighting) with slow response. The woofer cones were monitored at a distance of 0.81" from the dust cap toward the center. The three ports were measured at the center of the port, even with the baffleboard. The contribution from the cone and each of the three ports was logrithmetically summed and considers the difference in the size of the port appropriately (port dBL + (10*LOG((2^2*3.14159)/(7.5^2*3.14159)). As it works out, due to the size of the port relative to the woofer, the contribution from each port is 11.5 dBL lower than the woofer at the same volume. (I.e., if the woofer and a port each read 100 dBL, the actual contribution from the port is 88.5 dBA with a net volume of 100.3 dBL.)I for one, would like to know more about how the data were collected...
Even with these mic placements, cabinet resonences were notable. For example, both speakers would track various frequencies with dips and peaks observable at the port readings, but not so much at the cone. However, these tended to occur at frequencies where the port contribution is minimal and doesn't added measureably to the overall volume (<0.1 dBL).
Initially, the woofer was measured. The volume was adjusted until a sine wave of 180 Hz attained a volume of 100.0 dBL as measured at a distance of 0.81" from the woofer cap. The volume controls were left untouched and the frequency was swept to the various values both for the cone and ports. The sine wave generator is an older Bell and Howell unit from DeVries Institute. At the end of the test, the 180 Hz tone was verified still at 100.0 dBL.
As I said, if anyone is interested in the raw data, they're welcome to it.
The topic is technical because it examines the technical aspects of the two speakers being compared. This was not a DIY and the thread should have remained where I initially put it!
Er, actually, no.The topic is technical because it examines the technical aspects of the two speakers being compared. This was not a DIY and the thread should have remained where I initially put it!
Technical Help is specifically intended for JBL engineered goodies, in JBL engineered cabinets.
Interesting as they are, your cabinets are not JBL built, nor were these measurements collected by JBL using JBL authorized testing gear in a JBL sanctioned environement. Therefore, DIY is proper for this most interesting work.
For the beneifit of ALL our readers, it is hoped we can afffectively parse what is proper JBL, from what we try doing ourselves.
bo
"Indeed, not!!"
If you weren't a Moderator, I would ask you to point me to the reference for this this quote.
It ain't about being any Moderator - this is what Don discussed with all when the DIY area was proposed. It's to the core of the reason DIY was proposed. It doesn't mean any project is of lesser importance or anything like that.
It's just what I said:
For the beneifit of ALL our readers, it is hoped we can afffectively parse what is proper JBL, from what we try doing ourselves.
bo
"Indeed, not!!"
- So, are you going to create a new forum for Altec & Lansing Manufacturing ?
- Or are discussions ( involving those 2 names ) meant to occur in this "DIY" forum ?
Todd,
- In case you haven't read the "genesis thread" on the leadup to the eventual creation of this "DIY thread" read "this" for some backgrounding . Slog through the entire thread .
Heh. Zilch worked pretty hard to get Don's "unequivocal" spec nailed down there....
Todd is, in fact, using the W15GTI as a subwoofer in his system, along with multiple 2235H's. I don't think he's actively considering replacing those.
In any case, he demonstrates here that Keele's nearfield low-frequency measurement technique (1974) can effectively discern performance differences between these two 15" drivers in that region using simple measurement tools.
Todd - 'any chance on doing some outdoor measurements on a warm SoCal afternoon sometime? A situation where you were relatively free of reflective surfaces, where you could lay them flat on their back on the ground, projecting into this hemispherical environment. Then, somehow suspend the mic overhead, about 10 feet, like JBL used to do in their early measurements. This way, any of the port measuring anomalies, etc., wouldn't be so critical. You'd just be getting a final objective result of the sum of the parts.
John
Sorry but the time difference makes it difficult to follow the discussion.
It just seemed what you were doing was a bit jumbled up and misplaced.
The thing about the DIY and Tech Forums does take a bit of getting your head around.....
1a. Well that's a misleading statement and matter of opinion and presumptious at best.
1b. Refer to 1a.
2. I would like to see the W15GTI in the JBL specificed enclosure and measured again. The same applies to the other driver.
3. Perhaps if both drivers were tuned in individual ideal enclosures then measured it would prove to be a more satifactory comparison...woofers are not plug and play.
Nice bit of equipment though, would love to see some measurements of other know drivers and system under controlled conditions.
Ian
I am not going to create any forum(s). I will merely attempt to parse stuff as seemingly appropriately, as I can. There will be errors, but we all work together to right things.
To the latter, anything DIY will go into DIY.
bo
"Indeed, not!!"
bo
"Indeed, not!!"
Statistically, 23% of Americans are functionally illiterate. I don't think a new forum is the answer... but we can throw one in there to see what it does
http://indian-river.fl.us/living/ser...als/scope.html
Statistically, 23% of Americans are functionally illiterate. I don't think a new forum is the answer... but we can throw one in there to see what it does
http://indian-river.fl.us/living/ser...als/scope.html
I'm not sure these folks from Indian River are a good example to use, unless you were being sarcastic.
:dont-know
Apparently they don't know the literacy convention associated with acronyms, in which Office of Technology Assessment would become OTA, not OAT. Seems the US government at least gets it:The statistics make it clear: illiteracy is on the increase in the United States. The Office of Technology Assessment (OAT), an analytical arm of the U.S. Congress, stated in a 1993 report:
"Standards and requirements for literacy have increased over time and a large number of adults need to improve their literacy skills. OAT finds that at least 35 million adults have difficulty with common literacy tasks
http://www.access.gpo.gov/ota/
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