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Thread: Iconic Mfg. Co. to Exhibit @ Great Plains AudioFest in Tulsa 4/29-5/1!

  1. #31
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    Quote Originally Posted by Mr. Widget
    For the most part those guys are dead or retired. JBL, TAD, and Meyer are all still in business and discuss in their technical papers how the larger diaphragmed drivers produce less distortion. I'll go with the living.

    BTW: You can add Altec to the dead list... please take all of this as quite tongue in cheek. I doubt that we are talking about the same issue here as I doubt that there would be all that much disagreement between my group and yours. In simplest terms the large diaphragms yield lower distortion at higher SPLs since they physically move less to produce the same output. I have not yet read your link, but will.
    Might want to run this by Steve Schell.

    I once asked Thomas Dunker what to use...a two or four inch driver...his response was the larger driver is better for mid band and the smaller driver better for upper mid and HF. Perhaps then 3 inch is a good compromise.

    Thomas Dunker is still alive and very highly regarded in the field of horns, particuarly his work with VOT and exotic Japanese horns and drivers.

    Ian

  2. #32
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by Ian Mackenzie
    I once asked Thomas Dunker what to use...a two or four inch driver...his response was the larger driver is better for mid band and the smaller driver better for upper mid and HF. Perhaps then 3 inch is a good compromise.
    That was the commonly held belief and the reason I posted this excerpt from the JBL Tech sheet. In comparing say a LE85/2420 with a 375/2440 it does hold true, but with the newer designs it no longer seems to.

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  3. #33
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by spkrman57
    Is the JBL 1.5" driver throats and Altec 1.4" throats the same? Or is the .1" difference a matter of concern when mating up to the horns?

    I have a pair of 311-60's(wish I had a pair of 311-90's instead but..) and using them with 288C and wondering since 1.4" Altec is the only one I have seen, are they compatible with other folks 1.5" throat horns???

    I guess Altec wanted to be unique as everyone else uses 1.5".

    Ron
    You got me... I have used drivers and horns that were spec'd at 1.4" and those that were spec'd at 1.5" and have to admit I've never actually measured either and I haven't put the drivers of one brand on the other brand's horns. I have been more involved with 2" drivers and some of them actually are 2", but most are actually just under... the 2440 for example is actually 1 15/16".

    As far as Altec using 1.4" to be different... I think they picked 1.4" before the others picked 1.5".

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  4. #34
    Senior Member spkrman57's Avatar
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    Thanks for the info...

    I guess I have not seen any 1.5" horns yet. My 1.4" horns are the 311-60's and don't have anything else to compare to.

    Ron

  5. #35
    Webmaster Don McRitchie's Avatar
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    As a bit of trivia, the 1.4" throat diameter of Altec's large format compression drivers is a legacy of the Lansing Manufacturing product line that Altec bought in 1941. The Altec 288 was basically an improved, permanent magnet version of the LMCo 284 developed for the Shearer Horn in 1935. It used the same diaphragm and throat geometry. The exit throat diameter on the 284 was originally 1.5" in diameter. However, this dimension was at the end of a screw on horn connector that extended out past the body of the 284. Later, Lansing switched to a flush flange connection for their horns that was fastened by three bolts that passed through the flange. This resulted in cutting back the horn throat for a length equal to the height of the former threaded mount. The diameter of the throat at the point where it is flush with the compression driver body was 1.4" and thus this became the standard. You can see this in the picture below taken by Steve Schell of one of his 284's next to a 288D.
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    Don McRitchie

  6. #36
    Senior Member spkrman57's Avatar
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    Thanks Don for that info!

    That answers a lot of questions I had on the throat size.

    Ron

  7. #37
    Todd W. White
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    Yes, but WHY did Dr. Blackburn design it that way?

    There were REASONS Blackburn designed them that way.... hint-hint

  8. #38
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by Todd W. White
    There were REASONS Blackburn designed them that way.... hint-hint
    Maybe because back then it was very difficult to manufacture a driver with the required tolerances necessary to produce a viable 4" diaphragm, 2" throat driver?

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  9. #39
    Webmaster Don McRitchie's Avatar
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    There already was a viable 4" diaphragm 2" exit driver from that era, the WE 594. It was developed in 1933. The 375 was pretty much a copy of the 594, replacing the field coil with a permanent magnet. Hilliard stated in an article from the 70's that they investigated a 4" driver during the development of the Shearer Horn, but found that it did not have good "power response". I have no idea what he was referring to. Hilliard developed his own 4" diaphragm, 2" exit driver in the late 60's as part of the ill fated A10. The only reason it was not put into production was that he was overuled by management. Altec did not want a driver that obsoleted their current horn product line, not to mention validating the technology of their main competitor - JBL.
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  10. #40
    Administrator Mr. Widget's Avatar
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    I was speculating that perhaps Altec thought the 3" driver was the better compromise since it didn't require quite as large of a magnet to achieve the same gauss level in the gap and also as the diameter of the VC and diaphragm increases it gets significantly more difficult to maintain tolerances. (difficult but not impossible)

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  11. #41
    Senior Member Steve Schell's Avatar
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    Great discussion, guys. Todd's comments about the larger magnet on the 604 accentuating the midband response and making the bass seem shy make sense to me. When the bl is increased, it pushes the mass break point higher, raising the frequency at which the efficiency begins to roll off due to mass effects. Most high bl drivers exhibit a rising response anyway, unless they are loaded by a slow flare horn that affords adequate loading and directivity control of the lower frequencies. This is why small high bl drivers like Lowthers are often fitted to large rear loaded horns. My 20' path, 15Hz. subwoofer is fitted with an Altec 515-16G driver, and exhibits a very strong midrange response; it is almost as loud at 500Hz. as it is in the passband of 20 to 80Hz. unless it is steeply low passed. This was surprising, as the design has three almost 180 degree folds.

    The three stud mounting for the large format driver began with the Lansing 287. This occurred before the sale of LMCo. to All Technical, as I have seen 287s with the LMCo. metal tag. The three stud mount began with the small format 801 driver in 1937, and was likely adapted to the large format driver in 1940 or so. This method of driver attachment greatly simplified manufacturing of both driver and horn throat, and simplified alignment of driver to horn.

    The 2 1/2", 16TPI mounting standard of the early Lansing and RCA drivers was likely derived from Wente's original 4" diaphragm driver as built experimentally for the Fletcher System. Pictures of the Fletcher h.f. driver appear to show a 3" threaded mount with a 2" exit hole. In general, the earliest Lansing drivers as built for the Shearer Horn System bear a very strong visual resemblance to the Fletcher driver, though scaled down in size, and utilizing an assembled pipe pot structure rather then the cast pot structure of the Wente driver.

    Why did the Shearer team reduce the format size of Wente's driver? This is a question for the ages. The whereabouts of Dr. Blackburn's writings is unknown, and they are likely lost. The majority of Jim Lansing's papers were lost in a garage fire. What we do have is a reference to the issue in John Hilliard's article "Sound Reproduction", published in Audio magazine in March 1977. In the article he states that the 4" Wente driver with annular compliance "...proved to have poor power performance."

    We also have Dr. Hilliard's Technical Bulletin titled "A Study of Theatre Loud Speakers and the Resultant Development of the Shearer Two Way Horn System" which was published by the Academy Research Council on March 3, 1936. Here's a quote:

    "One of the features of the reference system is the use of a single diaphragm to reduce phase distortion. Inasmuch as theatres require parallel operation as protection in the case of failure of one unit, experiments were made with a Y throat and two units. As a result of these experiments, it is now recognized by all concerned that any increase in phase distortion which may be introduced by the Y throat is negligible."

    This bulletin was reprinted in modified form in the Academy's 1938 book Motion Picture Sound Engineering, in the Chapter titled Loudspeakers and Microphones. Material was added to the text, including this:

    "To obtain high efficiency energy transfer between the diaphragm and air column in an exponential horn loud-speaker, the acoustic impedance of the air must be matched with the mechanical impedance of the diaphragm. Such an impedance match is usually secured by the use of an acoustic transformer which provides a properly constricted cross-sectional area of the sound channel between the diaphragm and the throat of the horn."

    So what do we make of all this? The Shearer team did experiment with different high frequency driver format sizes. I bought an early Lansing driver a while back that was probably a part of this process. Model 199E, serial #10, it uses a 2" diaphragm and 1" exit. My theory of the moment is that the Shearer team found that the twelve square inches of Wente's diaphragm was correct for impedance matching. They likely also determined that they could achieve higher output with lower distortion by realizing that area with two diaphragms of six square inches each. This was also a more practical arrangement for their commercial system, in that the show could continue in the event of failure of one of the high frequency diaphragms.

    Todd, you have Dr. Hilliard's papers. Are there any mentions of this development process in his research notes from the period that could shed more light on the decisions made by the Shearer team?

  12. #42
    Senior Member spkrman57's Avatar
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    Steve, Thanks for the input!

    I hope everyone keeps coming up with info and we may all get a good history lesson at the same time.

    Just for the hell of it, I have to say that regardless of the diaphram and exit(throat) size, design engineering has more to do with the overall end product success than just the size. IMHO

    Ron

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