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Thread: "The Duplex Loudspeaker" presentation by James B. Lansing

  1. #1
    Senior Member glen's Avatar
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    "The Duplex Loudspeaker" presentation by James B. Lansing

    Originally presented by Jim Lansing at the 1943 Technical Conference of the Society of Motion Picture Engineers (SMPE then, now SMPTE includes Television Engineers). This is the presentation as reprinted in the September 1943 issue of the SMPE Journal.


    THE DUPLEX LOUDSPEAKER*


    JAMES B. LANSING**


    Summary.-The Duplex Loudspeaker is a combined two-way loudspeaker mounted in an integral unit so that the high-frequency energy is radiated from a small multicellular horn mounted on the face of the low-frequency diaphragm.
    Separate permanent magnets of improved magnetic material are now used for the fields of each voice coil.
    The crossover has been selected at 1200 cycles so that the high-frequency horn can be placed in the center of the low-frequency diaphragm.
    A signal input up to 25 w can safely be applied to the speaker. The intermodulation products are very low as a result of the two-way principle. The configuration of the high-frequency horn produces an angle of radiation which is 60 degrees in the horizontal plane and 40 degrees in the vertical plane. Due to the type of construction a high degree of uniformity between units can be maintained in manufacture.
    The unit is capable of efficient radiation beyond 15,000 cycles.


    The practical application of the Two-Way Multicellular Loudspeaker System for theater use began in 1935. Since that time there has been a gradual improvement in its quality and general performance. The wide acceptance of the high performance standard set by this two-way loudspeaker system indicated that the benefits to be realized by applying the same principles to loudspeakers for recording, monitoring, and broadcast radio work would be considerable.

    Since the large size of the theater system (Fig. 1) precluded its use in monitoring booths, the immediate requirement was that a substitute be found for the large folded horn used for the low-frequency band. Reduction in the size of the low-frequency horn called for a corresponding decrease in the size of the high-frequency horn in order to make the whole equipment compact.

    The first development to meet these requirements for a smaller system made use of a 500-cycle crossover network and a high-frequency horn designed to give proper acoustic loading at crossover.

    The folded-type horn, much reduced in size, using a 15-in. speaker was retained for the low-frequency end. While this design had adequate frequency range for most small rooms and is being used in large numbers by our armed services, it was still too bulky for the "cubbyhole" type monitoring room. The effect of separate sources for the different frequency bands was annoying when used in close quarters.





    FIG. 1.


    In 1937, the first two-way loudspeakers using the multicellular high-frequency horn in conjunction with a resonated low-frequency baffle were made available under the name of Iconic Loudspeakers (Fig. 2). A crossover frequency of 800 cycles was used with a corresponding decrease in the size of the high-frequency horn, compared to that used with the 500-cycle crossover systems. These loudspeakers were far more compact than those using horns of various configurations for the low-frequency band. Operating efficiency, while not as high as in the larger systems, was still high when consideration was given to the decrease in size.

    During 1941, intensive work was undertaken to find a method of producing a loudspeaker of still more compact form, retaining the same performance characteristics of the larger systems, and at the same time totally eliminating the tendency to radiate from split sources when used in close quarters.

    The intermodulation distortion effects produced by a single diaphragm, when operating at a multiplicity of frequencies simultaneously, precluded the use of a single diaphragm for all frequencies.1







    FIG. 2.


    A metal diaphragm designed to operate as a piston up to frequencies above the limits of audibility, was chosen for the high-frequency reproducing system. Aluminum alloy was used because of its high mass stiffness and high velocity of transmission. The resulting lightweight diaphragm is stiff enough to prevent its breaking up as a piston and thus introducing the intermodulation effects so common to the familiar paper and other fibrous types of diaphragms.

    Careful consideration was given to the type of high-frequency radiation system to be used. If the diaphragm was to radiate directly and was made small enough to avoid sharp beam effects at high frequencies, it became too small to handle enough power, near the crossover region, for practical purposes. Accordingly, the multicellular type high-frequency horn was chosen as the radiating medium.

    The final design for the high-frequency horn was a 2 X 3 configuration of 6 cells, with a 900-cycle cutoff, which could be enclosed by the low-frequency cone. The maximum angle of horizontal distribution was held to approximately 60 degrees in order to prevent interference from the mounting baffle at the high frequencies.

    Fig. 3 is a cross-sectional view of the completed Duplex .Loud-Speaker showing the arrangement of the functional parts in their proper relation. The high-frequency horn is shown mounted on the end of the low-frequency unit pole piece, which is bored out to permit the passage of sound from the high-frequency unit. A fine mesh bronze screen at the junction of the pole pieces prevents the entrance of foreign particles into the high-frequency sound chamber. Positive alignment of the bores of the 2 pole pieces and of the horn mounting flange avoids discontinuities which would cause destructive interference along the high-frequency sound transmission path.





    FIG. 3.


    The high-frequency horn is covered with a sound deadening material, but is not finished with a smooth surface which would set up a regular reflection pattern for sounds being generated by the surrounding low-frequency cone. The dome-shaped high-frequency diaphragm is shown in place over its transducer, which effectively prevents destructive interference from being set up in the sound chamber. The high-frequency voice coil is wound with aluminum wire to hold the mass of the moving system to a minimum. The low-frequency system consists of a 15-in. paper cone with its actuating motor system and surrounding mechanical structure.

    A frequency dividing network of the constant impedance type is used with a crossover frequency of 1200 cycles (Fig. 4). The selection of the 1200-cycle crossover point permits the 900-cycle cutoff horn to adequately load the high-frequency unit down to a frequency where it transmits little power. This eliminates any tendency to produce the distortion effects which would be caused if the acoustic loading were to cut off sharply at crossover, and effectively prevents any damage to the high-frequency unit because of unloading when the maximum rating power is applied in the crossover region.






    FIG.4


    Fig. 5 shows the Duplex Loudspeaker and its dividing network. These networks use iron cored reactors capable of being operated over a wide voltage range with negligible change in their inductance value. The networks are not affected by their proximity to other apparatus. The assembly shown has been used with various shapes and sizes of resonated baffles, but most satisfactory results have been achieved when a baffle with a volume of 6 to 9 cu ft was used. A 6-cu ft baffle when properly ported*** will permit good response down to 60 cycles. A 9-cu ft baffle will permit good response down to approximately 40 cycles. Care must be taken in the construction of the baffle to prevent "breathing" effects from the pressures built up in it at the lower frequencies. The inner wall of the baffles must be covered with sound absorbent material in order to prevent reflections which would give a "hang-over" or "echo" effect.

    Comparative tests of the Duplex Loudspeaker with the larger systems have been highly satisfactory as to reproducing characteristics and efficiency. At a distance of 2 ft from the new unit, all frequencies being reproduced appeared to come from a single source. The high-frequency radiation angle of 60 degrees by 40 degrees is small enough to avoid reflections from the baffle as the sound leaves the high-frequency horn, but is still ample to permit the listener to move about with considerable freedom.






    FIG. 5.


    The uniform characteristics which can be maintained from unit to unit should make the Duplex Loudspeaker ideal as a monitoring standard. The elimination of vertical spacing between the source of high frequencies and the source of low frequencies brings about a point source of reproduction which is found to be very realistic and
    helpful in the critical judgment of quality.

    * Presented Oct. 20, 1943, at the Technical Conference in Hollywood.
    ** Altec Lansing Corporation, Hollywood.
    ***The port is used to allow energy which is radiated from the rear of the cone to be admitted out the front side in phase with that portion of the energy coming from the front of the cone. The effect is to maintain a more constant acoustic impedance down to the cutoff of the enclosure. The area of the port is a function of the size of the box enclosure and the mechanical resonance of the loudspeaker unit.


    REFERENCE


    1 BEERS, G. L., AND BELAR, H.: "Frequency Modulation in Loudspeakers,"
    glen

    "Make it sound like dinosaurs eating cars"
    - Nick Lowe, while producing Elvis Costello

  2. #2
    Senior Member glen's Avatar
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    Quote Originally Posted by glen View Post
    Since the large size of the theater system (Fig. 1) precluded its use in monitoring booths, the immediate requirement was that a substitute be found for the large folded horn used for the low-frequency band. Reduction in the size of the low-frequency horn called for a corresponding decrease in the size of the high-frequency horn in order to make the whole equipment compact.

    The first development to meet these requirements for a smaller system made use of a 500-cycle crossover network and a high-frequency horn designed to give proper acoustic loading at crossover.

    The folded-type horn, much reduced in size, using a 15-in. speaker was retained for the low-frequency end. While this design had adequate frequency range for most small rooms and is being used in large numbers by our armed services, it was still too bulky for the "cubbyhole" type monitoring room. The effect of separate sources for the different frequency bands was annoying when used in close quarters.
    It would seem that the "smaller system" Lansing mentions is most likely the Lansing Manufacturing 500-A developed in 1937:

    http://www.lansingheritage.org/image...lletins/3a.jpg

    By the 1943 presentation this was still the only system I found in the Lansing Mfg. or Altec catalog using "a" single 15 inch woofer in a folded horn enclosure.

    I am also intrigued by the reference to the "smaller system" being "used in large numbers by our armed services" which supports the theory in a 1953 Life magazine article that the high-fidelity craze got it's start when soldiers in the armed forces were exposed to high quality sound reproduction during their World War 2 service:

    http://audioheritage.org/vbulletin/s...ad.php?t=13795

    At the time of it's introduction the 500-A would probably have been the first speaker used outside of a movie theater with reprpoduction that could be considered "high fidelity".
    glen

    "Make it sound like dinosaurs eating cars"
    - Nick Lowe, while producing Elvis Costello

  3. #3
    Dang. Amateur speakerdave's Avatar
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    Quote Originally Posted by glen View Post
    . . . . I am also intrigued by the reference to the "smaller system" being "used in large numbers by our armed services" . . . .
    I've read elsewhere that the much vaunted midrange clarity of the 755a was developed to make speech intelligible in PA systems on board ship.

    David

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    Senior Member Steve Schell's Avatar
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    Good job, Glen! I've had copies of this article and a couple of others for years now. When this site was established Don and I hesitated to publish them because of possible copyright infringement, but I doubt that this is any sort of issue anymore.

    Beneath the somewhat dry, factual language of the article, I can sense Jim Lansing's enthusiasm with the Duplex principle, and with high quality speaker systems of all sizes and capabilities. He had been immersed in the development and manufacture of arguably the world's finest sound equipment for the previous eight years, and the progressive downsizing of the systems greatly increased their possible uses. Jim lived to see the Duplex achieve great success, though of course he couldn't know that it would become the dominant monitor system for the next three decades.

    "The Duplex Loudspeaker" is, to my knowledge, the first article published by James B. Lansing. He soon followed with "New Permanent Public Address Loudspeaker", presented May 14, 1945 at the Technical Conference in Hollywood. The photocopy I have is indicated as "Vol. 46, No. 3", probably from the SMPE Journal. This article details the conversion of the Duplex to permanent magnet design, basically describing the 604 in detail.

    The only other article I have by Jim Lansing is "An Improved Loudspeaker System for Theatres" by J.B. Lansing and J.K. Hilliard, also presented May 14, 1945. This article describes the Voice of the Theatre systems, detailing the changes made from the Shearer type field coil systems to the permanent magnet drivers and combination bass enclosure of the VOTT.

    William Geiger once posted on these forums and cited an article by Jim Lansing on the design of a bass horn cabinet. I haven't chased this one down yet, but need to do this. This would make four articles by Jim; I wonder if there are any others?

    I am likewise intrigued by the mention of large numbers of Lansing Monitor systems being sold to the U.S. armed forces. He is undoubtedly describing the Lansing Monitor 500A and/or 500B. Your observation of their being the only high fidelity speakers available to the troops at the time is insightful and likely correct. These systems must have gone overseas and not come back, as Lansing Monitors are incredibly rare these days; I have only known of the existence of about five of them in the modern era.

    These articles are essential reading and probably should have been up on this site for years now. Thanks for getting us off to a good start, Glen.

  5. #5
    Senior Member Steve Schell's Avatar
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    Speakerdave, the 755A was part of Western Electric's postwar speaker line. I have no doubt that many 755As found their way into military service in the postwar era. The majority of speakers used by the U.S. military, at least on the ships and submarines during WWII, were phenolic diaphragmed cone speakers and compression drivers produced by W.E., RCA, Jensen, Stromberg Carlson and a few others. I have a variety of the RCA units and they crank! Most of them have a pronounced vocal range and a very high output capability. Some of the horn units incorporated a blast gate to protect the diaphragm from the concussions of the big guns on deck.

    You might be interested to read a bit more on the 755A here:

    http://members.myactv.net/~je245/755survey.htm

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