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Editor's Desk

About the D'Appolito MTM Loudspeaker Configuration

In this edition of The Audio Voice we have recovered one of the most interesting speaker projects ever published in audioXpress, the THOR Transmission Line, designed by Joe D'Appolito. For many years, Joe contributed interesting articles and projects for the titles that were the origins of audioXpress, and he has remained one of the magazine's contributing authors to this day. His THOR design uses an mid-tweeter-mid (MTM) driver configuration, also known as a "D'Appolito configuration."
The original Koss Dyna Mite M-80 speakers were manufactured from 1981 to 1984, when Koss Corp. filled for bankruptcy. The company recovered in 1985 but focused mainly on headphones for which was famous.
Following an email exchange some time ago regarding the MTM design, our friend and loudspeaker designer Paul Wilke commented that even though Joe D'Appolito is often mentioned as the originator of the MTM configuration, and the fact that in US Patent 8,170,23 83, assigned to Harman, D'Appolito is quoted in that regard, Joe never applied for a patent himself. He simply published an Audio Engineering Society (AES) paper in October 1983, "A Geometric Approach to Eliminating Lobing Error in Multiway Loudspeakers" (74th AES Convention, New York, October 1983. Preprint #2000). But, the Koss M-80 loudspeaker, produced between 1981-1984, used an MTM-configuration, even before that paper was published.
"Joe D'Appolito was certainly the first to describe in engineering terms what the advantages of an MTM-setup are. In his AES paper, he explains mathematically how an MTM-configuration can prevent lobing errors in the crossover region, even if there is a phase shift between the two sides of the crossover. Normally, in case of such phase shifts, a dominant lobe will be pointed upwards or downwards, but not into the direction of the listener. The MTM-configuration assures symmetry around the listening axis," added Wilke.
In that same email exchange regarding the MTM concept and the reference to the Koss M-80 speakers, we asked Joe D'Appolito himself (that's one of the privileges of being in charge of audioXpress) about the misunderstandings regarding who exactly "invented" the MTM speaker configuration and what the configuration encompasses. Since we have not yet had the chance to write a dedicated article about the topic, I thought it would be interesting to share a part of what Joe himself had to say.
"Regarding the MTM configuration, you are correct about the Koss M-80. I did not invent the MTM, but as soon as I saw the Koss M-80 I realized that it provided a universal solution to a problem with speaker radiation patterns brought to light by Siegfried Linkwitz. It is possible that Koss realized this, but they provided no explanation for their choice of geometry in their literature. My AES paper (Paper Number 2000 - published October 1983) got wide attention and led to the MTM popularity."

PreSonus E44/E66 nested MTM configuration monitors use dual Kevlar low/mid drivers (4.5" and 6.5", respectively) operating in parallel and covering the same frequency range so that they acoustically couple.

The following quotes are extracts from an article by Joe D'Appolito, "Understanding the MTM Geometry," and "The MTM Explained," a letter he wrote to The Absolute Sound (TAS) magazine, which he shared with us.
"To understand the benefit of the MTM array you must first appreciate the problem it solves. Conventional loudspeakers using two non-coincident drivers and odd-order crossovers produce asymmetric polar response patterns. In addition to asymmetry, the axis of maximum sound pressure level moves with frequency. This movement is greatest in the crossover region. This effect is called lobing error or polar axis wander. The sonic effect is to cause image instability and spatial spreading of the location of an instrument's fundamental and harmonic frequencies."
"When the woofer is placed below the tweeter, the principal axis of radiation is pulled downward through the crossover region. If the woofer is placed above the tweeter the principal axis will be pulled upward. A little thought should convince you that by placing woofers both above and below the tweeter, the upward pull of one will be cancelled by the downward pull of the other, producing a symmetric polar response with the axis of symmetry in the horizontal plane."
"Although the MTM geometry stabilizes the polar response pattern, the detail shape of that pattern does depend on the type of crossover used."
In the letter to TAS, Joe later clarified that one should not interpret this basic explanation with a specific crossover design. He writes: "The primary benefit of the MTM array is the elimination of lobing error. This benefit is solely a function of the driver geometric configuration and in no way depends upon the type of crossover used. (...)I think this misconception comes from a misreading of my AES paper on the MTM array."
In his excellent paper, Linkwitz[1] showed that using even-order in-phase crossovers would eliminate lobing error, but I felt this unnecessarily limited the choice of crossovers. The first part of my paper developed a mathematical model for the radiation pattern of an MTM array. The model contained an arbitrary phase angle between the woofer and tweeter acoustic outputs. With this model I was able to show that the resulting polar response pattern is symmetric and stable at all frequencies independent of the inter-driver phase angle and thus independent of the type of crossover used."
Although the MTM geometry stabilizes and symmetrizes the polar response pattern, the detail shape of that pattern does depend on the type of crossover used. Polar response patterns for all of the then popular crossovers were computed and included in my paper. The crossovers examined included the odd-order Butterworth's, the even-order Linkwitz-Riley networks and Small's symmetric constant voltage crossover[2].
The results show that the even-order crossovers produce strong off-axis nulls, while the odd-order networks have no off-axis nulls and provide the broadest and smoothest vertical coverage. I suggested that if such coverage was desired, the odd-order crossovers were the ones to use. I think this suggestion was subsequently responsible for the widespread belief that the "D'Appolito Configuration" comprised two components, the MTM geometry and odd-order networks.
Wilson Audio's Alexx speakers feature a modular MTM driver arrangement, adjustable on the time domain
For home audio application, the last thing you want is broad uniform vertical polar response. This pattern will produce excessive floor and ceiling reflections. The in-phase even-order networks produce large off-axis nulls that largely eliminate these reflections. For this reason the MTM geometry with even-order in-phase crossovers has become very popular in home theater applications.
I think the first sentence in the summary of my paper says it all. "This paper presents a simple arrangement of three non-coincident drivers in a two-way loudspeaker which eliminates polar response lobing error independent of inter-driver phase differences."

Yours truly,
Joseph D'Appolito, Ph.D.
1. Linkwitz, S. H., "Active Crossover Networks for Non-Coincident Drivers," Journal of the Audio Engineering Society (JAES), Vol. 24, pp. 2-8, Jan/Feb 1976
2. Small, R. H., "Constant-Voltage Crossover Network Design," Journal of the Audio Engineering Society (JAES), Vol.19 pp. 12-19, Jan 1971.


From the Vault
THOR: A D'Appolito Transmission Line
By Joe D'Appolito
One of the most important speaker building projects audioXpress has published, THOR was designed by Joseph (Joe) D'Appolito, who is world-renowned for his pioneering work on MTM-based (mid-tweeter-mid) loudspeaker configurations. The original THOR transmission-line design was published in May 2002 and Joe himself wrote about the project: "Transmission Line loudspeakers," which has long enjoyed a small, but dedicated following. The advantages of transmission lines (TLs) are well known. They are essentially non-resonant enclosures, producing a deep, well-controlled bass response. For a given driver, bass response will extend well below that produced with either a vented or sealed enclosure using the same driver. Above a few hundred hertz, the line filling material completely absorbs the driver backwave, giving the TL an open, non-boxy sound." For this online publication, audioXpress kept the original article unchanged, which has since been updated with availability of the original Madisound/SEAS kit and the publication of a complete kit project on SEAS own website. In a recent exchange with Joe D'Appolito, we asked him if he had considered revisiting the original project. Joe replied: "SEAS sponsored development of my THOR project, which used all SEAS drivers. I know it has become a popular speaker, but at present I have neither the time or resources to update the design. I believe the real contribution of the original THOR article was detailing the technique for experimental optimization of the Transmission Line."  Read the Full Article Available Here

Voice Coil Test Bench
Celestion CF18VJD Pro Sound Woofer 
By Vance Dickason
In this article, I review Celestion's 18" CF18VJD pro sound woofer. The CF18VJD is a high-power handling woofer rated at 1,600 WRMS (AES standard) and weighs a hefty 50.6 lb. The CF18VJD's features include a proprietary seven-spoke cast aluminum frame. However, Celestion's approach for this model does not include venting below the spider mounting shelf at the top of the front plate, a common feature built into high-power handling woofer frames. Instead, Celestion uses a vented front plate that exhausts toward a series of peripheral vents located on the top of the back plate. The cone assembly consists of an 18" carbon and Kevlar loaded paper cone and a large 6.5" diameter carbon and Kevlar-loaded paper dust cap. Suspension is provided by a three-roll M-shaped coated (sealed) cloth surround in conjunction with two 7.5" diameter flat treated-cloth spiders (dampers), which I would assume are mounted back to back to cancel out odd-order nonlinearity. All this is driven by a 5" (125-mm) diameter high-temperature non-conducting voice coil former wound with a two-layer inside/outside round copper wire winding. I immediately noticed the CF18VJD's massive ferrite finite element analysis (FEA) optimized motor assembly. Besides the eight 9-mm diameter peripheral vents in the back plate for cooling and transmitting air from the vented front plate, the T-yoke also incorporates a 52-mm pole vent. Also included in the motor structure are a set of dual shorting rings (Faraday shields) to reduce flux modulation and distortion. This article was originally published in Voice Coil, November 2013Read the Full Article Online

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