Friday, 4 November 2016

Horn Loud Speaker (European Triode Festival 2016 preload)

Ah, the horn loud speaker - so much has been discussed over the years and strong opinions of them are common. Opinions run strong because the performance is pronounced in either a pleasant or unpleasant way. When implemented well they are quite astonishing and like any powerful tool they are not easy to implement and can be hard on the ears. It is but one of many tools a loudspeaker designer can use. Certainly it has its place in sound reproduction since the very beginning, that is not an opinion - that is a fact.

Loudspeaker elements include the cabinet, driver, crossover and sometimes a horn. Theses combine to create one of the most complex interactive/reactive environments of the sound reproduction chain. Science is but a girl's phone number, you still have not called her, gotten a date and certainly numbers have not made you the beneficiary of her first kiss as Sakuma san so aptly pointed out. As virtuous as the Nerd in us can be he can only provide the phone number and then his job is done. Wazniac did not get the kiss -  Jobs got the kiss.  The ocean's ways are mysterious and its limits unfathomable. The intrepid must navigate not only with science, but also intuition and courage. The vibrational and AC analysis of a loudspeaker in action while reproducing music would look like more like a storm at sea than any other analogy that can be thought of.

Having designed a series of horn and more conventional speakers over the years and coinciding with the European Triode Festival, ETF 2016, we decided to create a horn loudspeaker for production that was not only well behaved in high performance but could also fit into a normal domestic space.

A fine early example of this was the Western Electric 753c which is known for its reasonable size, unsurpassed midrange and magical blending of horn and conventional woofer.

753s are rare and unapproachable to obtain, however, the elements and lessons it provides are not. 

Japanese Audiophiles had discovered a variety of good sounding horns, including the one in the 753c, all of which which combined well with the early Jensen/Magnivox 15 inch drivers. Thus it seemed that this horn and a similar driver to the Jensen/Magnivox would be appropriate to explore.

Horn Tastings

Since there is no current production of the Jensen or Magnavox woofer a ProSound manufacturer was commissioned to design and produce a proprietary driver using the Jensen elements combined with the modern horsepower of an overbuilt motor and chassis. The result is a 15" driver which can process 450 watts of continuous musical power while providing 97db/1 watt/1 meter of sensitivity. Obviously it is not the the 123db sound pressure level which can be generated that is of interest but the detail, nuance and power which can be expressed before the first watt of power is utilised - though it is fun to turn the high end room into a dance hall on occasion. The motor was researched in detail and it became clear that resources were best spent on performance factors other than magnet type. As much as I have been a proponent of the differences in magnet kind it became abundantly clear that the size and type of voice coil has an equally great or greater influence on sound - much as a circuit type has a higher tier in hierarchy over an output tube - the motor circuit has more elements than the magnet. Large, high quality voice coils have considerable significance by providing more area in interfacing with the magnetic field and the quality of the electromagnetic field it generates is of equal importance to that of the magnet. This new woofer carries the largest and highest quality voice coil that could be made on current tooling. There is no alnico or field coil tooling that could be found to provide the magnet required for it and this then presented a dilemma - the choice of two ways. A hard decision was made to buck the vintage trend and make a commitment to performance over marketing. 

The M15 Shaded Dog driver is the result.

The horn exploration of some years ended after hearing one of the oldest horns in continuous production. It originated with Western Electric and became part of the anti trust consent decree that broke Western Electric into smaller companies and was bestowed upon one of the resulting subsidiaries named Altec. It is an early exponential design with wide dispersion. It does not have the abrupt bends and transitions of other modern, commercial horns and as a result is presentation is more natural and "unhornlike".

The compression driver selected for initial prototyping is a vintage alnico unit known to work well with this horn. Beryllium diaphragm will be tried as will a field coil. A WE555 can be fitted and is proven to work well in this application.

These elements required a vessel to house and support them through the interacting vibrations inherent in their work. Load paths, wave propagation and vibrational dispersion were included in the cabinet design development. Over a year of CAD design work was undertaken with the help of Camila Barcha, who is an architect, to prove and execute the concepts in detail. We had a lot of fun discussing concepts and creating the braces and arches. The concept was architecturally inspired after all while walking under the golden means arch of Khan's Kimbelll Museum and noting the difference in ambient sounds where they became more quiet and even. It took some thought to understand how and why.

Light and rigid construction was indicated for low loss reproduction. This vintage approach can be seen in any many early designs. 

Golden means architecture for harmonically proportioned dispersion and distribution were implemented. 

Semi-Monocoque construction was utilised for lightness and rigidity for which it is also indicated for for boats, aircraft and musical instruments.

After trying to get several cabinet makers over the years to follow the design parameters it became clear the a prototype/model was required to demonstrate the concept and construction techniques of the design execution. I had to build them myself.

Guitarist, teacher and Luthier Eddie Freeman taught me some things about wood working, playing and building musical instruments. I began study at the age of six.

I do not believe a loudspeaker should be built like a musical instrument - they have different jobs to do although there are some similarities. It was not intentional that some details were influenced by this early education. I did not realise until looking back at the construction and remembering... He would always ask: "Catch on?" after showing me something.
My gratitude for his teaching and influence. 

Real world driver measurements were taken in house for cabinet tuning and design validation. It was rewarding to see CAD and real world practice come together.

The design has so far exceeded expectations and continues to improve during advanced voicing and tuning. 

We hope to show the production test mules semi-privately for the first time during the European Triode Festival 2016.

These test mules have interchangeable panels for quick changing ports, drivers and horns. Production units will have a single piece front panel without bolts and will be finished in Walnut, Maple or custom veneer. 

Here is a video of them playing music:

On the future agenda is a cabinet with Ultra-Flex/Onken inspired tuning, the possibility of tooling up for a field coil and transformers designed for individual tubes at specific operating points. 

- Photos curtesy of Camila Barcha, Copyright© all rights reserved.


Load Path - A load path can be defined as the assemblage of structural elements that "transfer" a load from its point of application to the point(s) of reaction.

Loads will follow the shortest load path because it is usually also the stiffest. The stiffest load path will also carry the biggest share of the load. Any change in direction of a load path will induce high local stresses due to bending, that is why the elements constituting a load path should be as direct and robust as possible.

Monocoque (/ˈmɒnəˌkɒk-ˌkk/) is a structural approach whereby loads are supported through an object's external skin, similar to an egg shell. The technique may also be called structural skin. The word monocoque is a French term for "single shell" or (of boats) "single hull".[1]

Semi-monocoque refers to a stressed shell structure that is similar to a true monocoque, but which derives at least some of its strength from conventional reinforcement. Semi-monocoque construction is used for, among other things, aircraft fuselages, car bodies, motorcycle frames, and musical instruments. 

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