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Focal Sopra Part 2- The Obsession with Midrange

Posted on 15 February 2016

Welcome to our second installment on the innovative technologies employed in the new Focal Sopra Loudspeaker, this one focusing on the design of the midrange section. Midrange frequencies are undoubtedly one of the most complex areas of a speakers design and the most demanding musically, it is also a key element of the sound signature of all Focal Loudspeakers. The performance of the midrange driver is critical. On one hand, there must be a smooth transition to the bass driver, while on the other, it must be tuned to the tweeter in terms of dispersion and acceleration.  The Sopra design team have approached this dilemma with a series of exciting advancements in midrange driver design which I will expand on below.


W COMPOSITE SANDWICH CONE

The "W" composite sandwich cone permits total optimisation of the frequency response curve, thanks to the total control of three key parameters: lightness, rigidity and damping. At Focal, the letter "W" means Glass/ Glass, as it usually has two sheets of woven glass tissue that are "sandwiched" onto the structural foam core. The cone structure is more homogeneous and its behaviour in flex is much more superior than Kevlar® or Aramid fibre .
         
These unique characteristics of lightness and rigidity provide control of the signal transmission speed inside the material. Adjusting the thickness of the structural foam allows a very precise control of the "W" cone's damping. The variation of the number of glass fibre sheets and the thickness of the central foam can easily optimize the cone's behaviour according to the desired frequency range. When listening, the sound of the "W" cone is entirely transparent, has an excellent phase response and has a very low distortion rate (rigidity 20 times higher than Kevlar® or Aramid fibre).

 

TMD - TUNED MASS DAMPER

For the past 20 years, Focal have been working on mastering the "break up" (the frequency at which the cone becomes deformed, leading to distortion) of their 3rd generation "W" cones. Today, thanks to the power of finite element analysis, Focal's design teams have developed simulation software to visualise the dynamic behaviour of the suspension which connects the cone to the basket, thus revealing the performance issues requiring attention.

 

The problem is now well known: there  is a sudden change in mechanical impedance where the wave passes from the cone to the surround, which itself radiates sound and has a "boomerang" effect, causing deformation of the cone. Many attempts to resolve this have been considered by Focal's competitors but none is satisfactory, since all are detrimental to cone travel and cause dynamic compression.

Following the discovering of these issues, Focal had to conceive the approach to solve them. The solutions already known for increasing the damping properties of the suspension all result in an increase of the mass which consequently alters definition. The answer came from a technology used in earthquake-resistant skyscrapers and which is also used for the suspension on racing cars!  This technology is called a "Tuned Mass Damper": an additional mass oscillates in opposition to the resonance frequency to control it.

It was used with great success in the suspension of the Renault R25 F1 car in 2005 and was quickly banned because it was considered anti-competitive by the FIA. It is also the basic principle of anti-seismic systems for modern skyscrapers. This device offers major advantages. The speaker engineers were able to:

  • Use an exponential cone profile that extends the bandwidth to more than 5kHz and thus achieve better transient response;
  • Choose a very light surround, eliminating the resonance that is even stronger when the mass is low;
  • Damp resonance in the direction of the sound radiation (the radial plane) whereas competitors' devices damp circumferential resonances of the suspension.)

Thus Focal are able to combine benefits which were previously irreconcilable: low mass, optimum damping and extension of the frequency response. This leads to several benefits at the listening level:

  • Improved transient response coupled with a flat frequency response and reduced distortion, of the order of more than 50 per cent in an area where the ear is highly sensitive around 2kHz.
  • This results in more accurate timbre, improved definition and better stereo imaging.

To clarify the last point, the resonance of a conventional surround blurs the soundstage, especially when the resonance is marked. TMD suspension eliminates the problem at its source.

Focal's Tuned Mass Damper (TMD) consists of two tubular rings moulded onto the surround. This simple solution, perfected thanks to new software, stabilises the dynamic behaviour of the surround according to resonance, thus avoiding deformation of the cone without afflicting the dynamics.

Two moulded circular beads in the surround form the tuned harmonic damper. While this appears to be a simple solution, over a hundred different configurations were tested to optimise the result.

A committed research project  thus enabled Focal to develop a remarkable solution by addressing the problem at its source, without limitation of cone movement, using a "TMD" suspension for which a patent was filed in the third quarter of 2014.

VISUAL ANALOGY OF THE SURROUNDS- Left shows no TMD or default configuration, Right shows with TMD suspension

NIC - NEUTRAL INDUCTANCE CIRCUIT


STABILISING THE MAGNETIC FIELD

The precision and detail of audio reproduction depends on the stability of the drivers magnetic field. This stability is compromised by three factors which tend to modulate the magnetic field:

  1. the movement of the voice coil (Lenz's law),
  2. the current passing through it (Foucault current),
  3. the frequency.
Consequently, the voice coil, and all the moving parts, including the cone, are located within the magnetic field which becomes too variable, and leads to loss of precision.

 

THE SOLUTION- NEUTRAL INDUCTANCE CIRCUIT TECHNOLOGY

After three years of research and the development of simulation software which makes it possible to visualise these complex interactions, Focal engineers have created an incredibly stable magnetic circuit for the Sopra line. The solution, (NIC technology), lies in a Faraday ring whose dimensions, materials and positioning were optimised to make the magnetic field no longer affected by the position of the voice coil, by the amperage or the frequency of the current passing through it.

 

To implement this solution Focal developed a new midrange driver magnetic circuit for the Sopra line. A central Neodymium magnet is topped with a ferromagnetic pole piece brought to saturation by a second Neodymium pellet above it. The field is looped by a ferromagnetic circuit dimensioned to avoid saturation. Finally a Faraday ring is carefully positioned further to reduce distortion below 1kHz.


The end Result ...

Focal have now applied two major developments to the Sopra's midrange driver:   the harmonic damper suspension "TMD" and the  new magnetic circuit "NIC".

It is interesting to see the combined effect of these two innovations on overall performance. The results can definately be heard in listening tests:
  • extended frequency response for better transient performance and thus better definition
  • high linearity in the critical region 1-3kHz for improved timbral accuracy
  • drastic reduction of resonance in the surround and non linearities in the magnetic circuit which are responsible for blurring the stereo image.

In the Part 3 we will explore the Bass section of the Sopra.
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