TMD - Tuned Mass Damper
The obsession with the midrange
The midrange register is no doubt the most complicated to control in a sound system. On one hand, there must be a smooth transition with the bass. On the other hand, it must be tuned to the tweeter in terms of dispersion and acceleration. This determines the homogeneity of the timbre and spatialisation.
For the past 20 years, we have been working on mastering the "break up" (the frequency at which the cone becomes deformed, leading to distortion) of our 3rd generation "W" cones and on drastically reducing the resonance of the tweeter with the IAL 2. Today, thanks to the power of finite element analysis, our 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.
Tuned Mass Damper
Following the discovering of these issues, we had to conceive the equipment 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 oscillate in opposition to the resonance frequency to control it.
Applied to the speaker driver, the solution consists in simply two tubular rings on the suspension whose dimensions and position have been judiciously determined. They form our Tuned Mass Damper (TMD) and they stabilise the dynamic behaviour of the surround according to resonance, thus avoiding deformation of the cone without afflicting the dynamics. This innovation is patented.
Visual analogy of the suRROUNDS
Our Tuned Mass Damper (TMD) consists of two tubular rings moulded onto the surround. On the left the default suspension, on the right, the TMD suspension. This simple solution, perfected thanks to our new software, stabilises the dynamic behaviour of the surround according to resonance, thus avoiding deformation of the cone without afflicting the dynamics.
Principle of the Tuned Mass Damper.
The principle of the tuned harmonic damper (TMD): The graph on the left demonstrates the principle of the tuned mass damper. In red, an m1/k1 system with very pronounced resonance. By adding a mass/spring system m2/k2 (upper section of the diagram), there will be two resonance peaks, as represented by the blue curve. The drop or anti-resonance being previously based on the resonance of the main system, we get the green curve. Finally, by judiciously adding a damper, we obtain the purple curve. The resonance has almost disappeared!
With Sopra hi-fi speakers, we opted for exponentially shaped cones in order to increase frequency response, and consequently impulse response. Another reason for increasing definition.
Measurement and analysis
Improved response linearity between 1 and 2kHz and the frequency extension provided by the exponential cone profile. Frequency response of our latest midrange driver (blue trace) compared to the previous-generation W-cone midrange, representing prior state-of-the art performance (red trace). Improvements to the magnetic circuit also contribute (see next section). NB: The dip at 3kHz in the blue trace is due to the tested driver not being fitted with a dust cap.
Effect of the harmonic damper on linearizing the frequency response between 1.5 and 2kHz (blue trace with TMD, red trace without).
Effect of the harmonic damper on nonlinear distortion, which is halved between 1.5 and 2kHz (blue trace with TMD, red trace without)