Drivers of sports and sporty cars want their engines not just to sing but to roar, say Ford. To make sure the new Focus ST does both, Ford engineers applied a new twist to an existing technology.
They added a special sound tube – called a sound symposer – to amplify the throaty frequencies enthusiasts crave in performance cars.
Specifically, they employed a composite "paddle" that vibrates with intake air pulses, naturally amplifying the lower range of engine frequencies, in the 200-to-450-Hz frequency range, that they found to be most pleasing to performance enthusiasts
While the sound tube concept has been used on Mustang in the past, the sound symposer used in Focus ST is unique because of its electronically controlled valve, which opens and closes based on driver inputs – engine speed, accelerator pedal position and gear selection.
In lower gears, the valve is mapped more aggressively, while in higher gears the effect is dialed back to enable quieter cruising. That variation isn’t possible with conventional, passive sound tubes.
Part of the reason Ford made these changes is that on Focus ST, for the first time, the symposer is attached directly to the intake manifold (as opposed to between the manifold and air intake).
"For ST drivers, it’s not enough to have a car that is fast or feels fast. It also has to sound fast," says Christopher Myers, Air Induction System engineer.
"Part of this is the design of the exhaust, but we went further and engineered the symposer both to dial up the nice sounds the EcoBoost delivers under the hood but dial back the interior sound volumes at part throttle," he explains.
"The turbo gives us great power across the rev range, but it presents a special challenge from a sound perspective as it absorbs much of the beautiful engine music," Myers adds. "The symposer helps us bring the throaty sounds that drivers love."
The secret to getting this sound right was developing the paddle to naturally amplify the ST’s natural engine sound. Ford engineers tested several different paddles. Eventually, the supplier developed a paddle with the correct stiffness that yielded the best acoustic response and ultimately, the best "flutter" and low-end frequency sound.
An international team from suppliers of the intake manifold, battery tray, electrical hardware and software, and electrical connectors came together with Ford to accelerate development of the symposer. All in all, 30 engineers from five countries had to balance NVH, materials, manufacturing and assembly considerations to bring the symposer to life.
"The sound symposer gives the Focus ST an aural split personality," says Lisa Schoder, Ford Focus ST Marketing manager. "In everyday driving, the car is composed and refined. But under full throttle, we unleash the sonic hounds. It’s a beauty and a beast."
Sounds a lot more complex than just adding an exhaust cut-out valve as we used to do way back when!