I was getting confused by speculative information about the 991 sound symposer and came across this article in SAE vehicle engineering online which I found to be very informative. Thought I'd share it.

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The complex exhaust system of Porsche's new 911 Carrera S has a wide-ranging acoustic capability.

The automotive engineer's vocabulary is typically broad and precise, which makes it intriguing to hear the word "emotion" used increasingly when new technologies are introduced. In the case of Porsche's new 911, emotion was cited by engineers as a key factor in developing the car's acoustic signature.

"Sound was a major design and engineering focus," explained Wolfgang Hatz, Porsche's Main Board Member for Research and Development, who also heads VW Group's powertrain development.

At a recent technology workshop for the 2012 Carrera and Carrera S, Hatz and other Porsche engineers noted that creating a unique sound quality "between the engine and the tailpipe" has been a leading criterion for the 911 ever since the iconic sports car's 1963 debut. For the new Type 991 series (see http://www.sae.org/mags/aei/10167), the development team focused on “mechanical engine sounds characterized by high frequencies with tonal elements” together with low-frequency intake noises.

So significant is the 2012 car’s symphony as deemed by Porsche that the interplay between induction and exhaust was part of the car's design and engineering specification. Computer modeling mapping, as well as analysis of the configuration and dimensions of manifolds, pipework, catalytic converters, and mufflers, was carried out at the company’s Weissach Development Center.

The required sonic profile had to satisfy all possible operating states—from engine starting and idling (conveying power without disturbing the neighbors) to wide-open throttle on a racetrack. The goal was not only to deliver the aural “emotions” expected of a 911 but also to provide the driver with feedback about the car’s mechanical status while meeting Europe's 74 dB(A) drive-by noise requirements.

Electronically synthesized noise is not a Porsche solution, so the engineers developed a new Sound Symposer that is standard on both versions of the car. An acoustic channel picks up intake vibrations between the throttle valve and air filter and a membrane incorporated in the channel reinforces the vibrations and transmits them as an engine sound into the cabin. The system is driver activated or deactivated via a “Sport” button that controls a valve ahead of the membrane.

The result is a direct acoustic link between the cabin occupants and the engine, providing optimal transmission of the load-dependent sounds. The sound of gearshifts can also be enhanced.

The Sound Symposer incorporates a tunable Helmholtz resonator to achieve an harmonious sound pattern, damping out unpleasant noise occurring at around 5000 rpm.

If the Sound Symposer is deactivated, an aperture in the air filter housing still allows the basic engine and exhaust tones to be heard—"because we don’t want it to sound boring!” explained Dr. Bernhard Pfäfflin, General Manager of the Noise and Vibration Department.