Technology explained
HELMHOLTZ RESONATOR
This little device helps manage the flat six soundtrack in the back of your water-cooled 911
Sound plays such a pivotal role in any 911. But the engineers at Weissach don’t just let the legendary fine-tolerance engineering simply riff away.
Instead, just like a music track, the sound itself is engineered. It’s massaged to maximise appeal, note and depth, while anything that detracts from the aural experience is minimised.
One of the tools they use to do this is the Helmholtz resonator, as first employed by Porsche on the intake tract of the 996’s M96 engine. The idea stems from the 1850s and the findings of German physicist Hermann Ludwig Ferdinand von Helmholtz. In his studies around sound analysis, he developed a way to analyse musical notes. Specifically, their pitch.
Sounds are pressure waves that cause our ears to vibrate. How high or low the note is – the pitch – is the frequency of the vibration and can be displayed as a sine wave. Helmholtz created a range of brass spheres with elongated necks, each of which would resonate at a set pitch as air was blown across the top, much like we’ve all done with a bottle. Each sphere would also resonate sympathetically when placed near a sound matching its specific pitch. However, it was realised that if you matched the pitch, but created a sine wave of opposing amplitude, you’re able to effectively cancel out that pitch. Effectively, a resonator could be utilised as a noise-cancelling device.
This is the concept Porsche applies when using a Helmholtz resonator, to attenuate or accentuate intake noise, therefore shaping the sound. Any unwanted frequencies can be cancelled out by passing air over the correctly tuned resonator inside the intake, as a way to reduce unwanted frequencies, as Porsche engineers see fit.
In the case of the 996, the Helmholtz resonator is in the air box ahead of the throttle butterfly. On the 997’s 3.8-litre motor, it’s again in the air box, but a vacuum-operated butterfly valve is able to vary the volume of the Helmholtz resonator. It should be noted, though, that some owners block off the Helmholtz resonators as a modification, in an effort to increase intake sound.