I wrote this up on another forum under a thread about drone and the sound symposer a few days ago, but decided the work would potentially be more broadly interesting to people, so here it is:
So I spent all afternoon figuring out mostly how the sound symposer works… this may be controversial, but I’m going to just report it like it is. FYI, I have boxster gts with oem exhaust with a Soul VC installed as open/OEM mode. I’m leaning toward buying a Soul valved rear (only).
Anyway, I started by removing the rear carpeting. Then I got a partner and we drove around for a long time running a series of experiments in various modes.
1.) I used a spectrum analyzer on the iphone to see if any of the settings made a measurable difference at idle. It’s pretty easy to stick the mic in the hole. Outcome: symposer does nothing at idle. You can sorta hear the valve activate (maybe) when you hit the sport exhaust button, but nothing at idle appears to alter the sound coming through that pipe in any way.
2.) The symposer has absolutely zero function when the sport exhaust is turned off. Zero. Tried everything. Symposer is silent. Sorry if anyone disagrees, but stick your ear up to it at any speed, any mode, any throttle position, any rpm. Nothing. I’m pretty sure the valve is just closed when sport exhaust is off.
3.) The symposer does function when sport exhaust is on. However, its output appears to be directly proportional to throttle input, at least after a certain point. It’s relatively silent at low throttle input, but if you floor it then the output appears to be generally proportional to the rpms. It is loudest above 5k or so under hard throttle. You can get it to be active in the 2k-3k “drone range” under full throttle, but its definitely not loud at all — and is most certainly unlikely to be causing any drone, unless you count drone as occurring above 5k.
4.) This may be a surprise to some, but it sounds good! Basically, it sounds like real induction noise (because it kinda is?). It’s easy to make it silent with your hand, so you can just simply sit there and control/modulate it with your hand. It’s amusing to control it with your hand actually. And the vast majority of the sound is above 4k rpms.
5.) There is no reason to go to great lengths to completely disconnect it. A plug of foam works as well as your hand, which just simply completely silences it. It’s not more complicated than that… I know someone used a tuning fork/audiology analogy early on in another thread, but that does not hold here; this is a flexible, low mass, little plastic pipe hooked to relatively well damped steel (there’s a raised ridge in the metal where the symposer comes in that will stiffen and kill most resonance in this area, and certainly any from a connected plastic flexible pipe).
Bottom line: it sounds good, and if you don’t like it, just push the sport exhaust button to completely disable it. No reason to pull the car apart. Lastly, and most controversially, this is definitely not the source of the high levels of resonance being reported (I can hear it too with valves forced open). For those who are hearing a difference after disconnecting, well, I think it’s not from this. But if you want to confirm, again, just use the sport exhaust button.
Also of interest, the rear carpet makes a huge difference in interior sound level. This car sounds like the engine is inside the cabin without that carpet and foam padding. You can hear the belts spinning and just more engine noise in general when it’s removed. The engine panel itself is not particularly well damped, so I added some CLD (dynamat) to the cover and to some of the other area that was not well damped before I reinstalled the carpet. The oem carpet has a plug of foam missing right where the symposer exits so that you can hear it better.
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Boxster GTS 4.0 Firewall
On the pic I have circled the symposer exit in red, area that is relatively undamped in yellow, and an unknown device in blue (anyone know what this is?). By the way, it’s easy to remove this carpet panel — only the speaker grille is in the way and it removes very easily with near zero risk of damage (pull from medial side)
My conclusion is that the Porsche engineers knew what they were doing and did not introduce drone via their symposer. They delivered a good solution to a problem and added in some real induction noise under high loads at high rpms (and they probably enjoy reading these threads).
So I spent all afternoon figuring out mostly how the sound symposer works… this may be controversial, but I’m going to just report it like it is. FYI, I have boxster gts with oem exhaust with a Soul VC installed as open/OEM mode. I’m leaning toward buying a Soul valved rear (only).
Anyway, I started by removing the rear carpeting. Then I got a partner and we drove around for a long time running a series of experiments in various modes.
1.) I used a spectrum analyzer on the iphone to see if any of the settings made a measurable difference at idle. It’s pretty easy to stick the mic in the hole. Outcome: symposer does nothing at idle. You can sorta hear the valve activate (maybe) when you hit the sport exhaust button, but nothing at idle appears to alter the sound coming through that pipe in any way.
2.) The symposer has absolutely zero function when the sport exhaust is turned off. Zero. Tried everything. Symposer is silent. Sorry if anyone disagrees, but stick your ear up to it at any speed, any mode, any throttle position, any rpm. Nothing. I’m pretty sure the valve is just closed when sport exhaust is off.
3.) The symposer does function when sport exhaust is on. However, its output appears to be directly proportional to throttle input, at least after a certain point. It’s relatively silent at low throttle input, but if you floor it then the output appears to be generally proportional to the rpms. It is loudest above 5k or so under hard throttle. You can get it to be active in the 2k-3k “drone range” under full throttle, but its definitely not loud at all — and is most certainly unlikely to be causing any drone, unless you count drone as occurring above 5k.
4.) This may be a surprise to some, but it sounds good! Basically, it sounds like real induction noise (because it kinda is?). It’s easy to make it silent with your hand, so you can just simply sit there and control/modulate it with your hand. It’s amusing to control it with your hand actually. And the vast majority of the sound is above 4k rpms.
5.) There is no reason to go to great lengths to completely disconnect it. A plug of foam works as well as your hand, which just simply completely silences it. It’s not more complicated than that… I know someone used a tuning fork/audiology analogy early on in another thread, but that does not hold here; this is a flexible, low mass, little plastic pipe hooked to relatively well damped steel (there’s a raised ridge in the metal where the symposer comes in that will stiffen and kill most resonance in this area, and certainly any from a connected plastic flexible pipe).
Bottom line: it sounds good, and if you don’t like it, just push the sport exhaust button to completely disable it. No reason to pull the car apart. Lastly, and most controversially, this is definitely not the source of the high levels of resonance being reported (I can hear it too with valves forced open). For those who are hearing a difference after disconnecting, well, I think it’s not from this. But if you want to confirm, again, just use the sport exhaust button.
Also of interest, the rear carpet makes a huge difference in interior sound level. This car sounds like the engine is inside the cabin without that carpet and foam padding. You can hear the belts spinning and just more engine noise in general when it’s removed. The engine panel itself is not particularly well damped, so I added some CLD (dynamat) to the cover and to some of the other area that was not well damped before I reinstalled the carpet. The oem carpet has a plug of foam missing right where the symposer exits so that you can hear it better.
Boxster GTS 4.0 Firewall
On the pic I have circled the symposer exit in red, area that is relatively undamped in yellow, and an unknown device in blue (anyone know what this is?). By the way, it’s easy to remove this carpet panel — only the speaker grille is in the way and it removes very easily with near zero risk of damage (pull from medial side)
My conclusion is that the Porsche engineers knew what they were doing and did not introduce drone via their symposer. They delivered a good solution to a problem and added in some real induction noise under high loads at high rpms (and they probably enjoy reading these threads).
