Exhaust drone - MLV at wheel arch

[Vigilant 0]

After fixing the trunk , had some leftover MLV and I suddenly had an idea of putting it in the rear wheel arch. If I can block the noise at the source, then it would be more efficient right?

Removed the wheel arch liner and placed the MLV around 1/3 all the way from the bottom(see pics). What surprises me is that there is more drone than before.
Help me understand this , how does placing a sound barrier on the wheel arch increase the drone, I can't wrap my head around that. It seems totally unrelated.  

 

[///M5 2,833]

Two of multiple possible answers.

The first being that you are incorrectly perceiving more, the second being that you changed the response of the system in an area where the drone isn't coming from having an amplifying effect on the areas that are.  As a general FYI, wheel arches aren't likely suspects for exhaust drone as they have to be adequately insulated as a barrier for road noise which occurs at similar frequencies.

[Vigilant 0]

I wonder if the close proximity of the exhaust to one of the rear wheels have something to do with this.

 

[Billy Jack 145]

You might have changed the resonant frequency of the car so the drone is more noticeable now. I know exhaust manufacturers are now measuring the cars acoustics and tuning the exhaust to avoid drone. But those are on the higher end of the market not flowmaster cherry bombs. 

[Rudeboy 152]

Stock exhaust?

[Vigilant 0]

37 minutes ago, Rudeboy said:

Stock exhaust?

Yes Stock exhaust. That picture is for illustration only.

Btw, the drone I'm talking about is nowhere as serious as those aftermarket setup but at certain engine rpm and at idle you can "perceive" more boominess in the cabin. You know that surround , bassy , echoish kind of sound.

Edited November 29, 2016 by Vigilant

[Rudeboy 152]

22 minutes ago, Vigilant said:

Yes Stock exhaust. That picture is for illustration only.

Btw, the drone I'm talking about is nowhere as serious as those aftermarket setup but at certain engine rpm and at idle you can "perceive" more boominess in the cabin. You know that surround , bassy , echoish kind of sound.

Do the rear seat backs fold down? If so, is it worse with them down?

[Vigilant 0]

I'l try to out it back again and simulate it again, this time I'll take some noise and FFT measurement.

[///M5 2,833]

Use at least 100 averages on the fft block or watch it until it stops changing.  Drone will have a lot of spurious signals you want to average out to lock in the real source.

[///M5 2,833]

11 hours ago, Billy Jack said:

You might have changed the resonant frequency of the car

The car has 1000's of resonant frequencies, not just one; every little part in the car actually has its own.  He locally changed the ones on the wheel well for sure, but it is unlikely that made a big difference on the overall cars modes.  What he could have done though is changed the transfer path for the energy which can have a pronounced effect on acoustic response.  No one in the automotive sector ever wants to run a transfer path analysis as it is brutal so these are rarely optimized all that effectively.  Definitely used for oh shit engineering, but normally it is one of the first areas of NVH that are sidestepped as much as possible.

[Second Skin 120]

The partial coverage you applied may just be causing the sound waves to move around the barrier layer, this movement around the barrier layer may be changing the perceived sound of the noise. Any 1" gap in the barrier layer will allow the same amount of noise through as not having any product applied at all. Partial coverage is sometimes not advisable. The best approach would be to get product everywhere between you and the noise, this may include applying product to the back seat wall since you already treated the trunk. Good luck with your project.