[flash]

yup - it's a natural "bowl". outdoor amphitheaters, like the hollywood bowl, are built just like that for a reason. i've often thought that the stereo would have sounded a lot better if the rear speakers were located farther back. they were obviously concerned about what it would sound like if the rear area were full of stuff.



there do seem to be a lot of members who know a bit about acoustics. it's a lot of what i have been doing full time since 89, touring for years with various bands as an engineer, and designing recording studios when i was off the road. i still tweak the odd room here and there, and every once in a while get to mix a show, but i am pretty much now sitting back and directing others. i've had enough cold pizza and warm beer for a lifetime.



as for percentages, actually it's almost all external, but not from the exhaust. this is easily shown by the difference in what you hear with the stock exhaust vs an aftermarket one. the sound pressure level change is minimal, but it goes from not being able to hear it at all, to easily hearing it in the cabin. the noises are mostly wind generated, either by passing over the body surfaces, or from the tires spinning in fairly enclosed resonant spaces.



the problem is there is no escape for the sound. it just bounces around in the cabin. there is nothing much to absorb it either. the roof and the floor are essentially parallel to each other, as are the doors and windows which doesn't help either.



the most important surfaces to cover are the big ones that are parallel to others. this will prevent the back and forth resonance. then, cover the large membranes that like to vibrate. unfortunately we can't easily get to the roof or the floor, but these would be huge in dampening sound. the wheel wells are important too, especially in a hardtop model.

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[Darryl]

Owning both a Cab and a Coupe and having an audio background, I can tell you that the Coupe is much louder than the cab and it is indeed from external sources. I've been driving the Coupe for a while now with no radio at all after removing all of the previous "top of the line" CASSETTE based head unit system. As Flash points out, I concur that most of the drone is from the rear tires and road noise reverberating in the "bowl".



I have finally collected all of the necessary parts and pieces to do the rear speaker enclosure mod. Since I'll be replacing the old speakers, one of which is blown anyway, with new 6.5" coaxial drivers and doing the mod at the same time, I won't really have a before and after but I know it'll be tons better. I plan to do the doors as well while adding the new 5.25 2 way component systems in each door. I opted for the B Quiet extreme since after doing the research, seemed like a very decent alternative to Dyna-Mat, and a forum member had a huge roll for sale...so I got about twice as much deadening material for about 1/2 the price I would have had to spend on the DynaM-Mat. Gotta love the forum!



- Darryl

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[bombfactory]

Definitely agree - road noise from the rear tires is often deafening, depending on the road surface. I've got some dynamat back there, and will eventually be covering just about every surface I can get to.



Recently my city has begun repaving streets using asphalt that contains ground-up recycled car tires. It's almost completely silent when you drive on it.

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[Rap]

Rear road noise have anything to do with ones selection of tires?

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[flash]

yes. it also has to do with the age of those tires.

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[bombfactory]

I think mine are from 2009. Probably time to start thinking about new ones.

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[flash]

yeah - they start getting hard after about 3 years. that makes them noisier. they will still perform for a couple more years after that, though not as well, but the noise really increases dramatically.



tread pattern will affect noise too. big blocks are much noisier than twisty grooves

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[tamathumper]

Yup, my 4-year old Conti's were much noisier than my new Sumi's. Tread pattern similar.

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[flash]

contis are noisy to begin with. i can only imagine what they sound like when they are older. i can't wait to get the contis off the SL550. i made them put new tires on the car when we picked it up 2 years ago. i didn't have a choice about the brand though. now i regret the decision and should have bought them myself. i just can't bring myself to yank good tires though. no worries. they are about half gone, and i will change them before winter, because in addition to being noisy, they have zero wet traction.

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[bombfactory]

I just put Sumi HTRZ 3's on the BMW. So far they're quiet and feel similar to the Michelin PS2's they replaced. Will have to try them on the 968, seeing as this car needs the quietest tires it can get. Still gonna dynamat everything in sight tho - or out of sight I should say...

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[flash]

one of these days i'm going to rip up the carpet in the blue car and do the floor. alternatively i may yank the panels underneath the car (the cab has an extra floorboard) and do that sheet and weld it back in

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[tamathumper]

I was wondering if there was a double-panel under the car... Certainly looked that way to me.

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[flash]

yup - part of the chassis stiffening they did. the good news is that the bottom of the cab is more aerodynamic than the hardtop. the bad news is that the same pieces make it heavier.

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[dwsjung]

I am starting my sound deadening project.  Does anybody know how to remove the rear quarter interior trim panels ?  I removed 3 screws on each side and I am stuck   Appreciate any help.  Thanks ! 

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