pontifex4

This appears to be a timely post, since I just noticed a mention in another thread from Walt (whose excellent reference on 924/944/968 stereos you almost certainly will have come across online if you're interested in the topic).

He's right, in my experience: I've only ever driven two 944s (one of which is my own, pictured below) and they were both very noisy on anything but super-smooth, brand new asphalt. There is quite a difference, though, between the sound of the engine when you're driving in a sporting manner, and the drone of the exhaust as you accelerate/decelerate in traffic. I'm also pretty sure I've never heard of anyone who is a tire roar or a road noise aficionado. In any case, I most certainly am not.

I've decided to kill a lot of these annoying sounds in my own car, while keeping the weight addition to a minimum. This is in anticipation of a fairly complex (non-bling) stereo build, but this work should pay dividends, even with the stereo turned off.

Also, if you're curious, this is my summer/track car, so I really do mean it about the weight-consciousness.

The first installment (reducing noise from the exhaust and transmission) happened this evening, since the weather wasn't good for painting.

These are the first sections of the damping material I applied, after degreasing the hatch area and spare tire well. Contrary to popular belief, you don't need to completely cover every surface -- doing so only realizes a tiny percentage improvement in damping force, at fairly considerable expense.

This first step was designed to damp structure-born vibration from the exhaust and transmission. The next one will provide a barrier against road, exhaust and transmission noise.

pontifex4

To actually block sound, you need non-resonant mass. It can be in the form of concrete, rubber, or even densely-packed teddy bears, but if it isn't both massive and non-resonant, it won't stop sound the way we want it to.

I offer this as a preamble to the second part of my first installment, primarily because there are several products in the car quietness market which rely on confusion over this point to boost sales. For example, a PVC closed-cell foam, Ensolite, is often touted as an inexpensive wondersolution to blocking road noise in cars. It isn't. It is very useful in car audio installations, yes, but its role is primarily to keep interior trim panels from rattling against door skins, etc. It is a (mildly) effective barrier to very high-frequency noises, but is powerless against road noise because it is very light.

Car audio is fairly unique, in that it necessitates that the listening environment be inside a massive (but resonant) chamber, and that's why you need to take steps to reduce the specific resonances which you can hear in the car's interior, as I did in the first post, above.

To reduce the overall level of sound transmitted to your ears, though, you still need to add something heavy between you and the sound.

Here's where I suspect you'll protest, "weight isn't cool on the track, man!" and I agree, so I'm pretty limited in what I can do about it in this project. What I can do, though, is specify mass loading which can be easily removed.

The product I used below is a sandwich of 1/8" vinyl (which is pretty heavy at about a pound per square foot) and foam, whose job it is to keep the vinyl from touching the surface from which sound emits ("decoupling" it, and thus cutting any resonance it may develop with the material below).

I pulled the trunk carpet out, spread the vinyl composite on a big cutting table and trimmed a shape out of the vinyl by tracing the trunk carpet onto it with a utility knife. The vinyl/foam mat fits under the standard hatch carpet, and weighs about what my slightly damp hatch carpet does, at just a little more than 10 lbs.

Crucially, though, like my slightly damp hatch carpet, I can take it out when I go to the track.

CurtP

Where did you get your barrier? I bought mine from Second Skin and went with their Luxury Liner Pro. It's pretty damn heavy, fairly expensive and they rape you on shipping. I need another sheet of it to do what you did in the trunk. The problem I had with Luxury Liner Pro is there's considerable waste. One whole edge has a taper and doesn't have the vinyl mat on it, so it has to be cut off. There's 2" thrown away. Their customer service and shipping speed leave a lot to be desired too. I went with 50-75% coverage with the deadener, with the exception of the tunnel which got 100% (mainly to try to keep the heat out), then topped with the barrier in key places.

dillon410021

good job, looks good and efficient

pontifex4

CurtP, the barrier I used is from B-Quiet (it's called V-Comp). I used to use their L-Comp (a version of the same thing, only with lead instead of vinyl) but they took the lead product off the market. They're a no-nonsense company, so long as you only need a couple of different mainstay items, as I did, here. In fact, the stuff used above is all leftover from other jobs, over the years.

I'm surprised to hear about your experience with Second Skin, though -- I used their spray-on product on a project, and found shipping to be relatively reasonable (considering it was liquid, and therefore heavy) but that was a couple of years ago, now.

By the way, yours looks like a really thorough job, and I wanted to ask you whether you had to pull off any old insulation on the transmission tunnel, and how you found the end result with all that work in the cabin. I've been eying mine, but don't know whether I should take the plunge.

I'm going to do the doors, next -- road noise now appears to come from them, rather than from the hatch, so I feel that I'm on the right track!

pontifex4

Thanks, Dillon, but I'm not done, yet. Indeed, as far as the original intention of the project -- cutting road noises -- I've only scratched the surface.

Tire noise is the major source of the din we hear in the cabin at highway speeds, and I haven't done anything to address it, specifically. It was certainly reduced indirectly by doing this, but to actually reduce noise from the tires, I'm going to need to mat the big, undampened areas around the rear wheels, and follow the plane perpendicular to the axis of wheel rotation (doors, body panels, etc).

Those will likely be parts 2 and 3.

Thanks for your interest, everyone!

JohnKoaWood

Different tires will also go a long way to reduce the road noise...

pontifex4

They would, indeed, as would a different width, or even running them softer (my rears are 255/40 PZero Rossos). That's something I'll certainly look at when it's time to change them, but I'm kind of pinned down there (as with the weight issue) because this car sees some track use, and I don't want to compromise fun for quiet!

V2Rocket

those are expensive tires for 188hp

CurtP

I used B-Quiet Ultimate deadener. Not the best product on the market, but I think it's a good compromise between weight and effectiveness and it's relatively cheap. I used to use Brown Bread, but it was replaced by the Ultimate. I looked at V-Comp, but I wanted a denser, heavier closed cell foam so I ended up with Second Skin instead.

Everything came out - all the old deadener and isolator was removed. I pulled the isolator off the firewall, laid down deadener, then put the factory isolator back in place. The tunnel was completely stripped. I even made the hole around the shifter tighter to try to keep the sound and heat out of the cabin. The doors and hatch area only have the stock deadener.

Overall I'm very happy with the results. It's not dead quiet, but I didn't want it to be. I wanted to keep the car as light as possible and be able to have normal conversation in the car. There is some exhaust drone (SFR cat-back and a test pipe), but it isn't bad at all. I also wanted to keep the weight as low in the car as possible.

I also replaced my headliner with foam-backed cloth. That seemed to help some too. There was a hard yellow foam dampener on the sunroof, but I took it off and recovered the sunroof panel with the same foam-backed cloth.

MM951

Thanks for the post, I'm very interested in quieting down my 951. It's not too bad, but I really like a quiet cabin, especially since my 951 is my only car and I do lots of driving. I'm not too concerned amount the weight either as my 951 has plenty of go....

How much did you spend on what you are showing in the pics? Do you have any measurement of noise level before and after (dB meter?)

Looks good!

pontifex4

Right you are! The tires and wheels come from a Boxster. They're posing wheels, anyway. I have a set of 16" phone dials which are getting more track-appropriate tires next year.

V2Rocket

bookmarked for the future when i can afford such things

for now its just lots and lots of carpet insulation lol

pontifex4

Curt, thanks for the details. I appear to have done exactly the opposite to you, so far (I've just finished putting a layer of Ultimate on the inner door skin, and have done the trunk).

You mentioned heat, which isn't normally a big problem for us in Canada, but I'm curious: how effective was what you did at blocking heat from the tunnel?

Also, I wouldn't mind knowing what you used on your roof. Do you have a picture?

alordofchaos

Curt, Charlie, good stuff! Thank you both, for the posts and pics. It reminds me what a slacker I am

subscribed - should have extra time and money next year