Sandyman

Motivation. My wife and I enjoy taking several 1200+ mile trips in our (base) C7 each year. I thoroughly enjoy it as a road car; love the handling, engine sounds (NPP exhaust), ergonomics, good audio system, luggage space etc. What I detest is tire noise on rough surfaced roads especially when it will persist for hours! We had endured tire noise in our previous C6 for 9 years. I was surprised to find the C7 tire noise not improved.
When I first saw threads of others removing the rear panels in virtually new Corvettes and installing sound deadening material, I though “what loonies”. But I eventually decided we would enjoy our C7 that much more if I could dampen tire noise emanating from the rear of the car. I joined the “loonies”.

An alternative would be to get quieter “touring” tires but I do not want to sacrifice handling.

Credits. I would never have attempted this without the help of a post on how to remove the rear panels and a suggestion on tools, by Wormwood (https://www.corvetteforum.com/forums...r-removal.html).

In selecting materials and an approach, I relied on the web site: sounddeadenershowdown. Their recommendations made sense to me and their material specs were useful. I found their delivery time, packaging, and product quality top-notch. The products I purchased there and used were,

- CLD Tiles™ - Used these self-adhesive panels to cover about 20% of the panel area to dampen vibrations.

- Mass Loaded Vinyl (MLV) – Used to block airborne sound. Is heavy (1 lb/ft sq) and rather stiff. As recommended I tried to make a continuous barrier using this stuff in the areas I treated, the three rear compartment areas listed below. I did not want the MLV strongly adhered to any surface (minimize its vibration) so for vertical surfaces it is only attached with a few heavy-duty Velcro® Strips, usually about 1”by 2”.

- Closed Cell Foam (CCF) – This was adhered to the back of the MLV using contact cement, to isolate the MLV from the underlying body panels.

- Heavy-duty Velcro® strips – Used so MLV can be easily removed and reinstalled.

The deadening material was applied to the rear side panels (under the interior wheel well covers), the “waterfall” behind the seats, and the horizontal luggage floor area.

Tools. I bought a set of plastic pry tools as recommended by others and found them indispensable. Even then, being inexperienced with panel removal I broke the plastic supporting one fastener on one of the rear side panels. I used epoxy to repair and reinforce that support and reinforced the matching fastener in the other side as a precaution for the future.

To measure any improvement, I used the iOS app, Audio Spectrum Analyzer by Black Cat Systems, running in my iPhone 6 while it was on a thick towel on the center console. This allowed capturing images while driving during four passes on a nearby, rough surfaced road. Due to traffic and the posted speed limit, I could only drive about 55 mph for the samples. I collected two “before” spectrum samples and two
“after”, taken about 300 yards apart on the same road.

I used Picture Format tools (Color, Artistic Effects) in Word 2010 on the captured images to allow overlaying the “after” on the “before” (while aligning the grid pattern) to easily see any improvement.

Results. Here, two “after sound deadening” samples are shown overlaid on their respective “before” samples. They are similar, just taken on different sections of the road.










In Figures 1 and 2, the green area represents the decrease in sound/noise power due to added sound deadening.

Before starting this project and while on a noisy road (i.e., with a coarse surface causing significant tire noise), the preponderance of noise clearly seemed to emanate from behind your head. Makes sense as the rear tires are much closer to your ears than the front tires, the rear tires being covered by composite wheel wells that protrude into the trunk area closely behind your ears, etc.

The noise power spectrum samples used here were collected by an iOS app (Audio Spectrum Analyzer by Black Cat Systems) running in an iPhone 6. This is not a professional quality sound level measurement system. My objective was only to assess the difference between various noise conditions, so no accurate absolute sounds levels are needed.

While a 3dB increase in sound power implies twice the power, that does not mean humans would interpret that as “twice as loud”. My understanding is a 10dB increase in power or intensity is required for a subjective assessment of “twice as loud”, and that an increase of only 3 to 6 dB of noise power should be discernible to people even if not viewed as “twice” as loud. So the project results should be noticeable.

Seat time in the car agrees there is noticeable improvement. The car is not now a “cone of silence” by any means on noisy roads but there is no longer the sensation that noise comes predominantly from the back; noise seems to not arise from anywhere in particular or perhaps to arise from the sides and front.

At some frequencies it appears the noise power is reduced by as much as 6 dB. (!) Even if the added rear sound deadening were “perfect”, I’d not expect more than a 3dB improvement provided the noise power at the microphone were equally split from the front and rear of the car. But with noise predominantly from the rear, greater than 3dB improvement is at least possible.

Sound Deadening Installation.

In Figure 3 below, the silver CLD Tiles™ are shown, covering maybe 20% of treated areas.




Figure 3. CDC Tiles™ (silver, ~20% of area covered)

Figures 4 and 5 show the black MLV over the waterfall. The MLV with the CCF adhered under it is only Ľ” thick. Still, the MLV is pretty stiff. I am amazed the original carpeting over the waterfall was able to be reinstalled on the first attempt; is a very tight fit!
I was impressed how thick and heavy the foam adhered to the underside of the original carpeting covering the waterfall is. Seemed a serious attempt to block heat and noise




Figure 4. Mass Loaded Vinyl Behind Seats





Figure 5. Mass Loaded Vinyl Extended up over Waterfall

Figures 6 and 7 show the MLV over the wheel wells. This is held in place by Velcro® strips so is removable. Trying to curve and fit the MLV over these compound surfaces was very time consuming.

While there is a thick fibrous material outside the car up in the fender well that probably provides useful sound deadening, there is virtually none inside the car between the (exterior) hard wheel well composite and the (interior) plastic wheel well panel (there is a few square inches of tissue paper-like material). I consider this area a prime candidate for deadening. Unfortunately it is the most difficult to treat given its shape and all the cables present.

For the flat area under the luggage compartment, my plan had been to install the MLV underneath the original carpeting. Given the complex shape of the composite flooring plus the fact that I would need cutouts for access to the fuses, the amplifier electronics, and the storage cubby, I decided the much better route is to simply cut a piece of MLV to lay on top of the factory carpeting (no picture on this). I purchased a thick Lloyd’s mat for the rear, used it as a template to cut the MLV, and simply laid the MLV between the Lloyds mat and the factory carpeting. I returned later and adhered the CCF under the MLV. This had not been done when the “after” spectrum measurements were made; therefore results could be slightly better than shown earlier



Figure 6. Mass Loaded Vinyl Over Left Wheel Well




Figure 7. Mass Loaded Vinyl Over Right Wheel Well


General Notes on the Install.
- This was not a trivial project; most steps were new to me. I just turned on some good music in the garage and took my time, trying for a good result. Was not unpleasant or fatiguing work just time consuming.

Fortunately I had access to another car for the ~6 days I had the C7 apart. I may have spent 35+ hours on this! Fortunately I am retired; that helped.
- Would I do it over? …..(pause)……Probably. Otherwise I’d frequently be bothered by high tire noise.

Hope this is useful to someone…..

Sandyman