NVH attenuation

NVH is the acronym for Noise-Vibration-Harshness. I installed a 1 inch closed cell acoustic foam mat ($54) on top of the carpet in the hatch area. I placed my Stingray cargo mat on top of the foam. The foam did reduce the decibel level. The drone at highway speed was reduced. I did not notice a reduction/change in the exhaust note. Here are the results in dBA measured with a sound meter.

Without acoustic mat
C7, Z51, 7 speed, performance exhaust, touring mode
Idle: 76 dBA
City street 40 mph: 89 dBA in 4th gear
Highway 65 mph: 92 dBA in 5th gear

With acoustic mat
C7, Z51, 7 speed, performance exhaust, touring mode
Idle: 76 dBA
City street 40 mph: 86 dBA in 4th gear
Highway 65 mph: 87 dBA in 5th gear

Lexus IS250, automatic, on Conti extreme contact DWS tires
Idle: 75 dBA
City street 40 mph: 82 dBa
Highway 65 mph: 85 dBA

The C7 highway sound levels of 92 dBA without the foam mat and 87 dBA with the mat means that the perceived loudness level is 1.4 times louder without the mat. Note that dBA is a log scale and you can't divide the numbers to obtain the 1.4 loudness ratio. There are online calculators to manipulate dBA sound level change to loudness ratio.

I did not attempt to cut the foam to fit all the curved spaces in the cargo area. I ordered a rectangular piece with dimensions 36” (tol +0, -1/4) by 42” (tol +0, -1/4).