NVH attenuation

3135 Views 12 Replies 6 Participants Last post by rdslon01, Apr 29, 2014

abshapiro

Discussion Starter · Apr 26, 2014

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).

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abshapiro

Discussion Starter · #3 · Apr 26, 2014

Chip said:
Art; This is interesting, but for me it is tough to truly understand / interpret exactly what a reduction in a couple of dBA would sound like?(in the case of the 40 mph example) I guess my question to you is how obvious is the noise reduction (@ 1.4 x) and is it really worth the expense of the material?

A factor of 2 (i.e., a 10 dBA) difference would be worth it. A reduction of 1.4 is a shoulder shrug. I don't think a passenger would say, "WOW, this car is really quiet since you added the acoustic foam". I probably would not notice the difference if I didn't have a sound meter to measure it. I may put some foam behind the seats and see if that helps.

Aside, I don't play the radio because I prefer listening to the engine. I was attempting to reduce the Freeway noise from the tires.

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abshapiro

Discussion Starter · #7 · Apr 27, 2014

rdslon01 said:
A factor of two is not 10 dB. A factor of two is 6.0 dB. You saw a decrease of 5 dB, which is a reduction of 1.8 times. I don't know where you got the 1.4 from.

If you are curious, to convert from dB to a factor reduction, divide your dB change by 20, and raise 10 to the power of that result. For example, 10^(5/20)=1.778 which is about 1.8. To convert that to a percent reduction, divide 100 by your reduction, and then subtract that result from 100: a reduction of a factor of 1.778 is a reduction of 44%.

So, a 5 dB reduction eliminated 44% of the noise.

You are correct. Doubling of the sound pressure corresponds to a measured level change of 6 dB. The equation is

(ratio for sound pressure) = 10**(dL/20) =10**(6/20)=2 where dL is the dB difference.

A dilemma is that there is no equation relating loudness (or volume) to decibels. Subjective (i.e., perceived by the ear) loudness data has been correlated by a log(2) equation. Using this equation, a doubling of sound volume corresponds to a measured level change of 10 dB.

(ratio of sound volume)= 2**(dL/10)=2**(10/10)=2**(1)=2

For my case
(ratio of sound volume)=2**((92-87)/10)=2**(5/10)=2**(.5)=1.4

This loudness equation is not precise. Therefor, I provided data for our Lexus as a baseline. The acoustic mat in the C7 does help, but I can’t say it was a “Wow! What a difference.” It does seem to my ear that the tire and transmission noise is reduced at highway speeds.
Lexus highway: 85 dBa
C7 without mat: 92 dBa
C7 with mat: 87 dBa

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abshapiro

Discussion Starter · #10 · Apr 28, 2014

Rodney - Ahhh! So you know the language of the ancient ones! I write code for the software package known as LS-DYNA. It is mostly FORTRAN with some C. The code is used to simulate car crash dynamics. LS-DYNA is used by car companies throughout the world, which means I get to see some really neat stuff. We model the car, seat belts, airbags, and place dummies (male, female, child) in the seats. The code has undergone extensive QA and is accepted as a replacement for physical testing. The car companies still experimentally crash cars but at a reduced amount, which means more C7's for the people instead of the junk pile. The code also is used for modeling manufacturing processes. I have spent the last few years improving the capability to model hot stamping of B-pillars. The steel is raised to 840C (1544F), stamped in a fraction of a second, and quenched. Sort of like making a Samurai sword known as a Katana. The high heat and quench transforms the steel from austenite to martensite. This provides the strength needed for rollover protection using a smaller cross section (i.e., saving weight). This is a very expensive manufacturing process and only a few critical car crash components are made using this method.

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abshapiro

Discussion Starter · #12 · Apr 29, 2014

Rodney -

I sent you a Private Message so we don't get off target here.

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