DugT

Has anyone tried to isolate noise measurement of their C7? I'm planning on doing that but if it has already done and the data is available, I'd love to see it and avoid the work. Eventually I will add some noise absorbing material at least to the back of the car but first I want to determine the loudest noise sources so that I know where best to allocate my effort and material. From this I might decide that a Blockit kit will be enough or not.

Here is my plan. First I will measure the noise while cruising steady at 55mph with no noise supression. (I have a good new meter that measures dbc and dba. I'm going to put pillows, sofa cushions, blankets etc behind the seats and over the entire cargo area including over the wheel well covers. This will give me an idea of maximum possible noise suppression. Then I will remove the noise supression just from the wheel wells and measure the noise. Then I will put the noise supression back on the wheel wells and remove it from behind the seat and measure. Then isolate just the flat cargo area.

I welcome any suggestions.

Bruceski

Good plan. The A-weighted (dBa) scale most closely replicates the response of the human ear so should measure most closely what we hear. The meter probably has a Fast and Slow [response] setting. The Fast mode will capture transients (like expansion joints or bumps, for example) while the Slow response should capture the average noise level. For things like road, wind and exhaust noise, the slow response should provide the most reliable steady-state data because it won't be affected by the occasional transient. Unless you have a helper to hold the meter and move it around, use the Max Hold feature (if your meter has one) to hold the maximum noise reading so you can look at it later. Do you plan to test the firewall area or the area under the console? Those areas will be difficult to muffle while driving. I may do the same kind of test eventually, but just bought my Vette and am not annoyed by the noise yet.

The Crazy Cowboy (forum vendor) may have some before and after data for his kits already. Post with Crazy Cowboy input

DugT

So far I'm enjoying my new to me Vette to much to care about the noise but I think this will be a fun and productive project. So far, just throwing the thick foam sofa seat cushion in the cargo area has made a nice improvement but it might be the placebo affect. Tomorrow I will measure the noise with the seat cushion in.

I plan to record both dBa and dBc. dBc is better for lower frequencies and I think lower frequencies are a big component of the drone that I hear. Also, I think higher frequencies are easier to supress. If I supress the lower frequencies, the higher frequencies will also be suppressed.

My meter has a record feature and the recording can be downloaded to a PC. It also has a Max feature. Tonight I did a trial run (I just got the meter.) and downloaded the results. The dBc average was about 97. The dBa average was about 75. This was taken while holding it above the console. The meter has a tripod mount so I just mounted it to a small tripod with flexible legs that can be attached to the passenger seat or almost anything. Hanging from the rear view mirror might be a good spot for it. As long as I am consistant my results should be usefull.

I think the rear of the car is the most accessable for adding insulation so I plan to work on that before the front. I've been studying insulation and the Blockit cargo mat foam design looks very effective. The foam looks soft enough for sound to penetrate but dense enough to cause level loss from friction. There is a denser layer near the middle. The top can absorb sound too, like sound reflected from the rear window, and then that sound will eventually hit the same denser layer.

For the front of the car, Lloyds Luxe matts look very thick and dense and they have a layer of foam on the bottom too. I haven't looked under the seats but there might be enough room to stuff a dead cat in there to absorb the noise. You would want to enclose the cat in a plastic bag to contain the smell.

The blockit kit doesn't include anything for the wheel wells which I thought would be a major culprit but today I looked above a rear tire and the wheel wells are made of a material that looks like it would absorb sound unlike steel. On the other hand, my Vette is a Z51 with wheel vents so sound could travel from the vents to inside the car.

My goal is to get substantial noise attenuation without removing the seats or the console. I'll know a lot more in a couple of days.

jimmbbo

I would call it a point of interest rather than a data point, but when I installed the Block It mats on the cargo floor and behind the seats, the noise level dropped on average 3dB as measured by the Decibel X pro app on my iPhone..
One evening I drove the same 10 mile route over smooth and rough roads, city streets and freeway. The best approximate reduction was 5dB over rougher roads, with a "knuckle and thumb" average of about 3dB.
I did not memorialize the data as my sole goal was to see if the mats helped...

SecondWind

Placing cushions, blankets, or pillows over specific areas may attenuate the noise coming from those areas but wont the material also absorb ambient noise from the cabin?
In other words, the Dba reduction observed may not be completely due to covering an area. Might be way off base here but thats what I think.

DugT

With an app on my phone, "Sound Meter", I took measurements of my Avis rental Corvette and the comprehensively padded Corvette of a friend. They were within about 3db of each other. His Vette sounded substantially better and 3 dB is substantial, but I wasn't confident in the meter or the method. I'm trying to determine if Block It mats might be good enough or if I should do the big comprehensive job of insulation installation. His Vette is a Z51 with magride so the total experience of his was more solid and that might have had a placebo effect on my perception ofquietness

My apps sampling rate was set too high so it was a little too busy to monitor well. Also, I read that phones mics filter out lower frequencies because lower frequencies are pure noise when picking up voice is the goal. A lot of the noise I hear in a car is lower frequency drone so I wanted a meter with dBc Because of that I decided to get a dedicated sound level meter. It has been a fun and interesting tool. "Memorializing" the data is very helpful to get a true average. My way may be overkill but it has been fun and interesting so far.

DugT

I think you are absolutely right about all of my padding absorbing reflected noise and all other noise coming from the roof and the glass. The BlockIt pad does the same thing. Based on a sideview photo of their cargo mat material, it looks like they have a sandwich of less dense foam on the outsides for sound absorbtion and a thinner denser foam layer in the middle. So, it would also absorb ambient sound from the cargo area. Plus I think most people cover the Blockit mat with carpet which should absorb some sound. A lot of premium mats, like Dynamat, have a reflective outer surface like aluminum. It is usually covered with something else like carpet but that aluminum looks like a mistake to me. It may be good visual marketing but I have doubts about its efficiency.

One of the reasons I took on this project is I couldn't find any objective test data comparing the different brands of sound absorbtion materials. I wish a compelling unbiased expert would post undeniable truth about the best way to make a Corvette more quiet but in the meantime I'm enjoying learning about this the hard way.

DugT

Today I measured the sound in my car with no padding and with overkill padding. The car was obviously more quiet with all of the padding. Maybe much less padding would get very similar results. The biggest pad was a 7" thick and heavy sofa seat cushion. I also put dense pillows behind the seats and a cushion between the two pillows. I believe this is more effective than Dynamat would be, for example.

Here are the measurements taken going 50-55mph on fairly consistent asphalt with the fan and radio off.

No insulation dBA 72.5 dBC 95
Extreme insulation dBA 72 dBC 89

It seems very strange that there was so little change in the dBA and so much change in the dBC. The implication is there was a huge reduction in low frequency noise. An increase of 6dB is the same as two times the amplitude.





The meter is bigger than I thought it would be.

Guard Dad

I did almost exactly what you are doing to your Corvette to my old 370Z only I really stuffed pillows right up the the hatch glass. The Nissan was notorious for excessive cabin noise with the hatch area being the major culprit. Stuffing the hatch on the 370Z made a real difference. I ended up putting maybe 50 pounds of Dynamat, carpet padding, mass loaded vinyl and additional layers of carpet in the car. It made a huge difference but it wasn’t cheap and it was a lot of weight. Would I do it again, probably but it moved the car in into more of a GT and less of a sports car.

I’m not entirely surprised at the modest difference in your readings because the C7 has always impressed me with its quiet-for-a-sports-car interior. In the absence of additional data your findings suggest that the hatch area is not a significant source of cabin noise which kind of backs up my unscientific opinion the the cargo area isn’t that loud.

The folks that seem the happiest are stripping the interior and applying a Dynamat type product to every accessible bare surface. If you put enough stuff in there it will make a difference. The question is how important it to you and how much time, money and added weight is it worth?

DugT

Thanks, Guard Dad.

For now, putting the sofa cushion in the back makes a nice differnce that agrees with the dBC measurement but not with the dBA measurement. I think there might be a variable that I am missing in my measurement technique. I think the minimum I will do is get the BlockIt Kit. It is fast and easy and it has resale value in case I decide to do a lot more padding.

Adding fifty pounds doesn't bother me at all. That is less than 2%. If the car accellerates 2% slower I probably wouldn't notice it. However, a funny thing that I notice when driving the sofa cushion around town is I catch myself driving faster than I think I am. Apparently I had already learned to automatically know the speed I was driving and I have to recalibrate when the car is quieter.

Fredtoo

One suggestion would be to make sure that you use the exact same section of road for the tests. Different road surfaces have a significant impact on sound.

I have been reading that Mass has a significant sound blocking ability, more so that just foam alone.

You may want to look into Mass Loaded Vinyl as a test variable.

There is a thread here where a guy was using a sound blocker that was a lead foil sandwiched between two foam sheets that showed significant improvement.

DugT

I've been using the same section of road but it's possible that it gets louder as it heats up and gets stickier. When testing I've been keeping my speed between 50-55mph. Maybe I should set the cruise control for a consistant speed.

That lead sandwich foil is called: Cascade VB-4 Acoustic Barrier Pad. It is hard to work with and expensive and I couldn't find any reviews of it so I suspect it might not be popular. It might be good for flat surfaces.

Guard Dad

Well it sounds to me that for you it would be well worth the time and expense.

FYI: I recently put a Dynamat clone on the floor of the cargo area of my C7 GS and really couldn’t tell much difference, so much for the ”low hanging fruit”.

Perf n Restore

Road Noise reduction and DB readings
2018 Z06 M7 2LZ
For me the road noise was unbearable. I could not listen to the radio, difficult listening to the passenger.....etc.

I used a DB App for my android phone to take noise level readings. All readings taken with cell phone at ear level.
All readings were taken on same stretch of road, same speed, same gear. These numbers are for reference. Actual DB reading from a certified DB meter would most likely be different.
The important thing here is the DB reduction.
3 sets of readings
-Stock
-With Insulation with Run Flats
-With insulation and Continental Extreme Contact Non-run flats

I installed the exotic vette extreme kit along with additional sound deadening material where ever there was room for additional material. Here I used .080" thick Kilmat from Amazon. For the top layer sound insulation, there was plenty of additional material in the Extreme kit to fill as many additional spots as possible. I took my time, 15 hrs total.
....................Avg*.........Min.... ....Max
Stock.............78............72...... ....85
W/Insulation...74............68..........8 2
W/ins + NRF....72...........67..........80
* the avg number is the average during the time of the test run, not the avg of the min and max.

I am extremely satisfied with my results. I can comfortably listen to the radio and have normal conversations with passenger.
I would do it again.

Guard Dad

The math experts on the forum will correct me if necessary but the last time I checked a 3db reduction is approximately a 50% reduction in cabin noise so a 3db reduction is really significant.

DugT

Here is a chart that shows the coversion of dB to % change in sound amplitude. I believe Perf n Restore's mod cut the noise in half which I think is huge. Any lower and you could forget you are driving a car.

DugT

Thanks for all of that helpful information! Crazy Cowboy said he measured noise levels of almost 10db less while cruising at 70mph. (I wonder if he rounded up from 6dB or 9.5dB.) At what speed did you take your measurements?

Do you have a link to any video's on how to remove the wheel well covers and the other side panels?

Fredtoo

Here is the thread where the guy used the VB-4

https://www.corvetteforum.com/forums...in-a-c7-2.html

JMII

As an ex-Nissan 350Z owner this is SO true... the Z was way louder then the C7. I dynamatted about 25% of my Nissan (hatch and doors) and it helped, but its a massive (and costly) effort to do it right, so for now I am leaving the C7 alone.

A lot of noise comes from the tires themselves, thus a less aggressive tread, non run-flat will be quieter.

DugT

I've been wondering if my new Sound Level Meter is accurate. To test it I downloaded a frequency generator and measured a range of frequencies from 50Hz to 9000Hz. Assuming the output of the app was at a constant level and my good PC speakers were fairly accurate, both my phone app meter and my "Quality" meter were inaccurate at some frequencies.

Here is the result:




The phone, which measure dB (not dBA or dBC) was fairly consistent until 4000 Hz and went deaf. The "Quality" meter was ok at reading dBA and usually it measured a little higher than the phone until the phone stopped hearing at 4000 Hz. Between 50 Hz and 200 Hz the quality meter read erroneously high levels which are coincidentally in the range of most of the noise in my car. I guess as long as the same meter is used for before and after comparisons it isn't important. I nice feature of the good meter is its ability to record the levels. That is useful for calculating the average and doing before and after comparisons of the min, max, and average.

What may be more useful is what I learned playing with the signal generator. It sounds to me like the frequency of the noise that I hear mostly is about 110 Hz. From idle to 60mph and maybe beyond, that low frequency drone is the biggest offender. My new mission is to find out what noise dampening material is most affective against low frequency noise.