Sound Reduction with Minimal Wt. Penalty

[RWebb]

I've been thinking about how to attenuate some sounds without incurring too great weight penalty for a while. Somebody sent me a PM on this, so I guess I'll jump the gun on it. This will be a thread of ideas and thoughts, rather than results.

Now, what we want to do is to have "good" sounds come thru to us in the passenger cabin, and to attenuate "bad" sounds as much as possible.

What we call Good sounds can vary -- with age, with the presence of a companion of the female persuasion, or with the type of trip we take (a 'fun' muffler might not be so interesting after droning on the hwy at 70 mph for hours on end - then when we get to our destination, it might again become 'fun' - say the next day on curvey roads.

For such changes we need some sort of muffler cutout, either a simple cap, or a solenoid controlled re-routing (which adds more wt.).

Other sounds we think of as good are:
- air fan noise
- timing chain noise (?)

Noises that are always BAD include:
- wind whistles, etc.
- tire drumming, etc.
- gravel strikes
- CV joint and transmission whine

The best situation would be if we could attenuate certain frequencies and not others. This is commonly done (or attempted) in many fields. We have other constraints:
- a sports car must be as light as feasible
- we don't want to screen out the "Good noise"

Generally, we can assume that any noise coming from the front or side should be reduced to zero (or as close to that as feasible). Sound from the rear needs to be filtered - and filtering by frequency is easiest.

For the front - a close inspection of every hole in the panels is useful.
- elec. wire feeds
- fuel, fuel vapor, cable, and hydraulic hose feeds
- heater hose feeds
- radio speaker (early cars have one in the dash)
- fresh air vent system
- radio cables, wires
- mechanical cable feeds
ALL of these need to be carefully sealed to prevent air and sound intrusion.

We also want to work hard on the side, front vent and rear vent windows, plus sunroof to eliminate sounds.

The door seals are another issue - I know some have put or plan to put the double SC seals onto their early cars.

[RWebb]

Here are some more (really undigested thoughts) from a MS Word outline file I started:

Filling Crevices

The problem with all mat materials is that they do not get down into crevices. NOW -- think about what a crevice looks like... it looks something like a funnel or horn shape, right?? Or maybe a long, thin slot.
- A viscous liquid is surely the best thing to fill such spaces.

What do we use horns for folks?? To magnify sounds, right?
Think of Miles's horn - or a tuba, or an old horn type record player before the electric ones came along, or a Klipsch horn speaker... or a sound reinforcement system at a concert...

So not filling crevices is more of a problem than you might think.

Places to be Concerned About
- holes for seat belt bolts
- approximately 8 stamped oval-shaped holes in the rear deck
- all corners & crevices in rear

“Holes” to Outside and Engine Area
Transmission Tunnel & Its Opening to the Rear
Sprayed Quiet Coat under trans. hump (and covered with clear coat for durability)
also sprayed Quiet Coat into interior of trans. tunnel above that area - coupler area; RonIn LB's idea
I covered it with a spray clear paint for protection

Transmission Tunnel Openings
The area just to the rear of the tunnel exit is very noisy. Noise from the transmission, the front part of the engine, the rear wheels and CV joints, as well as road noise is easily propagated into the tunnel through openings or by reverberation of the tunnel sheet metal.

Noise that gets into the transmission tunnel can travel out of it through the 3 large access panel openings or through any seatbelt holes that are unfilled.

Rear Triangular Cover for Shift Coupler
“Also, the coverplate for the shift coupler is another noise source. Did you know there's a 1/4" x 6" air gap near the coverplate? It is normally filled with carpet, but it's a gap nevertheless.”

Middle (Handbrake Cover)
Hole is filled by handbrake Sheet metal so may be fairly quiet. Leather and Rubber covers will further quiet noise from this area.

Front Opening to Tunnel
farthest opening from transmission, etc. but completely open except for carpet; make a small cover with foam on bottom to fill this area


Ventilation Slots Above Rear Window
The Factory apparently used an open cell foam blocking noise from outside there. This is something to be attended to when replacing the headliner, or possibly when R&R ing the rear window.

Air Leaks from Front Trunk
- air leaks can come through the glove compartment light housing & the speaker – see UpFixing III, p. 132


Wt. of Stock Sound Deadener
Yes -- my 11 pounds was front and rear floor only.

Here are some more weights for the dense panels that were glued to the tub in the areas indicated:

Rear seat buckets -- 2 @ 8.5 lbs each -- 19 .0 lbs
Rear wheel arch -- 2 @ 11 oz each ------ 1.3 lbs
Rear flat side panel -- 2@ 1.25 lbs each ---2.5 lbs
Rear shelf & upper fire wall --------- 1 @ 10.3 lbs
Floor (front & rear) -----------------------11.0 lbs

Total ------------------------------------ 44.1 lbs

I also seem to remember (I did this work 7 years ago) that there were the same type of panels glued to the front side foot wells behind the carpet pockets, but I don't have any weights recorded for them. Can anyone verify that there are sound mats in that area or was the carpet just glued to the tub?

How to Remove Stock Sound Deadener
to remove the thick hard factory floor insulation/sound deadening stuff on the floor before repainting try using a Fein Multimaster with the scraping blade.

misc

“Quick update: I have roughly 10 lbs. of Quietcoat on the rear seat, rear deck, and interior wheel wells. I was originally very disappointed in the noise. Well, the interior is bare (no interior, rear seats, or seat belts). The noise was coming through the inner seat belt mounts (through holes directly to transmission area). After covering the holes with masking tape (temporary measure), the noise levels decreased immensely.”

Also, there's significant noise coming from the transmission hump area, and the panel directly behind the rear seat backs. It would be smart to add more material in these areas, and use less Quietcoat near the inner wheelwells.

Yet another noise source is at the joint of the rear deck and the headliner. I haven't figured out how to attack this noise source.

I honestly believe attacking the noise sources will reduce noise immensely, even in a stock 911. Until all air gaps are eliminated, I cannot fully evaluate the performance of Quiet Coat. However, I have high expectations.


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I put in a layer of Thermotechs Thermo Guard FR on most of rear interior.
http://www.thermotec.com/products/full/14120/14120.html

The sound level with the thermo guard would be liveable but I'd like it quieter if possible without adding too much weight. The G50 is very noisy. The thermoguard 48x72 was 5.26lbs and I used pretty much the entire roll. I think two coats of quiet coat plus the thermo guard will do a better job and not add too much in weight. I removed 50lbs from the back and would like to put back 10 or less. With quiet coat, it might be more like 15lbs back in.

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Thank you for the inquiry. We have a floor barrier that will block road noise from entering the vehicle. However, the barrier is designed for interior use. I recommend using one sheet of VB-3.5 to mold over the transmission tunnel, and 3-4 sheets of VB-4.5 for the flat areas of the vehicle. I do not recommend exterior use of the barriers. Primarily due to the damage that can be sustained by exterior influences. If you need to put a coating underneath the vehicle, use our VB-1X spray vibration
damper, and cover it with a rubberized undercoating. (But remember, barriers are designed to block low frequencies, and dampers make things vibrate less. Use barriers on the floor, and dampers in the door.)
There are many other things to discuss, feel free to contact me if you have further questions.
Also, you can buy direct from us if you are unable to find a Cascade dealer near you.

Best Regards,
Linken Olsen
Customer Service Manager
Cascade Audio Engineering
PH: 541.389.6821
FX: 541.389.5273

[RWebb]

IN general, there are (now) two types of noise attenuation materials:
1. sound dampeners
2. sound attenuators

re #1 - these are heavy asphaltic or even lead based materials that act to lower the resonance frequency of panels in the car. These panels act like drum heads and vibrate at certain "resonant frequencies." The freq. is based on their size, thickness and configuration - just like a drum head. The original version of Dynamat is in this category, as is Brown Bread, etc.

materials in class #2 are designed to prevent sound energy from moving thru the material. For example, some are made of micro-beads and convert the sound into (tiny amounts of) heat energy.

One such is made by Cascade Audio.

The nice thing about a liquid is that it will tend to fill those tiny horns referred to above.

So, I would do that before putting big heavy pads on my car.

[t6dpilot]

Subscribed Randy. I am reading your findings with great interest as my LW is basically an empty oil can... that I will be driving to Treffen from IL with a sport muffler. I may not be able to hear you when I get there.

[old man neri]

Active noise cancellation fed into the stereo and then the speakers?

You can buy mini ones for airplanes that have an output and in input (for an ipod). If your stereo in your car has an 'aux' input you could simply plug it in. You could even listen to your ipod.

Actually my dad has one of those, I am going to see if I can borrow it the next time I see him, would be interesting to know my silly idea even comes close to working.

[dshepp806]

RWEBB: you're not messing around here, are you?

Being an audio engineer, I must say that your endeavor is quite large, as the holes (anywhere) will provide pathing of many frequencies..

Guess I should pack up the spectrum analyzer or TD spectro pieces and send it to you so that you can "quantify" your tasks? Are you, in fact, measuring this stuff?

I've designed and tuned MANY a room but NEVER a frikin car (and certainly had NO weight limitations along the way!!). An interesting proposition, to say the least. Sounds like you need attenuation approaches, and you seem to be on your way as to that regard.

I've a few rides that provide great listening environments with reasonably high $$ pieces. My studios are MANY levels above this... My '89 911 (environment) is absolutely (without question) the poorest of anything I've ever experienced as to audio,..and I've considered working on it to be much better.....HOWEVER,..(and I'm quite sure others will ring out),..that 3.2 is a band in itself,..but can supply fatigue just as much as anything else can...For now, she stays just as I bought her..I just have to crank RUSH a bit higher when need be....and I've plenty power for that, if necc. One thing this 911 WON'T outrun is the power reserve in the audio section.....

Interesting endeavor, you have there. I'll be watching this one.

BTW: mine's bone stock.


Best,

Doyle

[dshepp806]

I'm thinking of getting some baseline spectrum plots (both pink/white, Steely Dan) on what the 911 renders within the cockpit...I've the proper mics for this,..should prove interesting once the engine fires up.... ...

Best,

Doyle

[RWebb]

Thx Doyle. You could make a great contribution to this....

Altho I'd love to have your spectrum analyzer - we almost bought one to analyze bird song vs. ambient in noisy env.s but our research went a different direction.

re: messin' around -- I'm not saying how many pennies I would spend on this or how much of my labor... but it's good to take a pretty broad stance on paper or "in mind" before resources are committed.

I think active noise cancellation (or isolation - see some of Jack O's posts re his car) is not a good direction for me.

Two Queries for those out there:

1. Anybody know much about noise reduction materials in general aviation arena?

2. Anybody remember the issue of Road & Track where they presented color waterfall plots of noise from different sports cars? IIRC, they had Corvette & 911 (993). The latter had a much broader band of output from the motor...

[JP911]

Very interesting topic, Randy. I look forward to more suggestions and trials of the different materials. My car is LOUD (and I still have the factory sound deadening in the rear) to the point where I must wear foam ear plugs for long drives, especially during and after track days.

[RWebb]

heat transfer is an issue for the rear firewall - I thought it needed a separate thread.


for starters, fill in this chart:

[RWebb]

BTW - if was going to do something right now w/no more add'l research or thought, I'd:
- use factory foam pad
- strip stock gunk off interior of firewall & adj. panels
- get some of the Cascades Designs liquid and roll it all over everything with drippings going down into all crevices
- use foam to pad off the openings in the center tunnel & make sure the rubber seals there are all in good shape
- test drive & see what sort of panel resonances I got - maybe add strips of ashphaltic matting rather than entire sheets

do the front sealing stuff noted above.

[MotoSook]

I couldn't resists, but here's the best noise attenuating device with the least weight penalty.



Really though, my BMW has great noise level inside the car, but it's at a high weight penalty. I just don't think I'd be able to find a compromise between the sporting purpose results of my sporting purpose car and the luxo cruiser level of noise and physical comfort of my daily driver.

My 911 project will have a cutout for when I want low exhaust noise, otherwise everything will be nice and tight to eliminate the rattles....that's sporting and comfortable enough.

My biggest beef in any car is rattles and creaks....without that, everything else is tolerable.

[304065]

Randy, noise reduction in GA aircraft consists of quilted pads under the headliner and in the upholstery panels.

This is why we wear David Clark or Bose ANR headsets- just a few minutes in the combination of low-frequency exhaust rumble and the propeller spinning along with high-frequency air leaks will drive you crazy and make you want to land.

[dfink]

Wow I am really to lazy to read the whole thing so if this was covered sorry. You can really reduce the fan noise by polishing the fan. I am surprized at how lousy a job they did on cleaning up the casting. I sanded down all those nasty seams and now I can hardly hear the fan. The fan won't hold a high shine so unless you want to paint it or clear coat it don't even bother doing a complete polish in three months it will look like good old magnesium again. I am sure the fan is also more efficient now.

Also remove muffler and install bypass. You have now reduced all sounds to One.

[t6dpilot]

Randy,
As you can tell from my screen name, I fly a very loud airplane. The only sound deadening is the aluminum skin. Most of us wear noise cancelling headsets these days. All aircraft are loud - even the corporate stuff. I am sure the manufacturers use something, I just don't know what it is. I am debating on bringing noise cancelling headsets and and intercom system for the long ride west in September. Should be interesting... Carry on.

[304065]

Originally Posted by t6dpilot

Randy,
As you can tell from my screen name, I fly a very loud airplane.

To call that R1340 very loud is to call an SR-71 fast.

[Canada Kev]

This is a product designed for noise attenuation in vehicles. This company makes audio abatement products for buildings vehicles and whatever might need it.

It is applied as a liquid via spray or brushing. I have not personally used it as my project is currently on hold, so can not speak for its effectiveness.

http://quietsolution.com/html/quietcar.html

[Mike_Lettrich]

Randy,

While this is purely anecdotal, I had good results with a similar project last year. I removed all of the factory sound insulation on my SC Targa, including the asphalt mats from the floor and the kick panels, removed the thick rubber covering over the rear seats and firewall, the inside of the doors, and the for lack of a better word carpet pad type material behind sides of the rear seat area and the plastic kick panel covers. I mainly used a heat gun and paint scraper to get it all out.

I added two layers of quiet coat to the floor, firewall, doors and kick panel. A layer of HeatWave was added to the rear firewall. I also used a closed cell foam from Second Skin to replace the carpet type material on the sides of the rear seat area and the backs of the plastic kick panel covers. I also taped over the various holes in the rear seat and firewall area. I did not want to go crazy with the mats because of their weight, but in this small area it I think it was well worth it.

The steel kick panels, at least on a Targa, produce a lot of noise. Without the asphalt mats they are like a drum. The Quiet Coat noticably reduced the noise coming through them. I added a layer of Second Skin sheets to these areas to reduce their noise even further. With the Quiet Coat and the mats, there is a substantial reduction in front suspension noise coming through this area. This may be an area on which to focus your efforts.

Although I did not weigh the materials going in or coming out, the stock sound attenuation material was very heavy. The materials going back in were much lighter.

Again, I did not weigh the materials or use any instrumentation as to the noise inside the care either before or after. However, I am happy with the results. The car has less noticable noise from the suspension, creaks from the body and high frequency noise. Being a widebody Targa, the car used to have a noticable amount of chassis noise over rough roads. That is pretty much gone now. There is no noticable reduction of the sound of the engine over stock. But that is what I was looking for- just a reduction in some of the "bad" noise with the side benefit of losing some weight.

Good luck with your project, and thanks for a great thread.

Mike

[Jim727]

...moved.

Better?

[DG624]

Randy,

Thank you for doing the research.

use factory foam pad
- strip stock gunk off interior of firewall & adj. panels
- get some of the Cascades Designs liquid and roll it all over everything with drippings going down into all crevices
- use foam to pad off the openings in the center tunnel & make sure the rubber seals there are all in good shape
- test drive & see what sort of panel resonances I got - maybe add strips of ashphaltic matting rather than entire sheets

Have you tried any of the noise reducing liquids? I have used Dynamat with minimal results. I really don't like the added weight and it seems very crude. Your details about noise coming into the interior from holes in the body is very important. I can say that just improving the insulation and plastic barrier in the doors and replacing the weather stripping can make a big difference. I was considering Quiet Coat but are they all the same?