Sound Reduction with Minimal Wt. Penalty

[RWebb]

Here are some actual measurements presented by various magazines on noise levels under different conditions in various 911 models.- unfortunately the methodology was not given (direction mic was facing; averaging; position of windows, etc.). Methods likely differ among sources; methods would likely be the same for any single source of data.

But, FWIW:








The data are for all noises combined of course. Note the deep reduction in noise levels for the SC compared with the early S, and the reductions for the GT3 at idle without much reduction at speed or under acceleration.

[RWebb]

I. Overview
Engine noise is something we all like. Individuals may differ in how much engine noise they want to hear, and in which components they like to hear (exhaust, vs. induction howl, vs. chain noise, vs. fan noise). There will also likely be differences for each person depending upon situations. As you age, you may want less noise – for example, even Bruce Anderson eventually got rid of his 911 and bought a Boxster, though not all of that was noise issues. You may want less noise on a long freeway trip, if you are taking a female companion on a long trip, or are driving through a neighborhood. Cutouts offer a way to lower the noise level selectively.

Other noises are almost always “bad” – that includes wind noise, tire noise, noise from rock or grit strikes on the underbody, and noise from the transmission, wheel bearings and CV joints. Maybe some people like transmission whine??

[RWebb]

II. A Noise History for the 911
The early cars have very high wind noise and most magazine testers complained about it at the time. The difference between an early 911 and a well sealed modern car is even more glaring. The engine also made a lot more noise than the later SC, and there was likely more tire noise also, since modern finite element models for tire noise reduction were not implemented until about the late 1970s (nor did anybody have the computer power to run them).

There is not much an end user can do about tire noise except to change to a lower performance type of tire, such as the ones for performance sedans like BMW. Engine noise is often something we want to keep, so let’s talk about wind noise for a bit.

Wind noise is generated by turbulence – when the air breaks away from a smooth flow over the body (or loops back in an eddy). Noise is also generated when air passes through a hole for the same reasons.

Later, Porsche greatly reduced wind noise:

“Porsche has really gotten its act together regarding noise. [engine noise so low it's eerie]
... the dreaded wind noise that has plagued 911s since Day One is gone. Even the noise around the mirrors is subdued. [car is so tight now that it takes 2-3 tries to close the doors]”
- Road & Track, Porsche 911SC - What Could be Better?- April 1978 (last page of article).

So, why is the SC so much better? The thick and rounded edges of the mirrors no doubt play a part. The SC’s also have an “extra” door seal. A friend of mine added that seal to his 1970 E. He is a very careful guy, but he reports that although the extra seal reduced the wind noise, it is still high – and higher than an SC belonging to a friend of his. So there must be something else about the SC that lowers the wind noise. Perhaps it is the closer fit of the doors…

See also the graphs above - I updated them today also.

[Mitch Leland]

Good work Randy... Now tell us how to fix our old cars!

When I first got my car it had the original windshield gasket that was very dry and had shrunk, in fact you could have nearly stick your finger through the bottom part of the gasket. Even now with a new gasket I wonder how much wind noise I'm getting. I keep meaning when I have less things to do to the car is to run some duct tape around the W/S to see if the wind noise is reduced. Has anyone tried that?

Has anyone found the later style mirrors reduce the wind noise?

[RWebb]

yes duct tape around the entire seal will surely reduce noise - it would be similar to a flush mount windshield

[Mitch Leland]

Randy,

Then you're saying it's not that the W/S gasket is leaking air, it's the aero dynamic affects of the cross section that makes the noise... I might have run a test and deduced that the masking tape was holding out the wind and then silicone the gasket and still had the problem.

Thanks,

[RWebb]

no both cause noise - eddy formation of certain sizes & close to the window will create more noise than the "wrong" sized eddies created far away

- this is all based on the pioneering aerodynamics work of Prof. Edward E. Size

[RWebb]

One way to reduce noise is to rid the car of air leaks. Not nly that but they are annoying.

The rubber seals around the blue plastic lines above the pedal cluster area have gaps in them. The gaps are hard to see without shoving your hear way up into the compartment floor.

[RWebb]

They also can allow water and road groddu to enter the interior and cause rust (of course, it may not have been the brightest idea in the world to pierce the interior for the master cyl. and its mounting bolts either...)

An easy way to detect the gaps is to stuff a drop light up under the steering rack in this area and then look for light from inside the car.

This is a closeup of the area shown above:

[RWebb]

When thinking about panels of sheet metal, like the body of the 911 is made from, you have to realize that sound doesn't just go thru the panel and out the other side.

Instead, sound hits the panel, and some of it bounces back off the way it came; some of the sound will be absorbed into the panel and excite it. This means the panel will vibrate and radiate sound on both sides. The panel will vibrate just the way it did in your sophomore physics class textbook...

In other words, it will vibrate the most at certain resonant frequencies. This link shows some examples (using a panel that is specifically designed to vibrate nicely):
Harold A Conklin Jr: Piano design factors

Another example would be the head of a drum. The panel is the drumhead.

In your car, a crimp in the panel will change the way it vibrates - basically it will make the panel act like two smaller panels.

[RWebb]

One thing you can do is to put a pad on the side of the panel where the sound comes from (incident side) -- e.g. inside of the engine compartment. This has the advantage of preventing some sound from even exciting the panel in the first place.

Second, you can put some gum on that drumhead. You can demonstrate how effective this is at a concert some time. Go sticka nice wad of gum on the drum head and wait for the drummer to strike it - you will note that the sound is considerable attenuated. (Note also that drummers often have pretty strong forearms, so take care to not get caught when doing this.) It only takes a small amount of gum to .. ah.. gum up the works of the acoustical panel.

So, when using a dampening material on a panel, one trick is to only put a piece in the middle of the panel. The amplitude of the sound will be reduced and the frequencies produced will be spread out (i.e. the "Q" will be lowered).

25% coverage is what I usually see quoted for how much to cover.

some background:
http://en.wikipedia.org/wiki/Resonance#Q_factor

Third, you can put something on the inside of the panel (same side as your ear) to absorb any sound generated.

[RWebb]

The pic in this thread shows an idea about what to do - I might do it a little differently in the details but the general idea is correct for the Mexico Blue car.

Sound Deadening question.

[911pcars]

True, Kemosabe. Good info.

However, one must realize the muffler skin is also a major contributor to the decibels created. Using a more rigid shell might result in better noise control. In the end, droning is perhaps the trade off in exchange for a relatively lightweight muffler.

Sherwood

[Jonny042]

This is one of my favorite noise reduction threads, keep coming back to it when I search.

Single biggest improvement I have made to my car noise wise - replace cheap aftermarket engine mounts with genuine Porsche parts. No weight penalty, in fact my wallet is a bit lighter!!

[RWebb]

Originally Posted by 911pcars

True, Kemosabe. Good info.

However, one must realize the muffler skin is also a major contributor to the decibels created. Using a more rigid shell might result in better noise control. In the end, droning is perhaps the trade off in exchange for a relatively lightweight muffler.

Sherwood

Quite true - a light wt. muffler (using Ti and CF) is shown for the GS car made by Robert Linton. Go to RennList 964 section, and IIRC the thread is pinned to the top. Obviously, it is not something that ordinary mortals can do, but that thread is well worth reading in its entirely.