Hi,
I have been reading lots on the Internet, and the more I read, the more I find that no-one has done any real analysis.

What type of sound does a motor make?
Is it vibration, or soundwaves?

I guess what I am looking for is not the Use Dynamat, Use Brown Bread type of answer.

What if I combined both the ceramic spray on products, and the peal and stick products?
Aside from throwing alot of money at the problem, am I really stopping any more sound waves from coming through?

Are there any discussions of what sound waves Dynamat stops, and what sound waves a spray on ceramic sound deadening system stops?
Are they the same waves?

It seems to me that when you use two of the correct products, each designed to stop a certain type of sound wave, you will get a much better result.

But, where is the info telling me which sound wave is stopped by what product?

Rich

All sound is transmitted by vibration of panels. There are two types of sound deadening, blocking the sound waves from hitting the panel (think foam 'egg-crates') and keeping the panel from vibrating. Dynamat, BrownBread, and all the spray on sound deadeners work by adding mass to the panel. The more mass the panel has, the harder it is to move it, and thus, the less affected it is by sound waves. So sound hits the big heavy panel and is bounced back instead of passing through. The downside is that mass is weight. The weight issue can be combated by selectively adding Dynamat (etc). It is rarely necessary to completely cover a panel in that stuff. What you want to do is add mass at the most flexible point. That's why you see a 'small' piece of sound deadening on the back of a door skin. That's where it flexes the most.

You can kill the sound coming through the back wall. But you'll still have to deal with the sound coming through the rear window.

Does that help any?

I used quietcar on my firewall and floor. It's a spray on sound deadener. Now it sounds like all the noise is coming from the rear window. Since the rear glass is flat, it might not be that expensive to have a laminated safety glass version made. (like the front windshield) Those transmit sound a little less.

oh, now this makes the gears turn.




I used Quiet Kote, architectural sound deadening in my subaru. Excellent results.

OK,
To be a bit more specific.

I am considering DD a 356 coupe.

Ride in one, its loud.

So, I was considering all of these items.

First layer, ceramic spray on, inside and out of the rear passenger area, firewall.
Seconlayer, Dynamat (or equivelant)
Third layer, proper tar paper cut to size for 356 to give it the correct look.

But, I fear the window may be the big issue on this as well.

Currently, I can not hear my phone, nor have a conversation while driving.
This is my job, so there are no options, either I fix this, or buy a newer car.

My rational is like this.
The 356 gets 32 MPG all day long.
It looks cool
I can buy and build a really nice one, with AC for less than a ho-hum new car.
So, get a 356, convert to 12 volts, add AC, and use new technology to sound deaden the car.

Weight, yup, its is an issue, and will affect MPG, but I will still get better than an SUV.
Never gonna race it.

This is my approach.
Find all the new research, new technology and apply that to the sound issue.
Most 356 guys restore them to original, but dont drive them because of two major things.

Its a very hot car to drive, no AC and little air flow.
Its very loud (wife does not like it)
And for me, I cant use the phone.

I sell, on the road. I need the phone, I use it while driving, for hours at a time...
I get on calls between appointments, so pulling over is not an option.
Have lots of headsets, still no good.

So, looking for some real suggestions to tone down the noise.
Dual carbs and filters will not be removed.
And I want it to kinda look stock. Not to a trained judge, but to anyone else.
Rich

I use race quiet. very light and very effective
http://racequiet.com/

In that case, the more you can add to the panels, the quieter it will be. You have two noise generators: engine noise and wind noise. Wind noise is going to be hard to deal with, but engine noise can be eliminated. The panels between the engine an the interior can be loaded up with this stuff. I would put a layer of closed cell 1/2" thick foam into the engine compartment side to cut high frequency noise. I don't see any reason that you couldn't achieve your goals. A 914 with all the stock sound deadening in place is a decently quiet car.

I would say that if you sprayed Quiet Kote on the floor pans and rear firewall area, with a foam acoustical insul in the engine compartment you could call it done. You could buy a 2" slab of dense Mineral insulation for the passenger capartment side of the firewall to finish it off.

3 coats. Its water based though so your metal needs to be sealed properly. Then paint over it and add tar paper to taste. I would spray one coat on inside of the roof as well and in the doors. Limo companies use this and it works.

My subie went from tin can to really nice cabin noise in one thick brushed on coat.

good grief dude,

whats wrong with dynamat. I have the hood liner on one side of the firewall and I have regular stuff under the back pad, door pannels, floor mats, etc.

If you drive for your job, get a toyota. They are great cars and you will make it to all your appointments on time....

Strictly speaking it isn't the increase in mass that reduces the sound transmission, its the "de-Q'ing" of the panel. Think of a tuning fork making a nice tone. When you touch your finger to it, the fork stops resonating. Your finger has now become a lossy element in the structure. Its resistive to the motion of sound waves/vibration.

Whats happening is that a resonant structure (a panel of a car for instance) will absorb energy from the engine as either acoustical sound waves or as vibration through other panels or structures and then re-emit it as a summation of resonances. By adding dynamat or other sound absorbers, you are lowering those resonances in frequency and duration. The structure is now less "resonant."

Another way to think of it is that the speed of sound in metal is different than the speed of sound in air, the speed of sound in rubber, etc. etc. The more layers of dissimilar speed-of-sound materials you add, the more sound insulation you will get. Each time the sound wave hits a barrier of a material with a different characteristic velocity, it loses some of the energy as it reflects back. This is the thinking behind double and triple pane glass windows. When the sound wave is travelling through air it hits the glass, part is reflected and part is transmitted. Then when the sound wave hits the argon gas, it has hit another medium and then again and again as it hits the other pane(s).

So the idea of using ceramic and then dynamat etc, makes sense not just from a "what frequency do they stop best" standpoint, but also from the layering standpoint. The more layers of different materials you have the better. The order of layering may make a difference, but probably not easy to notice.

-Tony

Bravo Tony, I thought you were an electrical engineer? Oh well - Tony is right. The denser the material the quicker sound travels through it. What complicates a car is that not only do you have vibration that is transmitted from surface to surface through mechanical connections but you also have acoustic air space to deal with. It's fairly simple to deal with the acoustic air space; use multiple layers of materials with different absoption coefficients. Noise through vibration is much more difficult because in order to eliminate that you will need to "disconnect" the sources of vibration (engine, suspension, etc.)from your passanger compartment or find a way to dispell the vibration into air and then deal with it acoustically or cancel out the resonance of the part. Sometimes it's as simple as hanging a small piece of a Dynamat type product, other times it means redesigning a whole part. Or you can always just line the car with lead sheeting. Good luck with that one! Try to make the 914 ride like a sedan and you will have your answer; soft suspesion, larger quiet tires and compsite body panels. Like you, I find the rear window seems to pass a lot of noise. I have thought about trying a double pane glass of some sort of possibly some thick Lexan. Just haven't gotten to it yet. Air noise can also be a problem even with good rubbers in place. Look at a modern car; they will have 2 to 3 seals around the doors and you will still get wind noise.

We are honing in to my question, and thank you Tony for getting the idea to surface again in my head.
At some point in time I read an article that I cannot find again.
summary.

Sound travels in waves. Different frequencies have different wave lengths. Different materials block certain wave lengths.
This is why Dynamat is rubber backed with Aluminum. It provides two different materials which in turn block different wave lengths.
Leave the vibration out of this discussion, it is a bonus.

So,
Keep the stock look (no 2 inch close cell foam)
No good ideas?, dont reply...learn. that is what this place is for.

Dynamat blocks X number of wave lengths
Spray on ceramic blocks X number of wave lengths.
The net result is blocking more wavelengths than just using one or the other.

Now, the question remains.
What is the difference in products, and what wavelengths do they block?
If combined, do you actually block more wavelengths.

And the remaining question.
What wavelengths does a motor produce?

I know its a bit technical, but this is how good well thought out solutions are created.
Yes, I could buy a toyota, I know.
Ben Franklin could have just lit his whale oil lamp and called it a day.

Think outside the box here guys.
The new Caddy I rented, while it is quite large, was as quiet as a mouse inside.

I am looking to make the 356 the same way.
Not a 3-4 inch think wall of Home depot foam.
A good solid solution that implements the new technologies to solve a real old problem with this car.

Rich

First off, lets get a few things straightened out here... Sound is a vibration of air. So sound and vibration are the same thing. Sound is what you hear, vibration you can feel. In the end, they are both mechanical vibration.

The noise from an engine depends on a lot of factors, including;
1. RPM
2. Number of cylinders
3. Intake design
4. Exhaust design
5. Tons of other small mechanical features.

You get noises from explosions, friction, and movement of air (fans). You also get noise from things hitting other things (impacts). All of these are white noise generators, so they in general produce a spectrum of noise with some frequencies and some bands emphasized more than others. A good approzimation though would be to assume that an engine produces all frequencies within the human hearing range and beyond.

Here's the problem with trying to absorb things using frequency dependent materials (which are really frequency dependent structures)... The lowest frequency wavelength you can hear is somewhere around 20 Hz, which in air has a wavelength of 17 meters. The highest frequency you can hear (if you are lucky) is 20,000 Hz, with a wavelength of 1.7 centimeters. Frequency dependent structures (to do a good job) rely on 1/4 wavelength shorts. These are at best good for +/- 15% of their optimum wavelength. As you can see by the enormous range of wavelengths of sound, working with percentages like this is a joke in anything resembling a dynamat coating. In buildings these structures can be used (look up Helmholtz resonators).

So forget about thinking in terms of wavelength. The reason that dynamat has aluminum is almost certainly not for wavelength reasons. It is very liekly due to what I was saying about layering dissimilar materials. Aluminum and bitumen or whatever they use are very different from one another and have vastly different speeds of sound.

There definitely will be some variability of what materials do well as dampening at different frequencies, but because this is a white noise situation you will need multiple materials. So for the best results, use both ceramic and dynamat. Or whatever. But just layer things. And thick rubbery squishy materials in general will do a good job.

If you are trying to find spectral attenuation plots for any of these materials, good luck. The performance of this stuff will depend at least as much on the geometry of what you are attaching it to.

-Tony

edit:

If you are trying to attenuate airborne noise in a cavity like the engine bay, the best way to go is a contoured foam absorber like the stuff you see on the walls of recording studios, such as Sonex. The idea with absorbing sound is that you want a gentle impedance transform from air to a dampener. The wedges attempt to match the acoustical impedance of the air with that of the foam, thereby reducing reflection. Also, for smaller wavelengths, they act as a trap, causing multiple reflections against a lossy medium (the foam itself). But you mentioned you don't want that look, so stick with straight dampening materials. Layers of them.

Sound travels in waves. Different frequencies have different wave lengths. Different materials block certain wave lengths.
This is why Dynamat is rubber backed with Aluminum. It provides two different materials which in turn block different wave lengths.
Leave the vibration out of this discussion, it is a bonus.

So,
Keep the stock look (no 2 inch close cell foam)
No good ideas?, dont reply...learn. that is what this place is for.

Dynamat blocks X number of wave lengths
Spray on ceramic blocks X number of wave lengths.
The net result is blocking more wavelengths than just using one or the other.

Now, the question remains.
What is the difference in products, and what wavelengths do they block?
If combined, do you actually block more wavelengths.

And the remaining question.
What wavelengths does a motor produce?

Ben Franklin could have just lit his whale oil lamp and called it a day.

Think outside the box here guys.
The new Caddy I rented, while it is quite large, was as quiet as a mouse inside.

I am looking to make the 356 the same way.
Not a 3-4 inch think wall of Home depot foam.
A good solid solution that implements the new technologies to solve a real old problem with this car.

Rich
[/quote]
Rich,
Let me see if I can get you started on this since everybodies end result is subjective and thus is different so you'll have to do the analysis for your car.

Speed of sound at sea level = 1,130 feet per second. I always use 1000 feet since it is easier to compute with and most of us are higher than sea level which means sound travels slower.

To find the "size" of a wavelength divide the speed of sound by the frequency. The problem here is that it is very complex. Each individual engine will generate it's own unique "timbre" or frequency spectrum and that changes with RPM's. Add road and wind noise and your analysis becomes rather complex. What I suggest you do is get a hold of a frequency analyzer (if you lived in LA I could help since I own one), sit in the car and see what are the loudest frequencies at the different RPM's. This will at least tell you your problem areas. Bear in mind that this is only an acoustical analysis. This will not tell you what surfaces are resonating so while you can try to find materials to absorb the acoustical noise they might not translate to a physical barrier such as Dynomat. For example; you could line the firewall with Dynomat but in actuality the roof could be resonating causing all your problems. When I worked for BOSE the auto manufacturers paid big bucks to have BOSE solve most of the acoustical issues with their cars. The Cadillac that you drove is an example of lots of years of testing at Bose. They also found that composite body panels are the most effective way to eliminate a lot of noise problems.

Speaking of Dynomat, if you have an idea what frequencies you need to absorb, go to the technical section on their web site to see the absobsion cooefiecient of their product; http://www.dynamat.com/technical_technical.html. Other companies of worth will also have these same charts.

If you manage to procure a frequency analyzer you might be able to do surface contact analysis by using a pzm type mic on the surfaces. This would tell you the frequencies that the surface resonates at.

You have the right idea with using multiple layers of materials to absorb the different frequencies. However, high frequencies have very small waves (for example a 2000 Hz sine wave is about 1/2 a foot in length) and low frequencies are very large in length (for example a 500 Hz wave is about 2 1/4 feet in length). So, in order to stop lower frequencies you will need a very large surface, at least 3' x 3' (assuming it's the proper A.C. material). This is why most people who put sound absorbing materials in their car still have lots of low frequency "rumble". The surfaces in the car are not large enough to soak it up. The 356 is a large open cavity and does a great job of helping the noise to resonate. The best thing in my opinion to do is to use a material like Dynomat on one side of your noise barrier surfaces (firewall) and some sort of loose batting on the other - something like the wool carpet padding under the carpet. Use a good quality thick pile carpet and insulate the head liner. You might also try loosly stuffing the backs of the seats with something like rockwool. That's about all you can do to keep the originality of the car in tact. The next step would be to make sure all of the bushings in the suspension are good. Next would be tires that are not too low profile and have a good road noise rating.

I know this is a lot and hopefully it helps.

Tony... You be one smart sunofagun.

I am a firm believer in using the two layer method. Any of the Tar like dampers. (Bitumen, as per Tony) for the first layer.

And where possible, a second layer of closed cell neoprene. Much like wet suit material but stiffer and having a smaller celled structure. I buy 2' X 3' remnants from a place called AA yardage in Santa Ana, California. They cater to upholsterers.
I install it with 3M spray glue #77. Works like a charm. You can also put dynomat OVER it if you choose.

If you have any neoprene at home... Go set it on the hood of a car and give it a tap. It will kill a bunch of resonance pronto. It both absorbs and deadens. And turns the sound to heat.

If ya get hardcore. You find EVERY hollow space you can, and spray in NON EXPANDING spray foam in hollow spaces. This really helps keeping the
sympathetic resonance effect to a minimum.

This has worked for me many times. Just more stuff to think about.

Clayton

I use earplugs; can still hear the radio, but the car noise is minimized.

In another life I used to quiet down printers- noisy dot matrix printers. Spent many hours in a soundroom with a meter trying to see what worked.

You can get two types of sound to block. One is broadcast sound from a radiant source (motor) and another is from a sympathetic vibration- such as a large vibrating panel. To dampen the panel vibrations, a sheet sandwich of tar and foam and paper mats were used by the factory on the floors, and a sheet of rubber on the bulkhead inner wall. For radiant sound, dense soundfoam works best, and no, it doesn't have to be eggcrate.

Foam absorbs sound and convers it to heat. It was amazing what a well placed sheet of .50 inch to 1.50 inch thick layer of Soundcoat foam would do. Opposed sheets stop richocheting sound, and having one of the opposed pair at an angle is best. Foam for midrange, myar cover on foam for high frequency vibrations.

A really good way to dampen sound is to allow the sound to vibrate a mass. The mass is suspended on a spring (foam sheet or dampening sheet)) and the sound energy vibrates the mass, and the spring or (dampener material converts the vibration to heat and dampens the noise. I have used steel strips on gel or foam or vinyl as a dampener. Soundcoat used to sell a sheet of foam with a vinyl coated lead layer on top of the foam and a sheet of foam on top of that, and a layer of silver mylar on the very top. That stuff was awesome for sound deading but weighed a ton and was spendy. Ever wonder why the engine sound deading on the bulkheadwas a layer of asphalt paper on top of a layer of brown mat? The asphalt paper was the mass, the brown mat was the spring- until the glue crapped out.

Ken

So Ken, do you think adding some material to a stock rain tray would qualify as:

Mark, the answer is it depends on the materal and what you hope to solve. If you hang the tray from dampeners (such as springs), then the mass of the tray will transfer the vibrational energy to the dampeners. If you do that, make sure that one end of the "spring" attaches firmly to the engine lid and the other end attaches firmly to the rain tray and there is no fastener coupling the lid and tray together. I also think the stock tray qualifies as a large vibrating panel so adding a layer of dampening material (on the bottom) would help with that. The tray also seems ideally placed for a sheet of closed cell (non water absorbing) foam on the bottom of the tray (or the dampening material) to absorb motor noises.

These EAR guys make great dampening material and now foam. The "blue" vinyl materials and grommets are awesome. One of the aftermarket vendors is selling them with their non stock tranny mounts and I spotted the blue color and thought that it was a great use. The grommets, if used correctly can be used for the rain tray.

EAR site

Ken