Sound Deadening, A little more technical question Options

[r_towle]

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

[TonyAKAVW]

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

[ahdoman]

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.


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, (IMG:style_emoticons/default/clap56.gif) 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. (IMG:style_emoticons/default/av-943.gif) 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.