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]

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.