Sound Deadening - Structure-Borne Noise/Vibration Damping
While no one will mistake most SL-Cs for a Lexus, the SL-C can be made into a relatively quiet vehicle with careful attention to sealing and application of the optimum kind and amount of sound deadening materials. As with all engineering exercises, the goal is maximum attenuation at minimum cost. This wiki page addresses the steps a builder needs to take to make their own SL-C as quiet as it needs to be in terms of structure-borne noise reduction.
Introduction
To begin, what is noise? The informal definition is simply unwanted sound. So, for example, while the Harley rider may not think that the loud potato-potato sound is obnoxious, many others do. For most SL-C owners, noise is unwanted sound coming from the engine, or more commonly, vibrations from drivetrain or tire noise, road debris kicked up on the chassis or body, the sound of tumbling air over or through the body, or even unwanted secondary vibrations from a car stereo.
Other sources include airborne sound leaks from unsealed surfaces between the chassis and body, or between body panels. These can be attacked by several means which are covered in detail here.
However, the main source of unwanted sounds in the SL-C and similar cars comes not from airborne sources, but from the body and chassis vibrating to externally-induced sources. That is where, once you have all the leaks closed up, the real progress will be made. These externally sourced noises are a result of energy transfer from a vibrating object to another object that can be vibrated. That vibration comes through as noise.
Thus, the solution for stopping this kind of noise is to interrupt the vibrating energy and convert it to heat (as you may remember from school, you can't destroy energy, only convert it.). Luckily for us, this is a problem that has already had a lot of smart people thinking about, and consequently an array of products that are designed to solve this problem. In fact, all modern passenger vehicles are treated with materials that are designed to reduce structure-borne noise. The amount of noise reduction is a function of the amount of material used to reduce the noise, as well as it's placement. While OEMs validate their damping treatment using exotic tools like laser vibrometer testing, we have to piggyback on their testing, and use materials in locations and using methods that the OEMs and others have pioneered.
Constrained Layer Damping
The most often used vibration dampening treatment in automobiles is known as constrained layer damping (CLD). This approach uses a layered matrix of materials, beginning with the floorpan or panel to be damped, upon which a viscoelastic layer (usually butyl rubber) is applied. On top of that is a third layer of thin sheet metal, generally aluminum, which sits in and is constrained by the viscoelastic layer of butyl rubber. When a CLD is applied to a vibrating structure, the butyl rubber with an aluminum constraining layer converts vibration to negligible heat. Vibration amplitudes and structure-borne noise can be consequently reduced.
The physical mechanism of damping can be explained when the base layer is deformed in a mode of vibration, the surface away from the neutral axis elongates, stretching the viscoelastic material. The top layer, being a stiff elastic material, tends not to elongate, and thereby "constrains" the viscoelastic material. Consequently, the cyclic motions of vibration induce a cyclic shearing strain in the viscoelastic. This cyclic shearing strain, together with its associated hysteresis loop causes the vibrational energy to be dissipated as heat.
You got that? :)
So, with theory out of the way, how does the builder go about making the SL-C quieter?
The short answer is that you can apply any of the popular sound deadening products like FatMat, Second Skin, HushMat and others. These can be applied to all the interior chassis panels, and to some body panels as needed. You also need to seal up all openings from the outside, to keep heat and noise to a minimum.
However, there are substantial differences in cost and effectiveness in the products, as you might expect. The following spreadsheet, developed by Wayne Marov, shows that you can make your own CLD matrix that has a thicker butyl and aluminum component for less, and enjoy better performance to boot (as the performance of any given CLD matrix is improved with additional thickness, in the thicknesses we are discussing here). The commercial solutions are also shown in the spreadsheet, and compared on a cost and weight per sq foot.
Coverage
Discussionswith all the vendors and several universities with vibration control labs all seem to indicate the same basic principle: In general, more application of CLD is better. That's true, but it isn't always cost-effective.
For example, the engineers and university sources indicated that once coverage hits 30-35%, additional coverage produces less acoustical effectiveness per dollar spent. One engineer stated that 50% coverage would provide only 3 dB less effect than 100% coverage and that 25% coverage would yield only 6 dB less effectiveness.
In all cases, the thicker the viscoelastic material and the thicker the constraining layer the more effectiveacoustical effectiveness was observed relative to the base layer thickness.
It is worth noting that all of the commercial products use thinner butyl and thinner aluminum in their matrix. One obvious reason for this is cost, but another is that is conforms better to the complex curves found in most production vehicles. Since the SL-C chassis is mostly flat surfaces, this compromise does not help us as much as it would a builder with say, a Camaro.
Summary
You can make the SL-C be significantly quieter on the road by covering some part (at least 25-30%) of all exposed chassis panels and structures to reduce structure-borne noise and vibration.
More coverage is more effective, but the cost-effectiveness curve drops significantly after 30-35%. Those builders who want the most coverage will still see a noticeable difference with 100% coverage --up to 6 dB more noise reduction, still a significant amount. The commercial products are easy to acquire, but builder-created matrix of readily available butyl tape and aluminum flashing from home improvement stores is significantly less expensive (especially if you don't value your labor), and can be built to perform much better.
CLD can and should be used not only on metal parts, as on the chassis, but also on the inside of the body, especially where large flat surfaces exist. Places to add CLD include the inside of the doors (the added weight also makes the doors have a satisfying clunk when shut), and any part of the body near where noise or vibration can be produced. If you do add material to the door, the extra weight may require stronger gas springs. The standard springs are 130 lbs.
Wayne Marov has reported that he has covered every floor pan panel, foot box vertical panel, top of the foot box as well as both sides of the 2X6and all 2X2s with 35-45% coverage. He indicates that every additional piece of CLD he has added has resulted in a noticeable reduction of resonance. He expects to do the same for the fuel tank, the fuel tank bulkhead and other similar panels, as well as using sound deadening liquid spray in the roof air plenum.
(Written by Will Campbell from information supplied by Wayne Marov, who kindly did all the research and experimentation necessary to create this page)