|Don't Believe the Hype.Sound Deadening Fables.
By Paul M. Messett / Sales & Marketing Manager
All throughout this blog entry I'm going to have the audio track from the Public Enemy tune "Don't Believe the Hype" running through my psyche. Oh this is going to be a good one, where to begin? I'll just put a list together and start knocking them down one by one.
Read the lyrics.
1. Using thermal control compounds as sound control materials.
There are quite a few companies out there spread out around the automotive industry that offer paint on thermal control coatings (including ourselves). Many of these companies will try and tell you that these products will do three things which is reflect thermal energy (true), damp vibration (not quite) and block sound (absolutely not). Here are the reasons why.
a. Reflect thermal energy: Yes, these liquids are designed for this function because they consist of microscopic, air filled glass beads suspended in a latex emulsion. Air is the best insulator which is why the product works so well.
b. Damp vibration: Unfortunately, these liquids do not have much mass meaning they are very lightweight nor do they have any type of mineral load. If you were reduce the number of glass microbeads in an effort to add mineral load, then the material becomes less effective as a thermal control coating and not very effective as a damper. You could say at this point the material would suffer an identity crisis. To become effective as a damper there would need to be a serious amount of mineral load and it would not function as a thermal control coating at all.
c. Block sound: No way Jose! Airborne sound is blocked by products with serious mass (i.e. VB3 or VB4). Lightweight, thermal coatings are very lightweight and are not able to block sound.
TG-1 on the floor pan (41 Willy's).
2. Using damping sheets as thermal control materials.
What has been promoted is that a constrained layer damper (CLD) is also a thermal control material. Just because a product has a layer of aluminum does not make it a thermal control material. Why would you install a CLD over a hot exhaust component, allow the heat to permeate through the butyl layer, reach the aluminum layer, then reflect back through the butyl layer? Some have said the aluminum layer is there to dissipate the heat. OK, then why would you want to dissipate the heat into the passenger compartment? Why install the product at all for that matter? Don't buy into this, it's technically unbelievably wrong and simply marketing propaganda.
3. Applying a layer of foam inside door cavities.
Please don't ever do this! A thin foam does not have the ability to absorb high energy sound waves and a closed cell foam won't absorb anything at all. Thin foams are not dense so they can not block sound. They may absorb very high frequency sound waves, and that's great, but what about everything else? When we spec materials for commercial and residential projects, we do not spec ½ thick foams for the walls. The physics of sound do not change for automotive applications. The only thing a thin foam in a car door should be used for is for placement on the high spots between the panel and mounting surface, behind harnesses and linkages to eliminate squeaks, rattles and buzzes and to gasket speakers if necessary.
Foams will deteriorate over time and they will absorb water to some extent (approx 10%). What benefit does preventing the reflection of sound within a door offer? It's not a home theater. You'd be better off installing Deflex Powerpads and eliminating speaker backwave right off the bat!
Thick pyramid foam for studio applications.
4. Thin foam layers as "noise filters"?
Oh, this is a good one. Some companies promote their materials as having "noise filters". For example a floor barrier material has a foam layer bonded to a dense vinyl or rubber layer. These products are also used in commercial in wall and flooring applications to block sound transmission from room to room. Anyway, it has been said that the foam layer is a magical noise filter that will filter out unwanted frequencies. When blocking noise, which frequencies are wanted? And how can a foam actively do this?
The reality is that the foam layer is used as a means of decoupling the dense layer. As I stated earlier, air is the best insulator and in this case it "floats" the dense layer off of the surface of the floorpan. This combination of materials is what makes a floating barrier so very effective. The foam is way too thin to absorb or filter high energy sound waves and closed cell foams don't absorb anything.
5. Expanding foams - be careful!
Be careful with this one. Expanding foams can get out of hand real fast and they are also quite capable of making body panels buckle. Also, expanding foam can be a royal pain to remove if you ever have to perform maintenance on a body panel. If you do plan on using a spray foam, use the slow expanding type that remains soft when cured, not the hard stuff you spray around the pipes and in the crevices of your house.
6. Vibration dampers applied as noise barriers.
The primary function of a vibration damper is to reduce the modulation of a vibrating substrate. In the past, multiple layers of a vibration damping sheet has been used to reduce road noise, an airborne sound problem. I'll be honest, I am guilty of having installed 3 or more layers of a heavy damping sheets onto the floor pan of a vehicle. Yes, there is undoubtedly a reduction of road noise because at this point you are introducing a serious amount of mass in front of the offending sound. Vibration dampers do not have the air spring that is required to reflect high energy soundwaves because they are designed to be coupled directly onto a vibrating substrate. The reality of this technique is that it is very heavy, very time consuming and much more expensive than if you were to use a single layer of a specialized barrier material. Our tests have shown that in order to achieve the same level of noise reduction that is achieved from a 1lb ft² barrier, you would need to install 2.2 lbs ft² of a vibration damper.
Not all is lost, a single layer of a vibration damper installed onto a substrate underneath a specialized barrier will increase the STC of the floor system by 10%. Also, a vibration damping sheet installed onto a door panel mounting surface in replacement of the OEM plastic vapor barrier will block sound. It will only block sound if installed over the access holes leading into the door cavity. Granted the damping sheet material may not be as dense as a 1lb ft² barrier, however it is much more dense than the stock flimsy plastic or paper sheeting. The airspring in this case exists between the inner and outer door skins.
7. A noise control reality, the STC of automotive windows.
So many people are caught up in the concept that more is better. Let's take the doors for example, people will install layer upon layer of various materials to ever so gradually increase the STC, reduce vibration or seal the door the door cavity. The reality is that in terms of quieting the passenger compartment, the best you can do is match the STC of the windows. Otherwise, the windows become the weak link and currently, there is nothing you can apply onto an OEM piece of glass to increase the STC while maintaining the ability to see through it. So while it is possible to dramatically reduce airborne and structural borne sound, your weak link will always be the glass (unless you drive an armored vehicle).
8. Liquid dampers on top of sheet based materials, don't bother.
This technique has been recommended by a variety of competitors and really is more a waste of material and hassle to install than anything else. Liquid dampers do not reinforce sheet materials or any component of a sheet material. If your application requires structural reinforcement, then consider looking beyond standard damping materials and techniques. Liquid dampers are great from the installation standpoint if you are planning on treating a large surface area at once. Liquids can be sprayed which makes application fast and easy. They also work extremely well if attempting to treat hard to reach areas of within a passenger compartment. The only disadvantage (if it can be called one) is that liquids take time to cure and become fully effective. This is usually about thirty days after application. Sheet based materials offer instantaneous damping performance because there is obviously no cure time. Yes, they both have different damping characteristics and levels of performance, but combining the two is not the way to maximize performance of a damping treatment. Rather, consider using all of one material or the other. If you want to improve performance (this is recommended), create areas of high spots and low spots at various points on the treated surface. This will allow for the damping across a broader range of frequencies.
9. More is better? Not really.
More is not always better and the days installing massive amounts of material are long gone. Proper use of available materials and complete coverage is the key to an effective, lightweight sound control system that can be installed in a reasonable amount of time. Think about this, do you believe the "More is Better" theory works in aircraft or marine applications? No, it does not. How about aerospace? Uh, no. Would the builders of advanced military troop transport vehicles do some of things our competitors are suggesting? Absolutely not! So if other companies are claiming to be so advanced in the development of their materials and techniques, how is that their approach to automotive noise control is so outdated and misunderstood?
The world trusts CAE.
We have been in this industry for a long time and have done hundreds of vehicles (land, sea and air) in almost every possible configuration. We have also owned and operated a car audio shop and separate speed shop. What does this mean to you? This means that you can benefit from our experience in understanding what does and does not work. Our technical support is and always has been unparalled in this industry and we are very proud of this fact.
Our technical support does not cost you a thing. Without it, you could be spending more time and money than you need to.
Until next time.
Paul M. Messett
Cascade Audio Engineering
Sales & Marketing Manager