| Damping a Tin Can?
By Paul M. Messett / Sales & Marketing Manager
damp•ing [dam-ping] –noun Physics.
1. a decreasing of the amplitude of an electrical or mechanical wave.
2. an energy-absorbing mechanism or resistance circuit causing this decrease.
3. a reduction in the amplitude of an oscillation or vibration as a result of energy being dissipated as heat.
Daily, we are flooded with calls from customers asking how they go about determining where to begin installing an automotive noise control system. Typically, we ask a lot of questions in regards to what type of vehicle they wish to treat, how much time is allotted for installation, budget and so on. Initially, many of our customers are not so concerned about making the interior of the vehicle as quiet as possible, they just want to improve the overall sound quality of the audio system. This does not necessarily mean that they have to install several types of products all throughout the vehicle right away, it means that they may have to start by simply treating the doors.
Car doors are tin cans and some are worse than others. Newer automotive body panels are flimsy at best because of the thin sheetmetal, plastic or composite material construction. These panels will vibrate due to the presence of a speaker in the door itself. The sound energy radiating off of the back of the speaker cone is impacting the surface directly behind it and then continues to bounce around the inside of the door cavity. Furthermore, an active speaker can transmit vibration due to the fact that it may be directly coupled to a flimsy surface. Also, depending on the vehicle, structural resonance and vibration may caused by the drive train. Vibration and levels of intensity may also be attributed to the road surface. Often, OEM’s will use a minimal amount of a vibration damping material on the inside surface of an outer body panel. This can include liquid based damping compounds, baked on vibration damping sheets and CLD (Constrained Layer Damper) products. The introduction of aftermarket vibration damping materials will tighten up the structure of the door, reduce airborne sound transmission into the passenger compartment and dramatically improve overall stereo system sound quality.
The Outer Limits
Some would say that you have to remove these products and you can, but why? They are already functioning as a damper, rather than going through the hassle of scraping these materials, add to what is already there. Furthermore, good luck trying to remove a baked on product from the inside of a door cavity without leaving some remnants of your attempt on the panel (pressure dents). Apply damping materials (VMAX or VB1X) to the untreated areas and in most cases this won’t be hard to do depending on the door. Keep in mind that most damping materials become less effective below 200hz. You can install two uniform layers of damping material to improve damping performance but if you want to eliminate modulation entirely, structural reinforcement is required and that’s another blog entry.
VB1X on inside of outer body panel (Toyota Tacoma)
The Vapor Barrier
To make matters worse, most (not all) OEM’s choose to use flimsy plastic or foam vapor barriers behind the door panel. Although these materials may prevent or limit moisture from entering the passenger compartment, they are horrible dampers and even worse barriers. There are also numerous penetrations in this vapor barrier which allow the speakers back wave to pass through and interact with the speakers front wave. Can you say “acoustical short circuit”? When treating this surface, it is possible to install a damping material that acts both as a damper and a better barrier (VB2 or VB2HD). Now, it not possible to install a 100% penetration free barrier because there will always be linkage rods or cables and harnesses that need to pass through and onto the door panel. However, it is possible to improve upon the OEM design and reduce the amount and size of the penetrations. The VB2 and VB2HD vinyl based vibration damping material is highly moldable, features an extremely aggressive adhesive and will conform to the most irregular of surfaces. What does this mean? It means that when you are able to couple the material to almost 100% of the vibrating surface, the damping performance is maximized. Also, the VB2 and VB2HD damping materials are far more dense than the flimsy plastic sheet the OEM provided. When applied onto the surface and over the access holes, the damping sheet will act as a barrier effectively limiting the passage of airborne sound waves that would normally enter the passenger compartment. The VB2 and VB2HD products are also the only materials that can be painted to match the substrate for a truly custom finish. Yes, a heat gun or hairdryer is required to mold the material onto the surface, not to activate the adhesive. You can never get this type of a cosmetic finish from a CLD (Constrained Layer Damper)! Take a look at the following photos, the results speak for themselves.
Toyota Tacoma Door (Before)
Toyota Tacoma Door (After)
Don’t Stop Now
To finish off a door system properly, you’ve got to install something to fragment speaker back wave. Our Deflex PowerPad has been designed (and knocked off by many) to fragment the sound wave that emanates off of the back of the speaker and re-direct the energy sideways so that it cannot be reflected back into the speaker cone. Have you ever noticed that the speakers you just bought for a ton of money that are wired to the really expensive amplifier in your trunk breakup, pop and distort at mid to high volume? This is because the sound energy that comes off of the back of the speaker cone is always equivalent to what is coming off the front of the cone. When you put a speaker in a car door, you are essentially installing that speaker into a location that has a parallel wall (the outer body panel) directly behind it. The sound wave will reflect back into the cone off of that surface. The Deflex eliminates this reflection and improves sound quality tremendously by improving speaker accuracy, removing “hollow” tones and preventing breakup and distortion, that is if your amplifier, EQ and/or crossover settings are in check. If you have never used the Deflex, use it! There is no other way to eliminate this destructive backwave interference, it is caused by the physical location of the speaker. Other applications of the Deflex PowerPad will be discussed in future blogs.
Deflex PowerPad inside car door behind speaker location (Honda CRV)
Don’t Ever Do This, Please, I’m begging you!
Do not put foam on the inside of the car door on the outer body panel, ever! This concept has been floated around the internet and is the biggest mistake you can make. Some folks out there have suggested using closed-cell or open-cell foam to absorb airborne sound and speaker backwave. Here is the problem, closed cell foams don’t absorb much of anything. Although they have been recommended to prevent the absorption of water within a door, a closed cell foam will still wick about 10% of the fluid flowing across its surface. Also, closed cell foam cannot absorb high energy sound waves, period. In our home theater designs, we do not spec thin closed cell foam on the walls to absorb first reflection, rather we spec 2” thick cotton, fiberglass or poly material. Not much room for 2” of anything inside a car door now is there. In closing, foams in a car door will delaminate and/or fail for a number of reasons, allow me to list them…
For reference, take a look at what our competitors suggest as a treatment for the exact same door application…
1. Excessive moisture will destroy foam
2. Excessive moisture will cause spray adhesive to breakdown
3. High temperature will destroy cheap foam
4. Weak adhesive will cause delamination
5. Poor surface prep will cause delamination
The Force is Strong
I hope this brief description of treating a car door has helped you in some way. Realize that a car door can be turned into a more solid and tighter speaker enclosure and is easier to treat than you may think. Don’t be afraid to tackle this treatment yourself, some would have you think that it is more difficult than performing surgery, it’s not. I am more than willing to walk you through the steps and calm your install nerves. The effective treatment of your car or truck doors will result in better overall sound quality from speakers systems of all types, it does not matter whether they are cheap or expensive. For more information, you are more than welcome to call me and I would be glad to discuss your specific vehicle (chances are I have done a few of them). Until next time…
Paul M. Messett
Cascade Audio Engineering
Sales & Marketing Manager