How to: DIY Sound-Proofing with Professional Results
Disclaimer: This tutorial is intended as an informative guide. Do not carry out any vehicle repairs nor modifications unless you are technically competent. Apex2Apex accepts no responsibility for any actions undertaken by the reader.
There are very few vehicles on the road which cannot be improved with strategic application of sound-proofing materials. In particular, those of us who own “fun” cars or classic cars often find that sportier or older models can suffer from all kinds of issues related to unwanted noise. These can come from many sources, such as squeaks & rattles, tyre noise, wind noise and powertrain noise just to name a few. So, for those of us who don’t have stripped-out track cars, what can be done to improve things a great deal without spending hundreds, if not thousands of pounds on getting a sound-proofing company to do the work for you?
Reducing Road Noise: Theory
For the sake of simplicity for the DIYer, this guide will break down sound-proofing into two simple stages, mass loading or sound damping, and noise absorption
Car body panels are typically made out of thin sheets of metal with as low a mass as possible to save weight. As you drive, vibrations transmitted from the road pass through your vehicle and these large sections of thin, light sheet metal vibrate as well. Unfortunately, this is a major source of road noise in the cabin as they act like the thin, light membrane of an audio speaker.
Mass loading is the process of selectively adding mass to these sheet metal sections in order to bring their resonant frequencies right down and ultimately, to stop them acting as speakers which vibrate with the road and transmit this as a soundtrack to the interior of your car.
The second stage is to create a physical barrier to break up or deflect the energy of the sound waves, this often involves using layers of material such as foam. This is particularly important in areas of the vehicle where there are large cavities behind interior trim that sound can echo in.
Putting Theory Into Practice
The first step is to strip the interior of the vehicle, removing all interior trim panels.
For creating a sound barrier a very cost-effective and also extremely light material is laminate floor underlay, preferably one which is rated to a sound reduction level. This has the advantage of being waterproof as well. In the above picture, several layers of this gold material (with foam underneath) has been added to the boot floor. This physically blocks and absorbs some of the road noise coming up through the boot floor. On a hatchback, there is quite a lot of noise under the boot floor so even just adding this under the boot carpet makes a noticable difference, as this sound then echos around the cabin.
Next up, there is a large gap behind the rear quarter trim panels, below the window where the body of the vehicle can vibrate and cause sound to echo in the cavity:
This must be mass-loaded in order to reduce the resonant frequency of the outer body panel to stop it acting like a large, high-pitched speaker. For this, it is a good idea to use good quality, genuine sound-deadening materials. This tutorial made use of SilentCoat, but several other brands are also available. These materials do actually also act as a barrier to the sound in addition to adding mass to a panel, but using them throughout the vehicle as a barrier is not a good idea as they are both expensive and heavy. Therefore this tutorial uses them selectively to weight the centre of large panels. Below is the now mass-loaded panel:
In order to complete this section of the vehicle, the laminate flooring underlay was adhered over the top using Trim Fix spray adhesive. This is very strong and easy to work with. Other adhesives not intended for this purpose will fail under hot or damp conditions, so it is important to use the correct adhesive. This material completely fills the cavity behind the trim, which stops sound echoing in there and also has the added advantage completely stopping the trim from rattling:
The roof is another large, vibrating panel. This vehicle has a sunroof fitted, which reduces the surface area of the roof, but nonetheless, it is still a big area of thin sheet metal. First, we mass-load it:
Then we use strong adhesive to add the underlay to this to absorb the energy of the sound:
The doors are also a significant source of noise. Not only do the internal components and the window often rattle, along with the doorcard itself, but the outer skin of a door is a particularly large, thin panel, typically with very little support as can be seen in the picture below:
Fortunately, SilentCoat is self-adhering, so it can be cut to size, pushed through the gaps in the inner skin and stuck in the middle of the panel with some dexterity of the hands. This is of course necessary to mass-load the outer skin:
Adding underlay to the door is quite difficult. It cannot go between the inner and outer skin because this is where the window goes when it is wound down. It is often and very tight fit, but a layer can go on the back of the doorcard. In order to achieve this, put the underlay over the back of the doorcard and use it as a template to cut it perfectly to size, leaving gaps for any fixings or bolt holes that mount the doorcard. It may also need to be trimmed to accommodate things mounted to the inside of the door. This is then adhered to the back before refitting the doorcard:
There are plenty of areas of the vehicle not yet covered by this tutorial. These include but are not limited to: the rest of the vehicle floor, the area behind the dashboard and in particular, the wheel arches, which are a huge source of noise. A guide on how to reduce the road and tyre noise entering the cabin from the wheel arches is planned for the future. For now, there’s plenty of other noise-reduction for you to be getting on with, so get cracking, and enjoy the peace and quiet of your now silent vehicle, or perhaps just delight in being able to hear your loud exhaust and induction even better!