Following on from some good questions recently about various sound deadening or dampening techniques, and the mistaken belief that ‘Dynamats’ are the be all and end all, I thought a little piece looking at the various products and applications available may be appreciated.
For me, sound deadening is much more than just making sure we don’t have ‘panel rattle’ and the like, it’s a way of tuning the car to get the most pleasurable listening experience I can get, so we are also looking at reducing road and engine noise, as well as reducing reflections, preventing panel rattle and resonance. We are aiming to increase bass clarity, improve the dynamic range, reduce reflections and resonance, and generally get the best chance of having a system that sounds realistic whilst the car is both stationary AND in motion .
Ok, sound deadening or dampening in a car can be broken down into 4 basic categories:
Dampening / Mass loading
Blocking
Absorbing
Isolating
Each of these requires different products, and use different techniques to do their job, some products cover more than one of these, some don’t.
Dampening / Mass Loading
This is probably the most common approach, usually, and unfortunately, the only aftermarket approach to ‘sound deadening’ in a car. Resonance is the addition, or prolonging, of sound energy by the reflection or vibration of, in our case, panels. The idea of dampening is to reduce the resonant frequency of panels to below the frequencies usually played, by increasing mass. These are often bitumen based and is where the Marley tape and Aquaseal type products come into play, however, the best of the mass loaders are usually designed absorb vibration motion and convert it into low level heat, step forward Dynamat and Brown Bread etc. They usually come in sheet forms, or in easier to apply liquids.
Mass loading and dampening are usually used in the car to increase clarity of the bass lines by preventing panel resonance. Whilst it may help to reduce road noise, this is not it’s primary role and is actually a bit of a side effect, as we’ll see in a minute.
When applying we are looking for the best contact with the panel in question. Most effective panels to use it on are ones which have a ring to them when tapped and ones in close proximity to drivers, where, without de-coupling (see below) the chances of vibrations due to the drivers is obvious. It’s this ‘dampening’ that is going to give us the improvement in bass clarity, and the reason both the spl and sq lads drench their cars in it, to present the addition or prolonging of sound through vibration.
Blocking.
Blocking is pretty much as you’d expect, it’s simply applying a barrier to stop sound. These barriers can be made of metals, such as Lead and Aluminium, as well as textiles such as rubber and loaded vinyl. The ability to block sound is called ‘transmission loss’, and is usually directly related to the mass of the product. These products are ideally suited as they simply stop any sound from passing through them and have low resonant frequencies so reduce more low frequency energy from passing though. These products are usually available in ‘sandwich’ form where they are couple to a closed cell foam, (see below), such as Noise Killer. It is also worth noting that the more convential ‘Dynamats’ also provide some blocking ability….anyone ever wondered about the thin alloy covering to Extreme….
Blocking or using barrier matting is the stuff that helps reduce road and transmission noise for that ‘Mercedes experience’. It’s best used on the floor pan, the bulkhead and the transmission tunnel. When applying it’s usually in thick mats which due to the properties of lead, Ali’ etc is reasonably easy to cut and mould to the required shape, it can lay over, rather than be stuck to the metal work.
Absorbing
Sound absorption is, imo, one of the most overlooked, but important aspects of improving the acoustic environment that is the car. Acoustically absorbent material works by causing the sound wave to slow down as it passes through the material due to friction, so the sound wave is effectively turned into low level heat. The ability to absorb noise is has the catchy title ‘Noise Reduction Coefficient’, or NRC. Going back to what we have just read about the friction of the products obviously we expect thicker material to have a higher NRC than thinner stuff.
After market sound absorbing material usually comes in the form of acoustic foam, see Noise Killer, often specifically closed cell foam, however factory fit products include the carpet, and the underlay. Thicknesses of 15 and 25mm are fairly usual. It’s worth noting at this point that the Dynamats and Brown Bread type of products are completely independent and do not absorb sound at all. However, sound absorbing materials are often coupled to the barrier products such as Lead and Ali.
In automotive applications the most important use has to be, imo, for use on over and under dash pads, as well as trimming behind door cards etc, although the bonnet liner is also another extremely useful application. Absorption can greatly improve the midrange and tweets by reducing the nasty ‘near field’ reflections, as well as helping to reduce the possibility of unwanted echoic enclosures, such as the sound from kicks getting caught under the dash and pulling the stage down. Due to the texture of the products I would suggest sticking it (using some sort of spray glue), and then covering with a trim of your choice.
As an added bonus, and whilst related, is a pure by product, foam matting is great for stopping fixtures such as door cards, rattling against panels, such as doors, simply due to the movement of the vehicle..
Isolation
Finally, and an area that I think is quite cool, is the MOST overlooked area of sound dampening is de-coupling or isolation. This can either be done mechanically, through the use of shock absorbing sleeves or devices, see ‘isolatedsystems.com’ or material which keeps a vibrating source independent, or simply products such as silicone or neoprene. Most textiles come as part of a sandwich coupled with the barrier and absorbing products we’ve already discussed.
In an in-car scenario we would use these de-coupler’s to prevent vibrations from passing from a source, into panels, so they may be used between the midbass baffle and the door card, for example, or between the sub and the floor pan of the boot.
Summary
OK lads, there’s a good starting point, I have left some bits out, such as the use of expanding foam, which we can now catagorise thus: Doesn’t provide isolation, but will give absorption, mass loading and perhaps a little blocking sound deadening.
There is a cracking thread ongoing on Carsound by one of Geoff B’s pal’s ‘TheHummerGuy’ who has taken all of the above to the extreme
If anyone is interested in what I use: I have used Extreme on all the panels of my car, but with special attention to the roof lining, boot sides and doors. I have gone over this with Aquaseal, for good measure. I would have been better to use a decent blocking material to cover the bulkhead, transmission tunnel and floor pan, I used copious amounts of Marley tape – wrong product for the application, but the VW insulation is extremely good, and I ran out of cash The sub enclosure in my car when fitted, will be using some neoprene de-coupling sleeves when bolted to the floor pan, and the midbass drivers will be de-coupled from the doors using layers of cured silicone. I will be using closed cell foam for the under and over dash pads, as well as on certain areas of the door card where the horns are causing some nasty reflections.
Well, hoped you liked it lads, you know where the rep button is by know
Kindest,
Sound Deadening - definitive guide to products and application
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