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Interior guides and mods

Sound Deadening

The smart is only a small car and most of the engines are only 3 cylinder so they can be quite noisy

Modification Details

Sound Deadening - What, Why, When, Where & Who?


Sound deadening does exactly that, it reduces sound entering the car.
This can be from road noise, engine noise or just general mechanical noise.
It also stops noise from your speakers creating vibrations or booming on interior panels.


Not only does it reduce the noise in the cabin, it increases the sound quality of the stereo.
The sound doesn't actually pass through a panel, the vibrations are reproduced by the panel.
Sound deadening reduces the ability of the panel to vibrate so less of the sound is passed on.


What do you mean when? That doesn't even make sense. I'm gonna assume you mean How?

Each interior and exterior panel has a set resonant frequency depending on the size, material,
thickness, placement and fixing position. Vibrations from the engine or the road can resonate
at a similar frequency to a panel causing it to vibrate. This is called sympathetic vibration.

When a panel vibrates it is, in essence, mimicking a speaker and creating noise.


A panel is more likely to vibrate if it is large and flat so you should place sound deadening on
door panels, engine covers, roof (on the inside and not on the clear roof), and the bulkhead.


A lot of companies sell competing products that are all very similar. Generally they are a bitumen layer with a reflective foil layer on 1 side and an adhesive layer on the other. The foil reflects heat.

Other products use a closed cell foam, butyl rubber or carpet. Some are sprayed on.

How Do These Work?

The bitumen products work by increasing the weight of the panel and therefore lowering the resonant frequency below that of the vibrations being created. Butyl rubber products are the same.

Closed cell foam, carpet type and spray on products still increase the panel weight but they rely
more on absorbing the vibration like a shock absorber than increasing the resonant frequency.

What Do I Search For?

Here are a few brand names to get you started.
Searching for car sound deadening will find you more.

Dynamat (bitumen)
Brown Bread (bitumen, used to be carpet)
Roadkill (bitumen)
Anti vibe (bitumen)
Ground Zero (butyl rubber)
Audimute (butyl rubber)
Second Skin (butyl rubber)
Lizard Skin (spray on)
Silent Coat (both bitumen and closed cell)

Which Type Is Best?

Chances are you have heard of Dynamat so you might think that it's the best. This isn't
necessarily true. Car manufacturers have used a similar product (with the foil) on cars
for the last 60 years so all bitumen sound deadening products are just copies of that.

Dynamat was one of the first on the scene and has a lot of money to advertise which is why it is so well known. The fact is, car manufacturers use bitumen sheets for sound deadening because it is incredibly cheap. This is the same gear they make roads from.

However, the cheapness of the materials doesn't come across in the price of Dynamat.
Dynamat is very expensive, similar products are cheaper but still expensive.

Unfortunately the price is fixed so no one has to bother making cheap sound deadening.
You can buy rolls of sticky backed bitumen sheet from a DIY store, it's the same stuff.

Butyl rubber sound deadening is better generally.

Drawbacks With Each Type

Bitumen sound deadening has the most drawbacks. It becomes soft when hot which
can release the adhesive allowing it to fall off or move. It becomes brittle when cold so
vibrations can cause it to drop off or become detached. It smells like newly laid road when
it gets hot, you can't use it on the inside of a bonnet (hood for you 'mericans), it's messy
to work with and can only really be applied when the weather is warm.

Brown Bread used to make a sound deadening product that resembled a welcome mat.
A cross between coconut husk and synthetic coir. They have now moved to bitumen.
That can only be for a few reasons. Price, it wasn't as good or it wasn't fireproof.

Spray on sound deadening is messy, you need to strip the car to apply and panels will
need prepping to allow it to adhere correctly. You'll also need spraying equipment.

Butyl rubber sound deadening seems to be the best choice at the moment. The adhesive is
better so it stays in place when it gets hot. It is flexible rubber so it isn't affected by hot
or cold weather. It is easier to cut and lay plus it doesn't make a terrible mess.

The obvious drawback to all sound deadening is that it adds weight to the car.
Not really enough to matter but if you are a 0-60 freak, forget it.

I Have Seen Boot (Trunk) Carpets That Lay In

Nice idea but the smart already has foam and carpet in the car, adding more of the same
isn't going to do a whole lot and won't be nearly as affective as mixing it up a bit.
Also, the prices of these carpets are usually more than you'd spend doing your whole car
with decent sound deadening that is invisible when fitted (hidden under carpet and panels).

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