If you drive a current model car, the first thing that you notice when you
step into an older model is the level of noise in the cabin. Noise from the
exhaust, noise from the engine, noise from the passing airflow, and noise from
the suspension. It’s one area of car design where huge improvements have been
made. So what options are there if you have an older car and would like a bit
more peace and quiet in the cabin? (Especially if that upgraded exhaust has also
increased in-cabin noise levels!)
One approach is to fit aftermarket sound insulation. If you pick the right
materials and mount them in the right places, a significant reduction in noise
levels can be gained. But note that you’ll never turn a loud old car into a
Lexus LS430 – simply too much needs to be done at the basic design level stage
to be able to retrospectively make that much difference.
So you want to install some sound insulation? OK, what’s available, how much
will it cost, and how do you do it?
Types of Sound Insulation
Sound insulation for cars comes in three basic forms:
insulators – these comprise foam rubber backed on one side by a woven cloth (or
aluminised polyester) and on the other with pressure adhesive. As the name
suggests, they’re suitable for mounting under bonnets and also under
barriers – these materials use compressed layers of cotton-waste (or similar)
sandwiching a thin layer of bitumen. They’re used both to absorb noise and also
to prevent noise transmission. They can be mounted on the firewall within the
cabin (ie under the carpet), under the boot carpet and behind the rear seat in
booted sedans. This noise insulation is held in place with applied contact
materials – these insulators comprise low resonance (acoustically ‘dead’)
materials which are designed to stop panel vibration. In use they’re glued to
the panels. It is important that
the join between the insulator and the panel is continuous, with large amounts
of contact adhesive therefore needed.
In addition you can find more specialised forms of sound insulation,
especially in late model cars. For example, some cars use a sound insulator that
has been pre-shaped to match the transmission tunnel. It might be made of a
thick layer of bitumen with foam rubber underneath. (See the ‘Original
Equipment?’ breakout box below.) Other cars use bitumen that is sprayed on the
body during the car’s manufacturer, while still others use ‘sandwich’ type
firewalls, where bitumen is placed between the metal sheets of a double
If you drive a car that was available in many trim levels – and your car is
near the bottom of the luxury list – check out what sound insulation was fitted
by the factory to the top trim levels. This insulation will fit straight into
your car, in acoustic terms it will be designed specifically for that car body,
and at the wreckers it’s likely to be very cheap.
We decided to sound insulate a 1985 BMW 735i. Despite contemporary road tests
saying how incredibly quiet the car was, these days any new car is much quieter
inside the cabin. (Part of the BMW’s noise problem was also that some of the
original sound insulation was either missing or tired.)
We bought two pieces of sound insulation, both manufactured by Automotive
Carpets Industries Pty Ltd. The first was Sound Shield Under Bonnet Sound and
Heat Insulation, and the second was Sound Shield Floor and Firewall Sound
Insulation. These types correspond with categories #1 and #2 above. The total
cost was about AUD$100 from an auto parts store. Other places to look for high
quality sound insulation suitable for automotive use include marine stores – in
boats it is used to muffle engines.
The BMW’s bonnet originally had sound insulation stuck to its underside, but
age had wearied it. The middle panel was starting to flake off and the two side
panels were completely missing.
The remaining original insulation was scraped off, using a thin piece of
particle board as the scraper. Taking this approach got most of the insulation
off quickly and easily without damaging the paint.
The Sound Shield bonnet insulation uses a self-adhesive layer on one side and
an oil-repelling black cloth surface on the other. It is rated safe for
under-bonnet use, having appropriate fire safety characteristics.
To make sure that there was enough, a quick measurement was made of the
underbonnet area of the BMW. This proved that there was plenty of material in
the pack – it would need a big car indeed before there was insufficient
The remaining loose and flaking original insulation material was then brushed
...and then the middle point of the bonnet was marked.
The backing paper was then peeled away from the adhesive, starting with a
strip down the centre of the insulating pad.
The centre of the insulation pad was then lined up with the centre of the
bonnet and the exposed adhesive pushed against the metalwork.
Working outwards from the centre, the paper was peeled back and the pad
pushed against the inside of the bonnet. Taking this ‘from the middle and then
working out’ approach makes it easier to apply the insulator over the ribs on
the inside of the bonnet.
As can be seen here, the insulation material stretches and conforms well to
the shape of the bonnet inner.
The insulator was then trimmed at the edges. This is very hard to do neatly,
so mark the chosen cut line with a texta...
...before cutting along it with a pair of sharp scissors.
The finished result. You may be able to achieve a neater edge by placing a
piece of particle board temporarily under the insulation and then cutting along
the line with a sharp knife. (We thought of that technique only after finishing
with the scissors...)
Testing the Bonnet Insulator
Before fitting the rest of the insulation, we tested the car. Engine noise
was diminished – noticeably so when starting the car, but only a little in
normal driving. Given that there was previously an incomplete bonnet sound
insulator already in place, this result seemed reasonable.
We decided to cut up the second piece of insulation and use it in two areas
in the boot. The BMW had quite a lot of exhaust noise and this was being
transferred to the cabin through the rear deck and rear seat. Inspection inside
the boot showed that a sound insulator had once lived beneath the rear deck, but
other than glue, no remains of it now existed.
The first step was to remove the trim piece that sits between the wheel
arches, behind the seat. This panel already had insulation on it but we used it
as a template to cut out another piece that could go on top of the existing
The bitumen sandwich insulation is still quite flexible, although not as much
as the bonnet insulator. Cutting it can be achieved with scissors, but
heavy-duty shears are preferable.
Building adhesive was applied to the original sound insulation and then...
.... the new insulation could be stuck on top.
The underside of the rear deck was the next to be insulated. Glue was applied
and then the insulation pushed into place. The 1500 x 1000mm piece was enough to
do both jobs, with a little left over.
The easiest approach to applying the insulation was to get into the boot.
Final trimming could be carried out after the insulation was stuck into
Here’s the view from inside the boot. The red arrow points to the panel which
now has an extra layer of insulation behind it and the green arrow shows the new
insulation applied to the underside of the rear deck.
To neaten-up the exposed edge of the insulation, a U-shaped plastic moulding
was bought from a hardware store. Small V-shaped cuts were made at intervals
along the underside, allowing it to be bent to conform to the shape of the
The moulding was then slid over the exposed edge of the insulation, being
held in place both with glue and double-sided tape.
The finished job. The insulation is unnoticeable unless you feel the urge to
lie inside the boot.
Speakers and Vents
If your car has rear deck speakers and the undersides of the speakers are
connected to the boot volume, you will degrade speaker performance by placing
the insulation across the back of them. Cut a hole in the insulator to allow air
movement generated by the rear of the moving cone to still flow into the boot.
Many cars have cabin air outlets located at the back of the car – eg hidden
behind a bumper. The air reaches these vents through openings placed in the rear
deck – again, if you find ventilations holes or grilles, don’t block them
In this case there was plenty of clearance for the insulating layer that we
added. However, that’s not likely to be the case if you place a thick layer of
insulation under carpet. Because there’s not the clearance for the new
insulation, you’ll find that the carpet doesn’t fit nearly as neatly as it once
did, and holes in the carpet (eg for seat and seatbelt mounts) won’t line up as
they did. There are ways and means around these problems, but keep this extra
work in mind if you intend placing thick insulation on the floor and
Testing the Bonnet and Boot
With both insulators in place, the car is now obviously quieter. The exhaust
noise which once boomed in under acceleration is muted – the exhaust at full
throttle now sounds about as loud as half throttle did before! The noise from
the rear suspension is also reduced (although this is a small difference) and
with the reduction in engine noise caused by the underbonnet sound insulation,
the car is now clearly quieter than it was before.
Worth it? Yes, for about AUD$100 and an afternoon’s work, it’s a good
Sound Pressure Levels?
In addition to the subjective judgements made above, we also took along a
sound pressure meter on all our tests. We had figures for standard, with the
bonnet insulator alone, and with both lots of insulation. But the trouble is,
the readings simply didn’t stack up with what we could clearly hear.
With the sound insulation installed, the resonant boom of the exhaust was
absolutely and certainly decreased in level – but the SPL meter showed much the
Perhaps either a higher quality sound pressure meter or a full spectrum
analyser was needed – one thing’s for sure, the SPL meter’s readings simply
didn’t reflect the real-world improvements that were made.
has a good range
of products and some instructional graphics showing how to apply automotive