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Fitting Sound Insulation

DIY quietening of the cabin

Julian Edgar, pics by Georgina Cobbin

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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!)

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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

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Sound insulation for cars comes in three basic forms:

  1. Bonnet 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 bootlids.
  1. Noise 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 adhesive.
  1. Anti-vibration 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 firewall.

Original Equipment?

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.

Our Job

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 Bonnet

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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.

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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.

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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.

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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 insulation supplied.

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The remaining loose and flaking original insulation material was then brushed off...

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...and then the middle point of the bonnet was marked.

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The backing paper was then peeled away from the adhesive, starting with a strip down the centre of the insulating pad.

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The centre of the insulation pad was then lined up with the centre of the bonnet and the exposed adhesive pushed against the metalwork.

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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.

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As can be seen here, the insulation material stretches and conforms well to the shape of the bonnet inner.

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The insulator was then trimmed at the edges. This is very hard to do neatly, so mark the chosen cut line with a texta...

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...before cutting along it with a pair of sharp scissors.

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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.

The Boot

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.

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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 insulation.

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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.

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Building adhesive was applied to the original sound insulation and then...

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.... the new insulation could be stuck on top.

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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.

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The easiest approach to applying the insulation was to get into the boot.

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Final trimming could be carried out after the insulation was stuck into place.

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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.

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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 edge.

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The moulding was then slid over the exposed edge of the insulation, being held in place both with glue and double-sided tape.

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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 off.

Under Carpet?

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 firewall.

Testing the Bonnet and Boot Insulation

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 move.

Sound Pressure Levels?

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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 same readings!

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 sound insulation.

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