Bonnet Soundproofing - Phase 4

The Defender is a much noisier vehicle than most road cars, there are a few reasons for this including that there is no sound proofing around the bonnet and the bonnet sits over the front wings rather than closing into them.

I have tried to address both these issues in order to reduce the engine noise.

Before applying any sound proofing, the Defender was taken to an open space so that a relative measurement of the engine noise could be taken. The picture below shows a mobile phone running a sound meter app on a tripod placed 3 metres from the near side front wing:

Before testing, a reading of the ambient noise was taken. There was some variation because of a vocal bird population in the vicinity.

On starting the engine and letting it idle, the noise level rose to 70dB. 

With the baseline sound level taken, the work to sound proof the bonnet could begin.

The underside of the bonnet was wiped down with methylated spirits to ensure it was clean and oil free. It should be noted that the Defender already had some minimal sound proofing from a Noisekiller kit and a Noisekiller engine blanket (Shown below). Both of these were in place for the baseline testing. 

The entire underside of the bonnet was covered in 2mm silent coat dampening mat.

A piece of foil faced 10mm closed cell foam was cut to match the profile of the underside of the bonnet. This foam was sourced from Harrison's Trim Supplies

The foam is non adhesive, so a liberal coat of Trim-Fix adhesive was applied to the back of the foam.
A self adhesive foam is available, however it is only 7mm thick.

The underside of the bonnet was also covered in the Trim-Fix adhesive.

The 10mm foam was the applied.

A strip of 20mm x 10mm self adhesive foam was positioned along the edge of the bonnet to help seal the bonnet to the front wing.

The bonnet can be seen here closed against the wing.

At the front of the bonnet, a small cutout in the foam was made to allow easy access to the bonnet release.

After the work was completed, the sound measurement testing was repeated. The result was 67dB, approximately 3dB lower than before.

Overall the project was worthwhile, the engine is noticeably quieter from outside the vehicle and when driving with the window down. The noise level with the windows up, has not changed because the dominant noise sources are from the bulkhead and gearbox tunnel.