Land Rover Defender Blog

Budget Bonnet Sound Insulation

Today, FunRover Contributor Willem, looks at how you can reduce the road noise in your cab by adding some sound insulation material to the underside of the bonnet. You can also check out his excellent Cargo Rail Fitting Guide here.

Willem, 31 yrs, living in the Netherlands and a passionate Land Rover Driver. I own a 1998 300tdi Defender 90 hardtop which I drive on club days (LRCH) and in the weekends for recreational trips. I share my passion for Defenders with others on www.landrover-defender.eu (in dutch).

Sound isolation material will add a great deal of enjoyment to using your Land Rover. In my case the sound level is 87.6 dB (to give you an idea, a lawn mower is around 90dB) in the cab with the factory insulation installed.

But just like a cloth head lining, the sound deadening material has a tendency to start sagging after a while. It begins to look untidy, and it does make me concerned that potentially ripped and sagging pieces could fall into one of the belts or hot engine parts, causing further problems.

The factory sound insulation material on the underside of the bonnet.

Below: A previous attempt to fix it by strapping it with tie-wraps to the sides of the bonnet

Replacement sound-proofing is costly, so I'm trying a budget replacement to see how well it performs. I found insulation matting for €25, and that was for 2 pieces of 100 x 50 cm. The material is also heat resistant, which is a must for this type of application.

Here's the brand and material information.

Fitting

Fitting is pretty simple, you just need some basic tools and some patience. First I start with removing the old insulation by drilling out the pop rivets that hold the blanket into place and drill away the remaining pieces to get a flush fit.

Don’t forget to blow away the metal shavings from drilling, they might work their way into moving parts such as belts or prop shafts and cause extra wear or damage.

Then it’s time to clean the underside of the bonnet. This means the new adhesive we'll apply won't react with old glue and has a nice, clean surface to bond to.

A nice clean bonnet, time consuming but makes for a very tidy installation.

To get a good comparison before / after, I measured the sound levels of the bare metal bonnet. The reading came to 89.5dB. This is 0.5 away from being as loud as a lawn mower. Apparently, 8 Hours exposure to 90dB can cause damage to your hearing and anything above 85 means you have to raise your voice a little.

89.5dB is 0.5dB away from being physically harmful to your hearing. Hopefully now you can see the importance of sound-deadening material.

Now I need to fit the new insulation to the bonnet structure. Cutting with a knife gave the best result. I've also scored protective backing on the reverse side. This way I could tear it off in pieces which made it easier to glue on and align. It also means you don't have to stick it in places you don't want, reducing waste.

The material applied. Looks very neat and just like a brand-new factory fit.

Closer look.

I've used a paint roller to press the foam firmly against the bonnet. This worked well and gave an nice, bubble free finish.

With the replacement matting in place, it gives a great factory-finish look, but most importantly the sound level has dropped a whole decibel than the original Land Rover application and the difference between no insulation and the new insulation is massive. This makes it a very worthwhile job and for the price of the matting and a pot of glue, equally cost effective. There's a decrease of sound in the cab when stationary and the engine compartment is now tidier, and pieces of insulation are no longer likely to drop into moving parts. A very simple job, but highly recommended.

Insulation material is available from companies like LaSalleTrim (for an acoustic damping mat) or NoiseKiller provide pre-cut sound dampening kits.

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