Thread: How to make a Defender Quieter

Right, so that’s a basic description of the main noise sources, so lets now look at the other one, transmission path.
There are 2 transmission paths to consider – airborne and structureborne.


1. AIRBORNE – as the name suggests, air is the vibrating medium transferring the noise from the source to your ears. Reducing this transmission path can be as simple as putting something in the way, to either absorb the noise, or block it.
Absorption is generally using foams, felts, thinsulate or other porous fibres that dissipate the energy and works best on higher frequency noise that has a short wavelength. This is mainly because trying to use foam several metres thick to absorb low frequency noise is neither practical nor cost effective!
Blocking the noise, or creating a barrier, is more effective and usually more practical for lower frequency noise and this requires mass. The higher the mass, the better, because the transmission loss (TL) of the barrier follows the mass law, which means that every time the mass of the barrier doubles, the TL increases by 6dB.
For example, if you have a 70dB noise and a barrier with a TL of 10dB, then on the other side of the barrier would be 60dB. If the mass of the barrier is doubled, the TL goes up to 16dB and you have 54dB on the other side.
However, if you double the mass of the barrier by having 2 barriers, with a sufficient airgap between them, you get two lots of TL at 10dB.
Barriers.jpg

2. STRUCTUREBORNE – this path can be considered as everything except air. For example, engine vibration through the chassis rails and into the cabin, or the road noise transmitted through the suspension into the cabin. Reducing this path is generally by means of isolation, such as the rubber mounts (and/or hydromounts) on the engine and the bushes and spring isolation rings on the suspension.
Like the noise sources, it’s often difficult on a DIY basis to do much with structureborne paths, unless there’s something bleedingly obvious like knackered engine/transmission mounts or suspension bushes that can be replaced.


However, it’s at this point that it’s worth revisiting the modal separation that I mentioned earlier with the wheel example.
Essentially this means that the resonant modes of major structural items, major body panels and the frequency of major noise inputs are separated as much as possible. This attempts to ensure that any transmission path doesn’t act like an amplifier.
Other than our wheel/tyre example, another simple one to consider is the cooling fan. On most passenger cars, this will be an electric fan running at one, possibly two speeds and is usually mounted on the body with rubber mounts.
Let’s assume we have an 8 bladed fan running at a constant speed of 2000rpm. This will generate 2 main noise frequencies; one at 33.3 Hz (2000rpm/60) and another at 266.6Hz (33.3 x 8).
The noise at 33.3Hz is referred to as the first order (since it is the primary rotation speed of the fan) and the 266.6Hz is the 8^th order, or blade-pass frequency.
The 33.3Hz will be predominantly structureborne because it is a function of the imbalance level of the fan, imparting a force onto the mounts. There are quite a few items in a car that could have a resonant mode close to this, such as the steering column, the roof or possibly the rear hatch/tailgate. If the resonant modes of any of these panels were to align with the cooling fan, then whenever the fan was on, there would be a very loud, unpleasant low frequency boom in the cabin.
Let’s now assume that the cooling fan speed can’t be altered because doing so would jeopardise engine cooling efficiency (or more likely, you’ve inherited a carry-over fan from a previous model and it’ll cost too much to change it….). Fixing the problem is now limited to trying to improve the isolation of the fan mounts or moving the modal frequency of the body panels away from the input frequency of the cooling fan. This can be done by stiffening them, thus moving the resonant frequency higher, or by adding mass and lowering the frequency. If neither of these is possible, then other methods are used such as tuned, dynamic absorbers (this is a whole topic in itself…), or by the application of damping pads to limit the amplitude of the resonating panel.

The tin can nature of the body panels means that it’s this last one, the damping pad, that is of particularly relevance in a Defender, or Series.

OK, that’s a bit of an explanation of some of the relevant background, so let’s get to the practicalities of how to apply this to a Defender.
This is a combination of what I have done to my 2014 Puma Defender 110, what I would do differently if I were to do it again, great ideas I have shamelessly nicked off other people, and stuff that would work in theory, but I haven’t tried.

STEP 1 - SEALING
Before even starting to look at any sort of noise insulation, the most important thing is to ensure that as many holes, gaps, cracks, grommets, seals etc. as possible are either filled in, covered up or seated properly. Doesn’t matter how much insulation you fit, if there’s holes you will get noise!
Park the Defender in the darkest place you can find (I put mine in the garage. At night.) and then arm yourself with the brightest light you can get and put it inside. I started with the light in the back and, lying underneath looking up, anywhere I could see light shining through I set to with the Sikaflex covering up the panel gaps, seams etc. as I went.
From memory, particular areas to pay attention to are:
Rear door seal;
Around the fuel filler neck;
The panels under the rear seat/rear passenger footwell (in a 110);
The base of the B-pillars (and the seal of the wiring harness that goes through here);
The panel seam behind the front seats;
The driver and passenger seat box (take the seat base out and lid off and shine the light down here);
Seals/grommets through the engine bulkhead.

The age/condition of your Defender will potentially determine how long this takes and how much attention is needed. It took me a good few hours over 2 nights to do mine and it was brand new!
The one thing I couldn’t get sealing right was the rear door and I eventually resorted to applying a thin strip of adhesive foam around the inside edge of the door to take up the gap to the rubber seal on the body.
Talking of door seals, the later Defender seals seem pretty good, particularly the additional ones on the lower edges of the doors and are worth fitting if yours doesn’t have them.

The added bonus of doing this sealing is that it’ll help your AC, if you’ve got it, run more efficiently and it’ll help prevent dust ingress.

STEP 2 – DAMPING PADS
Right, first things first – you do NOT need to cover the entire surface of every square inch of bare sheet metal with dynamat, roadkiller, or whatever make of bitumastic damping pad (also referred to as sound deadener) is your favourite flavour. You are just adding weight and emptying your wallet.
Damping pads are just that. For damping. Think of them as the sheet metal equivalent of the shock absorbers (dampers!) on your suspension. Their function is to dampen the oscillation (the vibration - ringing, rattly, tinny noise) of the panels sufficiently so that they sound ‘dead’.
Admittedly covering everything in sight with damping pads does work, but by doing this it is no longer acting as a damping pad, it has become the equivalent of increasing the mass (increasing the TL and reducing the resonant frequency) of the sheet metal. There are better ways of doing this.

OK, now I’ve got that off my chest, here’s a list of places that benefit from the addition of some damping pads.

• Driver and passenger front floor
• Front face of the seat box
• The metal access panels covering the seat boxes
• The metal panel underneath the centre cubby box (I did this underneath, with foil backed pads to attempt to keep some heat out of the cubby box)
• Rear passenger floor (in a 110)
• The vertical and horizontal surfaces directly underneath the rear seat (in a 110)
• The rear wheel arch/wheel boxes
• The vertical panel above the wheel arches
• Around the big rear side windows (or the panel if a hard top)
• The rear door (take the trim off and try to stick the pads to the external metal, not the internal frame)

Admittedly this is nearly everywhere, but as a rough rule of thumb you shouldn’t need to cover more than half of each individual panel. Wherever possible, I tried to do this from underneath the car, however this isn’t always practical and needs to be done on the inside. This could also determine the amount of coverage – if you want a nice smooth surface, go for it, covering all you want.

There’s a few things that I didn’t cover, mainly the roof and the side doors.
I didn’t do the roof because I couldn’t do with the hassle of taking the roof lining off, but if it ever does come off, then I will put some pads on the roof. And I didn’t do the doors for a similar reason but felt that the doors on my Puma weren’t too bad. I have however put damping pads on the inside of the doors on my Lightweight.

I also didn’t cover the rear load floor. This has strengthening ribs, so relatively speaking isn’t as bad as some of the other panels. I also had plans for this involving a storage system, so I didn’t bother.

STEP 3 – ABSORPTION FOAM
This part is optional depending on how your Defender is set up. If you have a 110, particularly a hard top with nothing in it (or even a SWB hard top series), then this is worthwhile.
What is needed here is open cell foam to line the bare panels as this helps with noise absorption and stops the reverberation (echo) in the back. Basically, the vertical surfaces above the rear wheel arches, around the rear side windows (if fitted). Any non load bearing surface that doesn’t need to resist wear will benefit. The roof too.
The issue with open cell foam is that it absorbs water, so if your Defender isn’t too watertight, then perhaps give this a miss. Closed cell foam is marginally better than nothing but will not work as well as open cell foam. On the doors of my Lightweight I have fitted quite thick, densely piled carpet and anything similar to this (essentially soft and ‘open’ in texture) will work better than closed cell foam.

I haven’t fitted foam to the rear of my Defender as I have fitted a storage drawer system which I have covered in marine carpet. The back is also where the dog lives and she’d just rip the foam to shreds anyway!

If I were to choose a material, I would look at the car builders bonnet liner. This is open cell but has a scrim layer on the top which makes it look a bit neater and will help wear resistance.

If I ever get around to removing the roof liner, I will seriously look at putting a layer of 3M thinsulate on the back surface of the trim before I refit it. I haven’t costed this up (it isn’t cheap…) but this stuff has by far the best absorption coefficient of anything of comparable thickness and will do wonders for reducing wind noise.

STEP 4 - BARRIERS
This is where the panels are mass load to increase the TL.
There are a few ways of achieving this, but probably the best and easiest is with a mass loaded vinyl layer (not by smothering everything in damping pad material). These are available in various densities, but you need to be looking at least 4kg/m2 to be effective. Pretty much anywhere where the damping pads have been fitted (that hasn’t already had foam put over it) will benefit – the floor pans, seat box, rear load area, rear wheel arches etc.

The mass loaded vinyls are available on their own and are about 2-3mm thick, which is great for sticking over the front seat boxes, or they come with a foam backing and are approx. 10mm thick.
Where possible, the foam backed vinyl is the one to go for. The foam acts as a de-coupling layer between the body panel and vinyl layer to give the effect of an air gap and allow a higher level of TL (you were paying attention earlier, right?). Like the foam used for absorption, this should be open cell but as anyone who has experienced the soggy mess in the footwell of a series 1 Disco knows, this does have its issues! Hence most foam backings are closed cell.

I’ve put foam backed vinyl in the front footwells, under the rear passenger seat and all along the tops of the rear wheel arches.
For perfectly flat horizontal surfaces, an alternative to a vinyl layer is the good old carpet tile fitted upside down. I have done this in the rear load area underneath my storage drawers and in the rear passenger footwell.

STEP 5 – OTHER STUFF
There’s a couple of other things worth mentioning, one that I have done and one that I haven’t.
One of the ideas that I shamelessly nicked from another member on here was to fit plastic truck mudguards to the underside of the rear wheel arches to stop the machine-gun rattle of stones pinging off the metal. Can’t remember whose idea it was originally but a search for dynaplas truck guards should reveal the details.

The one that I haven’t done is to line the underside of the bonnet or fit an engine blanket. There were a couple of reasons for this. Firstly the 2.2 Puma is (by Defender standards) pretty quiet and has a very effective engine top cover. I was also put off by the experience of lining the bonnet of my old disco, which got covered in muddy water, sagged, torn and generally looked crap and left a mess when I tried to take it off. If I knew I could trust myself to do a good job and I could find the right material, I’d possibly give it a go one day!
Depending on how well sealed the bulkhead on your Defender is (or how good the screen vent seals are) will determine whether you would notice the difference of fitting a bonnet liner. A good, well applied, approx 10mm thick bonnet liner will help absorb some of the higher frequency ticks and clicks and whistles. A heavy engine blanket, if fitted properly so that it goes behind the engine a little bit, not just sits on top, will also reduce a lot of engine noise. But if you've still got big holes everywhere, then you'd not notice any difference....

Great thread DieselDan! Thanks for the time you've taken to put this up.

Since buying my Td5 Deafener last year I have been looking at various ways of getting the noise down to levels more like my D2. I have all of floor panels out to reseal them at the moment and I'm adding soundproofing as I go including a heavy exmoor trim seatbox cover.
I thought I'd share some images of the carbuilder bonnet liner you mentioned which I can definitely recommend. The Dynamat added some serious weight to the bonnet, one sheet of bonnet liner was the perfect size.

Cheers
Discofender

That is the best write up on the subject I have ever seen. Thanks - massive effort but one I will refer to as I improve my 110.