Originally Posted by
Offender90
I'll get back to this once I have a bit more time, however a brief summary of "quietening" a vehicle:
Noise in the cabin is the sum of:
- noise generated by the engine, drivetrain and wheels being transmitted into the cabin through the panels,
- vibration from both mechanical and aerodynamic sources transmitted into the body panels and re-radiated into the cabin as noise, and
- noise reverberating (bouncing around) inside the cabin, amplifying as it does (can add up to 4dB in a bare defender interior with only seats to absorb reverberated noise).
To effectively reduce noise in the cabin you need to reduce both modes of transmission as well as amplification.
Dynamat type products are effective at "deadening" the panels so they reduce vibration re-radiating as noise. For this to be effective you DON'T need to line the entire panel with Dynamat. It won't hurt (other than damage to the wallet and added weight in the car), but it won't really help either. Dynamat is most cost-effective when placed in patches, where each patch occupies 30% to 40% of the area generally placed in the centre of each flat panel. It is a viscoelastic material that works primarily by dissipating vibration energy into heat. It also adds some mass to the panel which shifts the panel resonant frequencies higher up the spectrum (but generally not high enough to be out of the audible range). I would use it in strips on roofs (several 10cm wide strips placed lengthwise between the stiffening ribs), with perhaps one widthwise strip across the centre, and on door sections (covering approx 40% of the centre of the flat area between the rounded section and bottom of the door). If in a wagon, a thin strip may also be placed on the rear quarter panel, below the rounded fold in the sheet.
Next we need to do something about airborne noise transmission through the panels & into the cabin. Dynamat is a REALLY EXPENSIVE sound barrier material IMHO. Sound barrier performance at low frequencies (i.e. the hardest to block out & therefore controlling) is determined by the WEIGHT of the barrier material. "Dynamat Extreme" weighs approx 2.2kg/sqm - Barium loaded vinyls come in sheet weights of up to 8kg/sqm, and at prices comparable to, if not better than, Dynamat. If you do a search for Sorberflex on here, you should find several of my earlier posts on what I would use, why, and where to buy it. On all panels that are between an airborne noise source and the cabin (e.g. firewall, footwells, transmission tunnel, seat boxes and load area) you need to ensure full coverage and use a heavy (4 to 8kg/sqm) vinyl. I would look for an 6mm to 10mm closed cell foam backed loaded vinyl (that dampens panel vibration and decouples the vinyl from the vibrating panel). Alternatively, strategically located Dynamat strips could be used under the loaded vinyl (wouldn't be my preferred choice).
Noise absorption inside the cabin. - Any "soft" surface will provide some absorption. Seats, people, carpet, roof lining... all help absorb some of the reverberating sound. Noise that's transmitted out from the cabin (through the roof and sides) also reduces the amount of noise that's left inside. Unfortunately, closed cell foam does virtually nothing to absorb sound in our cars, especially the high density type of foam that cheap sleeping mats are made out of. It will help with heat, and may provide reasonable panel dampening if surface glued to panels, but that's about all it's useful for (that I can think of). Noise absorption would probably be the last thing I would "get to", as we can hope to gain at most a 2 to 3 dB reduction.
to give an idea of perceptive differences of the decibel scale, a reduction of:
1 dB - not noticeable by over 90% of the population
2 dB - barely noticeable
3 dB - noticeable
5 dB - clearly noticeable
10 dB - noise perceptively halved
In order for the above measures to reduce noise, we first need to ensure effective sealing of doors, windows and vent flaps. By effective sealing, I mean reasonably airtight. In addition, all firewall penetrations should either have rubber grommets in good condition, or if these have perished, or for large openings with multiple wires running through them, you can tightly pack a "ball" of fibreglass or polyester insulation wrapped in a durable cloth or similar permeable membrane to prevent it from disintegrating and spewing out fibres everywhere, but allowing air to get through. Unless the cabin is effectively sealed, any additional measures are a waste of money.
Note - I'm not convinced that a "noise barrier" type material is helpful under the bonnet - if anything, I'd use an absorptive type foam or fluff to reduce the buildup of sound within the engine bay. (EDIT: Unless driving with windows / flaps open)
Firewall is a different story - I'd use a composite material comprising a thin layer of closed cell foam, a layer of loaded vinyl, with a thick layer of open cell foam, placed closed cell foam against the firewall. A range of manufacturers make it for noise control of firewalls and engine rooms. I've posted details of products I'd use in previous posts
As for quietening the engine itself, I wouldn't have a clue where to start, but have heard that later engines are quieter than earlier ones, and some are definitely quieter than others.
Isuzu appear to have had a reasonable go at insulating the engine itself, fitting an acoustic rocker cover (the outer steel skin's only purpose is to reduce noise), as well as decoupling the oil pan from the engine by using a double rubber seal with special rubber clad studs to isolate it from the engine. I'm led to believe the pump and injectors themselves make quite a racket and, if that is the case, don't understand why they were never dressed with a cover of some sort. There may have been an operational reason - I don't know.
Lastly, fitting a turbo will also reduce exhaust noise emissions by approx 6dB from memory.
HTH
Bojan
) turbo, p/steer, RFSV front axle/trutrack, HF, gullwing windows, double jerrys etc.
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plan on treating the others. Have a stick of silicone handy to plug up any apertures.
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