Quote:
Originally Posted by Laird Scooby
Try insulating the bulkhead/firewall on the engine side - it acts like a big "sounding board" and the vibrations that it reverberates from the engine will travel up into the screen. Giving those vibrations a soft pad to absorb them instead of a hard bulkhead to vibrate in sympathy will likely reduce that problem.
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When you say on the 'engine side' do you mean applying the insulation in the engine compartment? That would definitely preclude the use of something like 3M acoustic because of the potential exposure to moisture and engine fluids and I am not sure that your typical Mass Loaded Vinyls are rated for the temperatures that they might achieve from exposure to the radiant heat from an exhaust system. If you look at what modern OEMs do they keep their insulation on the inside of the car (with the exception of hood blankets which seem to be more about thermal insulation to prevent heat damage to the paint).
When I did my restoration I extended MLV with a 3/8" foam de coupler from the front floor wells right up the vertical face of the firewall. Volvo originally used what appears to be MLV with a foam de coupler; but, only on the vertical face of the firewall relying on the jute / felt stuff under the front part of the carpet for sound abatement. I didn't apply any sound control material (other than some damper pads) to the approximately horizontal surface under the front air inlet area below the windshield. I had experimented with trying to apply MLV to that location using some 3M spray adhesive; but, the MLV is quite heavy and would not stay in place (or the adhesive did not play nicely with the foam de coupler). In retrospect I should have tried harder and 3M acoustic might be a good solution for that location. The 3M is light enough that spray adhesive or double sided tape should keep it in place. I used double sided tape to attach the 3M to the inside or my roof and so far (6 years) it has prevented the 3M from dropping down on to the head liner.
With all the holes and mounting brackets on the vertical face of the firewall applying sound control material there is a real treat. I think MLV with a de coupler is about as good as you can do because things like the throttle operating mechanism (on LHD cars) and the heater cable and such make application of a thick material like 3M a problem.