Missing out on your new car stereo system due to road noise? Do you find
yourself yelling to your passenger to carry on normal conversations? Well, hereís
some good tips and ideas for a whisper quiet interior.
In the (early) Z-Car, there was pretty much nothing installed in the way of sound insulation. This is great for weight savings, but tough on the ears. Just to give you an idea, the first generation 300ZX in 1984 had 75 lbs to 100 lbs of sound insulation in it.
To start with, the easiest, cheapest single most effective noise-control step is to make sure the weatherstripping around the door is in good shape and is sealing properly. This is especially important in a Z, as the window frame is only inches from your ear. Permatex brand Black Rubber Sealant RS-9 works exceptionally well for repairing torn and aging weatherstripping. New door seals from Nissan are very expensive and typically seem to already be 15 or 20 years old (mine started tearing and coming apart only months after being installed - and the car is always garaged). The window channel felt can also be purchased through Nissan and will make a BIG difference in window sealing, and will keep it from rattling to boot.
The next step is to make sure all the holes in the firewall are sealed with a grommet or closed up with a plug or silicone sealant. In addition to noise reduction, this will also do a lot towards keeping exhaust fumes out of the passenger compartment.
Now for the sound insulation. There are three basic methods of noise reduction: damping, absorption, and barrier.
- Damping is a method of adding mass to body panels to change the harmonic
frequency, basically preventing the oil-canning effect of sheetmetal. In other
words, they reduce the vibrations of physical structures that, in turn, produce
noise due to that vibration. If you look at the floorboards of any early Z-Car,
you will see a thin layer of a tar-like substance. This is an early attempt of
damping, although for some unknown reason, Nissan decided not to paint or treat
the aforementioned panels, providing a haven for rust. Those that are in the
stereo industry are familiar with Dynamat. It is listed as a "high performance
insulation and vibration control sheet". Dynamat comes in three basic styles:
standard, premium, and super. The standard is the least expensive and is very
effective. The premium is slightly thinner than the standard, is more flexible
and has just slightly less dampening ability. It is good for roofs and highly
curved surfaces. The super is aluminum backed and is the most effective type. It
is also the most expensive.
- Absorption is a method of using a closed cell type of foam to ëtire outí the
sound waves as they try to pass through the medium. Their internal structures
offer a high degree of frictional resistance to vibrating air molecules for
effective dissipation of acoustic energy. The sound absorption ability also
increases dramatically with the thickness of the foam. The advantages of this
material are light weight, a fairly high temperature resistance, and moisture
resistance (the waste cotton blankets used by most manufactures become sodden if
water leaks into the interior). The disadvantage is that it only really becomes
effective with thicknessí of æ" to 2". For extremely high temperature, such as
areas over the catalytic converter, fiberglass blankets are both lightweight and
effective.
- Barrier is a method of blocking or reflecting sound waves, used to reduce noise when used between the noise source and the receiver. The most effective material for blocking sound is lead. The most common form is a material called ëlead vinylí or ëloaded vinyl'. Loaded vinyl is formed by combining powdered lead or an inorganic filler with a vinyl and fiber glass carrier for toughness and flexibility. It typically comes in two thicknessí, ëlightweightí (.025" thick) which is ‡ lb per square foot, and ëstandardí (.050" thick) which is 1 lb/sq ft. The advantages are effective reduction with minimal space. The disadvantages are high weight per sq/ft and only moderate temperature resistance.