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What about damping?
1/4" thick constrained
layer damper
two views of a typical floor mat
vinyl barrier (left) goes inside;
felt with hook & loop (right)
goes towards floor
1/8" closed cell foam (tan) &
1/8" open cell foam (white)
1/4" felt
The concept is to reduce the total amount of vibration just like turning down the volume on a radio reduces the movement of a speaker (but not the frequency).
We first tried to keep the vibration from entering the structure with isolation mounting but once it has entered, it is the job of dampers to reduce the amplitude of the vibration.
One of the most common and effective dampers are the "Visco-elastic" dampers made from a gummy foam. As the name implies, they are elastic so they recover when they become distorted by the movement of a panel. But the "visco" (as in “viscous” liquid or gel) part means that they return to shape slowly and out of phase with the panel they are attached to.
A slightly more advanced concept is "Constrained layer damping". These have a stiff face (probably a relatively thick aluminum skin) opposite the panel side to prevent movement thus forcing the soft gummy foam core to absorb more energy. Either one MUST be well bonded to the skins being treated to most effectively stop it from vibrating.
It should also be pointed out that dampers tend to be frequency specific. Higher frequencies are best damped with stiffer materials. Since aircraft have most of their noise in the lower frequencies, we tend to use thick, soft damping treatment.
One of the nice things about dampers is that they do not require a lot of precision in installation. Generally, we treat only about 50% -75% of a panel (although I like to cover a firewall completely with damper to add mass). The reason for this is a panel (defined as a flat structure framed by surrounding stiffeners such as longerons and stringers) is acting like a drum. The center of the panel has the most deflection, is creating the most noise and is the most effectively damped. Near the edges of the panel, deflection is much less due to the stiffeners. If you have a panel framed by stringers 10" apart and frames 20" apart, (200 sq. in.) you should have between 100 and 150 sq. inches of damper approximately in the middle.
Note that while visco-elastic dampers are very effective, other materials can also add damping. Carpet pad laid on top of a floor will damp the vibration of floorboards as does felt and rubber floor mats. These same materials however, will provide almost nothing as dampers if laid vertically and not held in intimate contact with the structure, but they might add some effect as absorbers or barriers.
One esoteric kind of passive damper is the tuned mass damper. For some aircraft (or busses or just about anything else that vibrates) with very bad noise problems of a specific frequency, tuned mass dampers may be a good solution. These are weights bonded to a base via an elastic mount. These are in turn mounted to the rigid frame of the aircraft at specific locations and are designed NOT to vibrate at the problem frequency of the aircraft. They can be very effective but the down side is that they are heavy and require a lot of analysis to properly design. The Beechcraft Kingair is one type of aircraft that has a mass damper kit just for this purpose.
