In many helicopters, they say they have reduced the vibrations significantly, but how is this isolation being achieved?
Passive reduction techniques involve designing the powerplant, transmission, gearbox and particularly the rotor(s) to minimize vibration. Designers carefully arrange all the moving parts to be as closely balanced as possible. They can be mounted using flexible parts and springs to damp vibrations (the same principle behind the shock absorbers in an automobile). The sophistication of helicopter design has evolved over time as engineers learn more about the characteristics of helicopters and how mechanical parts behave. Computer-assisted design has also allowed for huge improvements here as moving parts can be simulated in great detail to optimize each individual part for minimum vibration.
Active systems work by using accelerometers to sense the vibration of the helicopter at certain points in real time. This data is then used to shake a reaction mass back and forth to create vibration opposite to that of the rest of the helicopter. The two opposite vibrations cancel each other out. Effectively, the active system is using electrical power to dump all of the vibration energy in the helicopter into a small mass, so the helicopter stays still while the reaction mass in the active system shakes to absorb the energy. Such systems are already in common use in large transport helicopters and are becoming more prevalent in smaller helicopters as well.
Despite all that, some helicopters still vibrate significantly. Some helicopters, mainly designed and built by Eurocopter/Airbus can be adjusted to be flying without vibration (EC155, EC225, EC175 for instance).
One of the main items to get right is the so-called rotor-track and balance (RTB). Older or smaller helicopters need specific maintenance for RTB using a stroboscope to see if all the blades follow the same track. This can only be done on the ground.
Some helicopters have a camera-system fitted that is able to capture the track of each blade. A computer system knows which blade is in the camera view and upon return can make a recommendation how to adjust the trim-tabs. This can only be done by day and won't work under specific lighting conditions (camera pointing too much into the light for instance).
Some of the newest helicopters don't even have a camera anymore, but accelerometers on the gearbox and the computer knows the position of each blade and calculates the necessary adjustments to be made.
If the RTB is done right, the active anti vibration system has little work to do. This is rarely done to perfection as most operators accept it when it is within 'manufacturer tolerances', so there still is vibration. To get it perfect costs time (and money).