Northwest Airlines has spent millions installing shock absorbers on the engine mounts of its DC-9s to reduce the noise levels . The system is similar to the one used in the propeller planes. It makes vibrations to counter the external forces that rattle the plane. That system offsets some of the jet's vibration, but not as much as the propeller system. On average, it appears that the devices have cut the noise in the back of the plane from 95 decibels to 90.
Any added weight means that an airline is spending more money by burning more fuel to fly a plane.
Airlines, for instance, choose their paint carefully to reduce weight. So Northwest spends more on fuel to fly planes equipped with noise-reduction devices. The airline has stopped installing the devices and is considering what to do next. Boeing's lead sound engineer, Larry Craig, says any commercial jet could be made quieter, but those airlines would have to pay for that luxury. Where our levels are, there's no compelling reason to add that weight.