Turboprop engine airplanes; Despite low fuel consumption, they are used only for cargo transportation and domestic short term flights.
The vibration and loud noise caused by the propeller anatomy cause fatigue in the structural elements of the fuselage also give rise to great disturbance to the passengers.
This loud noise is approximately 20 dB more than turbojet engine aircrafts and can increase as much as 125 dB. In some researches, it is stated that sounds of 87 dB and above cause hearing loss. For this reason, some systems, components and insulations are used to prevent high noise.
First of all, thanks to the reduction gear which is connected to the turboprop engine shaft provide that propeller rotates in a slower RPM (Revolution Per Minute) than the shaft. Also increased propeller blade diameter can decrease the propeller ends speed. In addition, the noise in the aircraft can be reduced with passive and active systems.
Passively, for example, insulation elements which is located between the fuselage linings and the structure reduce the noise by 5 dB. Actively, the TVAS (Tunable Vibration Absorber System) and ANS (Anti-noise System).
ANS system includes a set of amplifier and microphones. These microphones located in the fuselage determine the phase of the sounds created by the vibration of the propeller and send it to the control unit of the system. The control unit eliminates the ambient noise by creating anti-noise from the existing amplifier according to the existing noise level.
Another system, TVAS, consists of a control unit, an internal power amplifier, inertial actuators, actively controlled electromagnetic deflectors and microphones.
Both of these active control methods try to prevent loud noise by creating an ”anti-noise” that has the same frequency and amplitude as the noise but it is 180 degrees out of phase with the noise.