Getting to Grips with Aircraft Noise

2.1.2. FAN NOISE
Flight Operations Support & Line Assistance
Getting to grips with aircraft noise
2 - AIRPLANE NOISE SOURCES
The noise produced by the fan results of the superimposition of a wide-band noise (as
for the jet) and noise with harmonics.
• The wide band noise is due to the boundary layer developing on the fan blades,
and more generally to the airflow around them.
• The harmonics are originating in the intrinsic cycling character of the fan motion
(spinning motion). The most remarkable frequency is the fundamental, the value
of which is the number of blades times the fan rotation speed. The harmonics are
multiples of this fundamental.
• When the engine rating is high (during takeoff for instance), the airflow around
the fan blades transitions to supersonic and these multiple pure tones are at the
origin of a the so-called “buzz saw noise”.
2.1.3. COMPRESSOR NOISE
It is of the same kind than the fan noise, but the harmonics are less emergent due to
interaction phenomena.
2.2. AIRFRAME NOISE
The airframe noise would be the noise produced by the aircraft, if all engines were
made inoperative. It is generated by the airflow surrounding the moving plane. The main
sources are the discontinuities of the aircraft structure, such as high-lift devices, landing
gear wheels (when extended), trailing edges where there is a speed shearing (aircraft
speed versus still air).
It was empirically determined that the noise emissions are dependent on the sixth
power of the aircraft’s true airspeed. This noise produced from aerodynamic
phenomena is most sensitive during approach, when engine power is the lowest.
2.3. EXAMPLE OF SHARE BETWEEN ENGINE AND
AERODYNAMIC NOISE
The following sketches illustrate the share between engine parts and airframe regarding
noise emissions.
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