Introduction to Acoustics

Reduction of airplane engine noise during departure
operations (as illustrated in Fig. 23.19) was achieved
primarily by increasing the bypass ratio, and hence the
diameter of the fan stage at the front of the turbofan
engines. Bypass ratio is the ratio of the mass flow rate
of the air that passes through the fan-discharge ducts
to the mass flow rate through the turbine stages in the
core of the engine. Design thrust is provided by moving
a large amount of air at a lower exhaust-gas velocity
and hence with lower levels of jet-mixing noise at high
engine-power settings. Higher bypass ratios also result
in better fuel efficiency for the same thrust, although at
an increase in engine weight and diameter.
Discrete-frequency noise from the fan, compressor,
and turbine stages is reduced by means of soundabsorbing
linings in the engine inlet and discharge ducts.
Tonal components in the sound from the fan stages are
minimized or eliminated by careful selection of the number
of blades and vanes and by increasing the spacing
between the fan stage and the fan-outlet guide vanes. Inlet
guide vanes ahead of the fan blades are no longer
used in modern turbofan engines.
A summary of technology for engine noise-control
designs is available [23.112]. A summary of retrofit
applications intended to allow chapter 2/stage 2 airplanes
to meet a phase-out deadline of the year 2000
for stage 3 (ICAO chapter 3) compliance has been
published [23.113].
23.3 Propagation Paths
23.3.1 Sound Propagation Outdoors
Sound propagation in the atmosphere is discussed in
Chap. 4, and only general information will be presented
here. Geometrical spreading is the most important effect
that reduces the sound pressure level as distance from
a source is increased. There are, however, several other
factors which influence outdoor sound propagation.
1. Atmospheric absorption. At long distances and at
high frequencies, the effects of atmospheric absorption
are significant. These effects are usually
described by an attenuation coefficient in decibels
per meter Chap. 4.
2. Ground effects. When the sound source is located
above a ground surface, sound waves that reflect
from the ground will constructively and destructively
interfere with those propagating directly from
Noise 23.3 Propagation Paths 991
Interior Aircraft Noise. The engines and pressure fluctuations
in the turbulent boundary layer outside the
fuselage of an aircraft during flight are sources of
noise in the interior of an airplane. Vibration from
jet engines or propellers can cause the fuselage to
vibrate and be radiated as sound into the cabin of
an aircraft or helicopter. The air-conditioning system
is often a source of noise in the interior of an
aircraft.
Noise from external sound sources can be partially
controlled by the design and construction of the fuselage.
Sound-absorbing material is installed between the
skin of the fuselage and the interior trim panels; this
material also acts as a thermal insulation barrier to
the cold outside air. Some propeller-driven airplanes
have successfully used active noise and vibration control
systems.
Control of noise in the interior of an aircraft requires
a balance between the desire to achieve low cabin interior
noise levels for the comfort of the passengers and
the flight and cabin crews, while maintaining a degree
of privacy, and an increase in airplane empty weight and
maintenance costs.
Cabin noise levels vary greatly with seat location
and operation of the aircraft (takeoff, cruise, and landing).
Standardized test methods are now available for the
specification of procedures to measure aircraft interior
noise under specified cruise conditions [23.114, 115].
the source. In general, the ground is partially reflecting;
the reflected wave is modified in amplitude
and phase by its interaction with the ground surface.
The amount of attenuation attributable to this
ground interaction, and its variation with frequency,
depend on the surface characteristics, the source and
receiver heights, and their separation. The effects of
the ground are largest for intermediate frequencies
(≈ 500 Hz) when the source is above the ground (1 m
or more). If the source is very close to the ground,
all frequencies above about 500 Hz are highly attenuated.
3. Temperature gradients and wind speed gradients.
The speed of sound is proportional to the square root
of the absolute temperature of the medium. The normal
temperature lapse with height above the ground
means that a sound wavefront moves more rapidly
near the ground surface. This causes the wavefront
Part G 23.3