Chapter Acoustics, 2017) (Sengpie, 2018). Level Change Volume Loudness

Chapter 1 : Introduction & Literature

1.1  Background

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1.11 Aircraft Noise

The aviation industry has
drastically reduced the noise by a factor of 10 from commercial aircrafts in
the past 50 years from the de Havilland Comet 4 to the Boeing 777. This reduction
is due to the progression of aircraft engine technology with a big push from
governments setting new legislation that put restrictions on noise. Although
aircraft noise has reduced the public perception has been exacerbated over
time, this is due to several reasons, two of which are from communities and
airports moving closer to each other either by airport workers wanting to be
closer to work or by airport expansions (e.g. new runways & terminals). The
second reason is due to globalisation, which means that more of the public want
to travel which of course encouraged the growth of the aviation industry
through more flights. In 1950, 25 million passengers travelled whereas in 2014,
there was over 3 billion passengers a year travelling which creates more than
100,000 flights a day (Garfors, 2014). So due to the
Public’s demands for quieter flights the policy makers and the aviation
industry are committed to reduce the noise produced from aviation.

Noise pollution has been found to
be very disturbing to the communities near airports, aircrafts taking off and
landing have been found to cause depression, hypertension, cardiovascular
diseases and other small psychological problems (CAA, 2016).
A Boeing 737 from 1.2 miles away can produces up to 90dB while landing, which
means the engines are on low thrust so the airframe is making the majority of
the noise. Research has found that an annoying noise is 70dB, with the pain
threshold being at 110dB. Due to the decibel (dB) scale being a logarithmic
scale 90dB is in fact 4 times louder than 70dB. (IAC Acoustics, 2017) (Sengpie, 2018).

Level Change


Sound pressure

Acoustic Power 
Sound Intensity

+60 dB




+50 dB




+40 dB




+30 dB




+20 dB




+10 dB

2.0 = double

3.16 = ?10


+6 dB

1.52 times

2.0 = double


+3 dB

1.23 times

1.414 times = ?2

2.0 = double

– – – – ±0 dB – – – –

– – – – 1.0 – – – – – – –

– – – – 1.0 – – – – – – –

– – – – 1.0 – – – – –

?3 dB

0.816 times

0.707 times

0.5 = half

?6 dB

0.660 times

0.5 = half


?10 dB

0.5 = half 



?20 dB

= 0.25



?30 dB




?40 dB




?50 dB




?60 dB




Log. size

Psycho size

Field size

Energy size

dB change

Loudness multiplier

Amplitude multiplier

Power multiplier

 (Sengpie, 2018)

The original problem of noise
pollution for aviation was the turbojet engines that were being used, the jet
produced a large amount of noise due to the engines movement of a small amount
of air very fast, and this was more of a brute force engine. Turbojet engines
work by fast moving compression blades squeezing air, the compressed air is
then sprayed with fuel which is ignited by an electric spark, this mixture
expands as it burns through the turbine and blasts out the exhaust nozzle. These
engines were also ridiculously inefficient because the thrust was simply
produced by the high velocity of exhaust gases, the most efficient speed was
Mach 2 (Loftin, 1985).

Through the innovation of turbofans,
jet flight became more fuel efficient by combining the ducted fan and the jet exhaust
gases to create thrust. Turbofan engines draw air into a high and low pressure
compressor through large fan blades, the compressed air continues through the
combustion chamber where it is sprayed with fuel, the mixture of air and fuel
is then ignited where is expands through the high and low pressure turbine then
out of the exhaust nozzle, this process is very similar to the turbojet, the
turbofan however also draws the majority of the air through the large fan
blades at the inlet and through the outer bypass duct. Turbofan engines move a
larger amount of air slower than a turbojet which creates more thrust. (Loftin, 1985)


Turbofan engines were originally
used in 1960s with a low air bypass however by the 1990s most engines were
using high bypass ratio which were much more efficient and reduced the jet
noise, however the noise of the fan started to create more noise than the jet
as seen in Figure 1. Consequently the reduction of aviation noise through using
a bypass on an engine has reached its limit, there will need to be a
technological breakthrough to reduce engine noise further therefore reductions
of aviation noise will need to be devised by improving noise production from
other areas of an aircraft. (Dobzynski, 2010)


Figure 1Evolution of high to low by-pass ratio engines (T.Chong, 2017)


As flight paths increased there was
a development of new and expansion of existing airports, this became a massive
issue for communities around these environments which is why plans to reduce
noise pollution started to be developed by governments. Concerns and issues
brought forward to the council about aircraft noise can have a detrimental
effect on the growth and prosperity of an airport, with councils stopping
airport expansions. Airport such as Birmingham, London City, London Heathrow.
In the case of Heathrow airport, Hillingdon council have rejected the expansion
through a third runway, the council have blocked this runway expansion due to
fears of destroying communities, adding noise and air pollution. The first
serious consideration of the building of a third runway was brought forward in
June 2001, however Hillingdon council have battled hard to block this new
runway through petitions. After 16 years it looks like Heathrow will finally be
getting their desired third runway as the expansion is building momentum with
the launch of the 10 week consultation for the public to have their say.
Restrictions such as night flight curfews also heavily cap the total capacity
of many of the airports worldwide, with Heathrow airport only allowed 5,800
take-offs and landings a year, which on average is only 16 aircraft movements a
night where 80% of these happen between 4:30am and 6:00am.  (Hillingdon Council, 2017)

The FAA developed the CLEEN program
in 2010, this program was created to accelerate the innovation of new aircrafts
and technologies that will reduce noise, emissions and fuel burn. The FAA
shared costs with industries to aid the integration of the technologies into
existing and new aircrafts,  the FAA
invested $125 million, which when added to the companies included (Boeing, GE,
Honeywell, Pratt & Whitney, Rolls-Royce) brought the total investment  to just over $250 million (FAA, 2014). Due to the success
of bringing new technology into aircrafts by 2016 by the first phase of the
CLEEN program the FAA and the companies involved have invested $200 million in
the second phase the CLEEN II in 2015 which hopes to bring more new technologies
into new aircrafts by 2026. (FAA, 2016)

Major Projects






Unconventional aircraft configurations


Adaptable trailing edges & ceramic matrix composite acoustic nozzle

Structurally Efficient Wing (SEW) & Short engine inlet


Fuel burn reduction using integrated flight-propulsion control

Flight management systems & Alternative fuels


Alternate 100% bio fuels, new compressors, advanced material and
Air-Air sealed turbines

Compact combustor & Advanced turbine system

Pratt & Whitney

Geared turbofan propulsion efficiency

Geared turbofan Motor efficiency


Fuel burn reduction through dual-wall turbine airfoils and CMC turbine
blade tracks

Advanced combustion system



Short thrust reverser & advanced tailored acoustics


Flightpath 2050 is how the Advisory
Council for Aeronautics Research in Europe (ACARE) backed by the European
commission in 2011, envision the aviation industry to develop and improve by
2050. This publication discusses ways to improve, financial systems, scarcity
of resources, the challenges of globalisation and climate change. The European
commission hope to reduce the perceived noise emissions of a flying aircraft by
65%, Perceived noise is that of which is measured by comparison to the sound
pressure level at minimum perceivable noise of a human (European
Commision, 2011).
In 2001 ACARE initially set up the programme Vision 2020 which planned to
decrease perceived noise levels of 2001 by 50% by 2020. However this project
was found to be too ambitious that is why the Flightpath 2050 was created. (ACARE, 2001)



1.2 Literature Review

1.21 Aerodynamic Sound


The study of aerodynamic noise and
the science of aeroacoustics began around 65 years ago with Sir James Lighthill
while he was at the University of Manchester. Lighthill was the first person to
identify the source of sound from aerodynamic turbulence (Lighthill, On
sound generated aerodynamically, 1951) (Lighthill, On sound generated aerodynamically, 1954). This work laid the
foundation for future aerodynamic noise and aeroacoustics, which have been used
to solve both civil and military aerodynamic noise issues over the following 65
years. Present day research into aerodynamic noise and aeroacoustics is
research into reducing aircraft self-noise from high lift devices such as
wings, flaps and slats, also the reduction of the aircraft turbofan jet

1.22 Airfoil Self-Noise

The Aerodynamic noise produced from an airfoil is called trailing
edge noise or airfoil self-noise. This noise is caused by the effect of a
turbulent air flow within the boundary layer of the airfoil interacting with
the sharp trailing edge of the wing.