1. Introduction to Aeroacoustics
Fernando Lau

2. Introduction to Aeroacoustics
Noise
Sound travels as a wave:
in air sound travels at a speed of 340 m/s
in water it travels at a speed around 1500 m/s
Audible human range: 20 Hz – 20 KHz
Human ear more sensible around 1Khz – 5 KHz
Acoustic Power: large range!!
Human whisper: 𝟏𝟎 −𝟏𝟎 watt
Human shout: 𝟏𝟎 −𝟓 watt
Large jet at take-off: 𝟏𝟎 𝟕 watt
Introduction to Aeroacoustics

3. Noise Logarithmic scale used : decibel (dB) Sound Pressure Level (SPL)
𝑝 𝑟𝑚𝑠 - Randon Mean Square (rms) of sound pressure
perturbation
𝑝 𝑟𝑒𝑓 - Reference Pressure:
Atmospheric Air: 𝑝 𝑟𝑒𝑓 = 10 −5 N /m 2
In water: 𝑝 𝑟𝑒𝑓 = 10 −6 N /m 2
SPL = 10 𝐿𝑜𝑔 𝑝 𝑟𝑚𝑠 2 𝑝 𝑟𝑒𝑓 2 =20 𝐿𝑜𝑔 𝑝 𝑟𝑚𝑠 𝑝 𝑟𝑒𝑓
Introduction to Aeroacoustics

4. Introduction to Aeroacoustics
Noise
Pain Threshold:
130 – 140 dB
Sound Pressure Level
above 150 dB:
Acoustic Fatigue
Continuous Exposition to
75 dB noise:
- Stress;
- Hipertension & Heartbeat increase.
Introduction to Aeroacoustics

5. Introduction to Aeroacoustics
Aircraft Noise
Aircraft Noise Predictions:
- Steady growth rate of 2 – 3% per year
- From 1990 to 2015 the number of airplanes will double in number.
Necessary to reduce take-off and landing noise.
Aircraft Noise reduction less that Air traffic Increase
Introduction to Aeroacoustics

6. Introduction to Aeroacoustics
Aircaft Noise
Easy way to reduce noise in aircraft:
- Take-off weight limitation, by reducing number of passengers, payload and/or fuel.
- Less engine powwer and/or speed
Flights less profitable
(more fuel stops and less passengers)
Efficient Solution:
Prediction Methods of aircraft noise generation during early design phase.
Introduction to Aeroacoustics

7. Subjective Units of Noise
Noise nuisance depends on its duration, periodicity, etc.
Human response to noise varies with the noise frequency
Units used in Aviation, taking into account the human ear sensibility and the specific noise characteristics:
dBA – “A – weighted decibel”
Does not take into account time variations
PNdB – “Perceived Noise decibel”
Used to meadure Aircraft Noise (in particular, it considers the high pitch noise components of jet engines).
EPNdB – “Effective Perceived Noise decibel”
Uses as penalty not only the fundamental frequencies of jet engines, but also the time duration of the Aircaft flight.
Resposta relativa (em dB) da escala dBA
Introduction to Aeroacoustics

8. Aircraft Noise Certification
Worldwide, three Organizations can be singled out for aircraft certification:
ICAO – International Civil Aviation Organization
United Nations Agency, for international certification
FAA – Federal Aviation Agency
FAR – Federal Aviation Regulations, divided em parts
JAA – Joint Aviation Authorities (European Space)
JAA – Joint Aviation Requirements
EASA – European Aviation Safety Agency
Replaced the former JAA
Created in 2003; in full operation in 2008
Aircraft Authority in European Union
(as opposed to JAA, that depended on each European Nation)
For noise:
FAR – 36
JAR – 36
EASA - Form 45
Introduction to Aeroacoustics

9. Introduction to Aeroacoustics
Noise Certification
As Aircraft noise become less popular, FAA proposed in the 70s a noise certification scheme
for all Aircraft, later adopted by ICAO, using three noise certification reference positions:
Noise under approach flight;
Noise under take-off (also called flyover);
Sideline noise, with engines at full power.
Noise limits are dependent on aircraft weight and on the number of engines.
Noise could not exceed 108 EPNdB, which was DB less than the noise generated by the
operating aircraft.
Introduction to Aeroacoustics

10. Stage IV has been introduce, which will force Aircraft to be quieter.
Noise Certification
Current three stages for Noise Certification:
A Stage I aircraft generates noise levels during take-off, approach and sideline, that exceed Stage II limits.
- A Stage II aircraft obeys to the limits given by FAR 36 - C36.5 (a) (2), tendo contudo (Noise levels above those imposed by limites impostos pelo Stage III.)
A Stage III aircraft generate noise within the limits given by FAR 36 - C36.5 (a) (3).
Stage IV has been introduce, which will force Aircraft to be quieter.
Introduction to Aeroacoustics

11. Noise Certification for Stages III and IV
Stage 4 Certification Noise Limits, EPNdB
Approach Limit: 98.7 dB
Sideline Limit: 94.8 dB
Takeoff Limit: 89 dB
Measured Approach: 89.8 dB
Measured Sideline: 89.1 dB
Measured Takeoff: 82.4 dB
Cumulative Margin = 21.2 dB
Measured Global Express, EPNdB
Stage 3 Certification Noise Limits, EPNdB
282.5 dB Total
261.3 dB Total
Approach Max Limit: 98.7 dB
Sideline Max Limit: 94.8 dB
Takeoff Max Limit: 89 dB
Maximum = dB Total
Total Reduction = 10 dB
Cumulative Margin =11.2 dB
Introduction to Aeroacoustics

12. Number of current Aircraft
Introduction to Aeroacoustics

13. The Green Aviation Challenge
Aviation currently accounts for 2-3% of man’s contribution to global warming.
Air travel is predicted to increase by 4-5% each year over the next 20 years.
Aviation will have to find ways to meet the growing demand for civil air transport whilst reducing its environmental impact, specifically to the level of carbon emission.
The International Air Transport Association vision is for:
carbon neutral growth by 2020, 50%
absolute reduction in carbon emission by 2050
and zero-emission aircraft within 50 years.
The aviation industry in Europe has long recognised this challenge.

14. EU 2020 targets: Horizon 2020
In 2001 the Advisory Council for Aeronautical Research in Europe (ACARE) established the following targets for new aircraft entering service in 2020 compared to those entering service in 2000:
reduce fuel consumption and CO2 emissions by 50% per passenger kilometre
reduce NOx emissions by 80%
reduce perceived noise by 50%
(from ‘European Aeronautics: A Vision for 2020’, January 2001)

15. Aircraft Noise Sources
The Aircraft flight forces the movement of considerable amounts of air, which
generate high noise levels.
Noise Sources
Airframe Noise
Sonic Boom
Propeller noise
Jet Noise
Helicopter Noise
Air flow interacts with all aircraft structures:
Windows structures (dimensions in centimeters)
Wings and Fuselage (dimensions in meters)
Broad frequency range (broadband noise), sendo o
Significant Noise increase when landing gear is down.
Introduction to Aeroacoustics

16. Aircraft Noise Sources
Sonic Boom
Noise created by Aircraft flying at supersonic speed.
Nowadays, only used by military Aircraft (Even the Concord only flew at Mach >1 over the Oceans).
Supersonic flights over Land are not allowed by most Countries:
Even a sonic boom of a small aircraft, at a long distance, can have a significant impact on the human and wild welfare.
Introduction to Aeroacoustics

17. Aircraft Noise Sources
Propeller Noise
Although first Propellers were very noisy, it was no cause for concern untill WWII.
With the increase of Turbine Propulsion, Jet Noise became much more relevant.
However the noise restriction increase Brought a new concern in noise research, when generated by Propeller Engines.
Noise characterized by well defined harmonics, that are multiple of BPF (Blade Passing Frequency)
Monopolar Noise (Thickness Noise) and Dipolar Noise (Loading Noise):
Efficient Sound Sources
Introduction to Aeroacoustics

18. Aircraft Noise Sources
Jet Noise
Considerable increase in efficiency, as larger volumes of air are drawn at considerable less speed.
Bypass cycle: requires 50% more air per unit of Thrust unit, which reduces the mean exhaust velocity.
Divides the larger entry airflow into two almost equal streams, using only one to supply a smaller but more efficient engine core, which then has the same fairly exhaust as in pure jet.
- Turbo-Fan: Another step forward in efficiency by even higher levels of bypass flow via a large fan.
Lighthill Law:
Introduction to Aeroacoustics

19. Aircraft Noise Sources
Jet Noise
As bypass ratio rises and jet velocity falls, so the external jet mixing noise falls also.
Introduction to Aeroacoustics

20. Aircraft Noise Sources
Helicopter Noise
Several characteristics that are different from Aircraft:
- The rotor is aligned with the flight direction, rather than being normal to it.
- BVI (Blade Vortex Interaction) – characteristic noise generated by the blade interaction with the flow coming from the previous blade.
- Tail Rotor Noise.
- Asymmetric noise field: substantial difference in relative tipspeed between the blade on the advancing side and on the retreating side of the rotor disc plane, with respect to flight direction.
Introduction to Aeroacoustics

21. Introduction to Aeroacoustics
Future Perspectives
Number of passengers tends to increase 4% per year.
Given the continuous increase on the aircraft dimensions, we can predict an increase of 2-3 % per year on the global number of aircraft.
Given the number of flights, noise increases 1 dB each year.
However:
Individual Aircraft Noise tends to decrease.
Twin-engine Aircraft have been replacing Four-Engine Aircraft. (The reliability of the former even led to their use in transatlantic flights.)
Introduction to Aeroacoustics

22. Introduction to Aeroacoustics
Questions?