Copyright © A. Filippone (1999-2003). All Rights Reserved.

Aerodynamic Noise


Sound (or more appropriately noise) generated by airflow has been familiar for a long time. The increased use of fluid machines and engines has led to an increasing level of noise generation, and hence to an increasing interest in this area of research.


Aeroacoustics deals with noise generated by aeronautic systems or their components. Aerodynamic noise is one of the main concerns in the design process of any new jet engine (or re-engineering of old ones).

Other fields of research are the following:

  • blade-vortex interaction (when a blade chops a vortex)
  • turbulence-related noise in inlet flows (fans, compressors)
  • sonic-boom created bu high-speed flight
  • noise from turbulent wakes and shear flows
  • noise from internal combustion engines
  • noise from supersonic jets and rockets
  • explosions, detonations, etc.

In hydrodynamics it is of particular interest the noise from bubble cavitation.

Fundamental studies in aerodynamic sound were started in the 1950s by Lighthill and Ffowcs-Williams in the 1970s. General theories of sound are much older (Rayleigh, 1896).

Acoustic Energy

As reported anecdotically by Ffowcs-Williams (1977) and Crighton (1993), it has been calculated that the first generation of Boeing 707 at take-off produced as much sound as the world population shouting in phase together!

A Boeing 767 of 30 years later (with four times as much thrust per engine) produced as much sound as the city of New York shouting in phase. However, the energy radiated by such sound in the first 45 seconds is just enough to cook one egg !

Some Noise Levels

The only supersonic jet transport in service, the Concorde, exceeds by far the noise regulations for transport aircraft (this is one of the reasons why the Concorde is still banned from most of the world’s aircraft). The noise generated at take off is well above 100 dB (that is the level of a rock band noise).

Frequency Ranges

Dynamic levels of sound range from about 0 dB (threshold of audibility) to about 120 dB (threshold of pain). The range of frequencies perceived by the human ear is only 20 Hz – 20000 Hz. However, the ear is much more sensitive in the 2-4 KHz range.

Experimental studies in aero-acoustics are very expensive. They require anechoic tunnels (insulated against external sources) and delicate instruments able to measure the high-frequency pressure fluctuations. On turns this instrumentation has to be coupled with flow visualization techniques in order to understand the relationship between aerodynamics and acoustics.

As the computational resources increase and the numerical techniques are refined, it has become more interesting, and certainly more efficient, to employ computer simulations. Computational aero-acoustics is that branch of computational fluid dynamics concerned with aerodynamic noise.

Doppler Effect

The Doppler effect is characterized by frequency changes due to a source of sound approaching an observer at speed u. In this case a single frequency Eq. (having period Eq.) travels a distance cT (c = speed of sound), while the source travels a distance uT. The apparent wavelength is changed to (c – u) T. and the frequency is shifted to its Doppler-value.

Thus, a train approaching a station at a speed of 100 Km/h, and emitting a single frequency sound of 8 KHz would be heard by the passengers on the platform at 8.7 KHz.

Consider now a body of finite dimension, with characteristic length L in the direction of motion.

A source of sound at rest travels the distance Eq. in the unit of time. The volume involved by the sound is therefore proportional to T³ The same source travelling at subsonic speed u < c after the time T will be close to its front end, Fig. 1. At sonic conditions the source will be coincident with the front end of the wave. The volume involved is the infinite space behind the source. At supersonic speed the source will always be ahead of its sound. The waves radiate from the source along characteristic lines making an angle Eq. (Mach angle).

Mach Cone

Figure 1: Supersonic source radiation

Sounds emitted by s source travelling at supersonic speed u > c are heard by the observer on the ground in reverse order: pap pep pip pop pup will be heard pup pop pip pep pap. At sonic condition u=c all the sounds are heard at the same time: this is the sonic boom.

Sonic Boom

A sonic boom is a loud noise caused by an aircraft travelling faster than the speed of sound (M > 1). The sound propagates along the Mach cone, Fig. 1, which represents a surface of discontinuity for most of the thermo-aerodynamic variables (pressure, velocity, density, entropy).

The boom is due to a combination of volume and lift. While the boom due to volume can be virtually eliminated (Busemann, 1935), the boom due to lift can only be minimized.

Minimization is not straightforward, because it is constrained by structural, aerodynamic and design parameters, and not least by the variation of the thermo-dynamic properties of the atmosphere.

The minimum sonic boom generally does not correspond to the best aircraft. There is among others: sonic boom minimization at given drag; minimization at given volume, etc. (Seebass, 1998). Because the shock energy is nearly conserved as the shock radiates, its strength decays only slightly with the distance from the aircraft.

Over-Pressure at the Ground

The boom is characterized by a typical over-pressure signature on the ground and a total impulse (time integral of the over-pressure).

The wave associated to the over-pressure is generally N-shaped, which is a result of a sonic boom due to two shocks (front and rear). The N-wave is a function of the aircraft geometry (length, volume, etc), flight altitude, speed, atmospheric conditions.

Boom Waves

Figure 2: Waves from Sonic Boom Optimization

Figure of Merit

Seebass-George (1961) defined a figure of merit, FM, to characterize the sonic boom levels. This figure is proportional to the aircraft weight divided with the three-halves of the aircraft length W/L^(3/2). The lower this value, the better the aircraft.

Some values are given in the following table.

Table 1: Figure of Merit; (*) forecast
Aircraft M FM
Concorde 2.0 1.41
NASA-Boeing 2707 (1972) 2.4 1.9
Supersonic business jet (*) 1.6 0.4

The B-2707 was a project aborted in 1972. Forecasts for a supersonic business jet (Seebass, 1998) flying at M=1.6 would give a FM = 0.4, that is considered acceptable.

Related Material

Selected References

  • Goldstein ME. Aeroacoustics, McGraw-Hill, 1976.

  • Smith MJT. Aircraft Noise, Cambridge Aerospace Series, Cambridge Univ Press, 1989.

  • Hardin JC, Hussaini MY (editors). Computational Aero-Acoustics, ICASE/NASA LaRC Series, 1993.

  • Ffowcs-Williams J., Aeroacoustics, in Ann. Rev. Fluid Mech., Vol 9, pages 447-468, 1977.
[Top of Page]

Copyright © A. Filippone (1999-2003). All Rights Reserved.