What is Aeroacoustics?

“Aeroacoustics is concerned with sound generated by aerodynamic forces or motions originating in a flow rather than by the externally applied forces or motions of classical acoustics. Thus, the sounds generated by vibrating violin strings and loudspeakers fall into the category of classical acoustics, whereas sounds generated by the unsteady aerodynamic forces on propellers or by turbulent flows fall into the domain of aeroacoustics."

This is the definition given by Goldstein (1976) in the introduction to his book entitled "Aeroacoustics" which is probably the only single text which brings the theoretical aspects of the subject together under one cover although there are many other texts on acoustics, fluid mechanics, flow induced sound, flow induced vibration, etc. which collectively cover the same ground. The multidisciplinary nature of aeroacoustics has probably been responsible for the slow recognition of the subject, particularly in Engineering where a comprehensive understanding is required if problems associated with environmental noise, industrial injury due to hearing loss and some types of machinery failure are to be avoided.


The Nature of flow noise

There have been many attempts to develop mathematical models for the prediction of what is often referred to as "aerodynamic noise", in some cases acceptable correlation with experimental data has been achieved for particular situations but no universal theory has yet emerged. It is generally accepted that broad band sound will be generated in turbulent shear layers and Lighthill's theory was developed to explain sound generated in the shear layers between jets and the surrounding air. Of greater interest in connection with aeroacoustics is the instability of the wakes of immersed bodies and the consequent formation of vortices as this is frequently the source of excitation of mechanical and/or acoustic resonances which often result in Sound Pressure Levels up to 165 dB being generated by flow at low subsonic velocities.

The resonances observed in the airflow model of the Hinkley Point circulators were described by Forster in a written contribution to the discussion of the paper by Rizk and Seymour (1964-5). The excitation was clearly shown to be associated with the wakes of the diffuser centrebody support spokes and a survey with a stethoscope identified a number of clearly defined acoustic modes. These observations led to a series of experimental investigations of flow induced resonances in various test rigs designed to facilitate investigations of the acoustic properties of the flow passages, the effects of mechanical vibration and the excitation mechanisms.


Current Issues

Low-Noise/Low-Detectable Aircraft, Engine and Helicopter

Structural Fatigue/Damage of Supersonic/Hypersonic Vehicle

Failure of Space Launcher/Acoustic Damage on Satellite

Operations of Fighter/Bomber at Transonic Speed



Computational AeroAcoustics (CAA)

Time Marching Free Wake

Anechoic Wind Tunnel Test

Actual Noise Control/Actual Flow Control by Sound