Aircraft noise

Aircraft noise

Basics of acoustics:

Physics of sound waves. Harmonic waves, sound speed, wave front and rays. Reflection, refraction and diffraction of sound waves. The dB scale for acoustic power, sound intensity and sound pressure level. Interference, adding sound pressure levels and the standing wave. Directionality of sound sources. Periodic and broadband noise. Doppler effect and shock waves.

Wave equation and its basic solutions:

Derivation of the wave equation from conservation of mass, momentum and energy. Plane waves, acoustic resistance. Harmonic solution to wave equation. spherical waves, characteristic acoustic impedance.

Propagation of sound in the atmosphere:

Geometrical spreading and sound attenuation due to friction. Sound pressure level calculations as a function of distance from the source. Derivation of reflection and transmission coefficient of sound when going from one medium to another medium. Critical angle. Effect of temperature gradient on sound propagation. Calculation of shadow zone distance. Ray tracing. Ground effect.

Sound propagation - special situations

Sound transmission through a layer, e.g. a wall. Mass law. Propagation in enclosures, room acoustics, diffuse sound field, reverberation time, Sabine's law. Acoustic filters. Attenuation of sound in ducts (with changing cross-sections), e.g. exhaust systems. Helmholtz resonator. Acoustic lining and its application in turbofan engines. Noise barriers.

Acoustic signal analysis:

Fourier transform (continuous and discrete), power spectral density and spectrum level. Octave band and terts band analysis, pressure band level. Effect on bandwidth on measured aircraft spectra. Overall sound pressure level. Spectrogram. Examples of aircraft noise measurements, e.g. from flyovers. Properties of microphones.

Noise metrics:

Human perception of sound, loudness and the phone and sone scale. Equal loudness level contours. Overall loudness level for broadband noise. Equal noisiness curves, overall noy value and perceived noise level. A-weighting and overall A-weighted sound pressure level. Effect of the duration of sound on human perception, equivalent A-weighted sound pressure level and sound exposure level (SEL). Single flyover noise contours. Noise indices for total noise exposure (Lden). Noise certification. Dutch aircraft noise model NRM.

Directional acoustic sources:

Monopole, dipole and quadrupole source. Line array of monopoles. Rayleigh integral, loudspeakers.

Acoustic imaging:

Principle of beamforming. Imaging aircraft noise data.

(Note: this topic is only touched upon in this course, the details (including application) are treated in AE4463P-23)

Aircraft noise sources:

Mechanism of propeller noise and blade passage frequency. Mechanism of turbo engine noise. Effect of bypassing on exhaust jet noise and effect of acoustic lining on fan noise. Properties of airframe (aerodynamic) noise. ANOPP modelling method.

Programming assignment using real microphone data, consisting of measured pressures as a function of time. From these data both the overall sound pressure level (OSPL) and the A-weighted OSPL should be determined as a function of time.