Acoustic source characterization of simulated subsonic jet noise using spherical harmonics.

As subsonic jets remain one of the major contributions to aircraft noise emissions, near-field flow simulations should be included in aircraft design at an early stage using quantitatively predicted sound pressure levels and the time-domain signal properties of the noise data. In this regard, the interface from the near-field data to the far-field radiation-under consideration of acoustic reflections from objects such as fuselage and wings-remains as bottleneck. This study presents the computation of a spherical equivalent source model of jet noise with minimal complexity by means of spherical harmonic (SH) coefficients. Using spherical Hankel extrapolation of sound pressure data from virtual, concentrical microphone arrays, the results of the determination for the radius, in which all acoustic sources of a flow field are confined, indicate the source radius around the end of the potential core to be equivalent to five times the nozzle diameter. The result of the SH transform shows the dominant energy contribution to be related to nine elementary sources. The resulting equivalent source model of jet noise provides a convenient format for further use in large-scale computational fluid dynamics simulations.

Aircraft noise-Auralization-based assessment of weather-dependent effects on loudness and sharpness.

This paper deals with the question of how specific weather conditions affect the perception of aircraft noise. Auralization is a suitable method by enabling parametrical decompositions of the overall aircraft noise scenario into source and propagation components. Considering influences on the auditory perception, the signal processing chain contains different virtual receivers and post processing using psychoacoustic hearing models. For broad coverage, generic standardized as well as measurement-based atmosphere models with variation of ground impedances such as soil data are evaluated. These variations are given aircraft noise measurement values based on A-weighted sound pressure levels LA and psychoacoustic measures regarding loudness, N, and sharpness, S. The results show an immense influence of weather conditions on A-weighted sound pressure levels and on psychoacoustic perception of aircraft noise, too. The weather-dependent differences of A-weighted sound pressure levels are up to 15 dBA and relative differences regarding loudness of factor 1.6 and sharpness of factor 2.0 occur. The approach can be used to get a better understanding of how the temporal statistics of specific local weather conditions and their perceptual consequences may lead to improved taxation of actual noise events and to an improved basis for long-term averages of aircraft noise effects.

Association Equilibria of Organo-Phosphoric Acids with Imines from a Combined Dielectric and Nuclear Magnetic Resonance Spectroscopy Approach.

Aggregates formed between organo-phosphoric acids and imine bases in aprotic solvents are the reactive intermediates in Brønsted acid organo-catalysis. Due to the strong hydrogen-bonding interaction of the acids in solution, multiple homo- and heteroaggregates are formed with profound effects on catalytic activity. Yet, due to the similar binding motifs-hydrogen-bonds-it is challenging to experimentally quantify the abundance of these aggregates in solution. Here we demonstrate that a combination of nuclear magnetic resonance (NMR) and dielectric relaxation spectroscopy (DRS) allows for accurate speciation of these aggregates in solution. We show that only by using the observables of both experiments heteroaggregates can be discriminated with simultaneously taking homoaggregation into account. Comparison of the association of diphenyl phosphoric acid and quinaldine or phenylquinaline in chloroform, dichloromethane, or tetrahydrofuran suggests that the basicity of the base largely determines the association of one acid and one base molecule to form an ion-pair. We find the ion-pair formation constants to be highest in chloroform, slightly lower in dichloromethane and lowest in tetrahydrofuran, which indicates that the hydrogen-bonding ability of the solvent also alters ion-pairing equilibria. We find evidence for the formation of multimers, consisting of one imine base and multiple diphenyl phosphoric acid molecules for both bases in all three solvents. This subsequent association of an acid to an ion-pair is however little affected by the nature of the base or the solvent. As such our findings provide routes to enhance the overall fraction of these multimers in solution, which have been reported to open new catalytic pathways.