Andrew Christian

Annoyance to Noise Produced by a Distributed Electric Propulsion High-Lift System

Results of a psychoacoustic test performed to understand the relative annoyance to noise produced by several configurations of a distributed electric propulsion high lift system are given. It is found that the number of propellers in the system is a major factor in annoyance perception. This is an intuitive result as annoyance increases, in general, with frequency, and, the blade passage frequency of the propellers increases with the number of propellers. Additionally, the data indicate that having some variation in the blade passage frequency from propeller-to-propeller is beneficial as it reduces the high tonality generated when all the propellers are spinning in synchrony at the same speed. The propellers can be set to spin at different speeds, but it was found that allowing the motor controllers to drift within 1% of nominal settings produced the best results (lowest overall annoyance). The methodology employed has been demonstrated to be effective in providing timely feedback to designers in the early stages of design development.

Initial Investigation into the Psychoacoustic Properties of Small Unmanned Aerial System Noise

For the past several years, researchers at NASA Langley have been engaged in a series of projects to study the degree to which existing facilities and capabilities, originally created for work on full-scale aircraft, are extensible to smaller scales — those of the small unmanned aerial systems (sUAS, also UAVs and, colloquially, ‘drones’) that have been showing up in the nation’s airspace of late. This paper follows an effort that has led to an initial human–subject psychoacoustic test regarding the annoyance generated by sUAS noise. This effort spans three phases: 1. The collection of the sounds through field recordings. 2. The formulation and execution of a psychoacoustic test using those recordings. 3. The initial analysis of the data from that test. The data suggests a lack of parity between the noise of the recorded sUAS and that of a set of road vehicles that were also recorded and included in the test, as measured by a set of contemporary noise metrics. Future work, including the possibility of further human subject testing, is discussed in light of this suggestion.

Evaluation of the perceptual fidelity of a novel rotorcraft noise synthesis technique

A human subject experiment was recently conducted at the NASA Langley Research Center to evaluate the perceptual fidelity of synthesized rotorcraft source noise. The synthesis method combines the time record of a single blade passage (i.e., of a main or tail rotor) with amplitude and frequency modulations observed in recorded rotorcraft noise. Here, the single blade passage record can be determined from a time-averaged recording or from a modern aeroacoustic analysis. Since there is no predictive model available, the amplitude and frequency modulations were derived empirically from measured flyover noise. Thus, one research question was directed at determining the fidelity of four synthesis implementations (unmodulated and modulated main rotor only, and unmodulated and modulated main and tail rotor) under thickness and loading noise dominated conditions, using modulation data specific to those conditions. A second research question was aimed at understanding the sensitivity of fidelity to the choice of mo...

A Psychoacoustic Evaluation of Noise Signatures from Advanced Civil Transport Aircraft

The NASA Environmentally Responsible Aviation project has been successful in developing and demonstrating technologies for integrated aircraft systems that can simultaneously meet aggressive goals for fuel burn, noise and emissions. Some of the resulting systems substantially differ from the familiar tube and wing designs constituting the current civil transport fleet. This study attempts to explore whether or not the effective perceived noise level metric used in the NASA noise goal accurately reflects human subject response across the range of vehicles considered. Further, it seeks to determine, in a quantitative manner, if the sounds associated with the advanced aircraft are more or less preferable to the reference vehicles beyond any differences revealed by the metric. These explorations are made through psychoacoustic tests in a controlled laboratory environment using simulated stimuli developed from auralizations of selected vehicles based on systems noise assessments.

A real-time method for computing confidence intervals for trial-based psychoacoustic tests

The vast majority of psychoacoustic testing involves presenting a subject with a series of short, simple questions (trials). A common desire is to track the level of statistical confidence in the results after each trial so that one can stop the test once a threshold of certainty has been surpassed. Typically, this certainty statistic will take the form of a confidence interval (CI). Past methods to compute CIs involve assumptions that may impact the performance of the resultant test-stopping criterion. This talk proposes a method to compute CIs that is free of assumptions regarding: the testing procedure used (e.g., up-down staircase, PEST, non-adaptive), the testing modality used (e.g., n-alternative forced-choice, yes/no), and the model psychometric function used (e.g., logit, Gompertz). The method involves a fixed-quadrature integration of the likelihood function of the model parameters given the data retrieved after each trial. The execution time is demonstrated to be low enough so that the method ma...

A Recording-Based Method for Auralization of Rotorcraft Flyover Noise

Rotorcraft noise is an active field of study as the sound produced by these vehicles is often found to be annoying. A means to auralize rotorcraft flyover noise is sought to help understand the factors leading to annoyance. Previous work by the authors focused on auralization of rotorcraft fly-in noise, in which a simplification was made that enabled the source noise synthesis to be based on a single emission angle. Here, the goal is to auralize a complete flyover event, so the source noise synthesis must be capable of traversing a range of emission angles. The synthesis uses a source noise definition process that yields periodic and aperiodic (modulation) components at a set of discrete emission angles. In this work, only the periodic components are used for the source noise synthesis for the flyover; the inclusion of modulation components is the subject of ongoing research. Propagation of the synthesized source noise to a ground observer is performed using the NASA Auralization Framework. The method is demonstrated using ground recordings from a flight test of the AS350 helicopter for the source noise definition.

Establishing the response of low frequency auditory filters

The response of auditory filters is central to frequency selectivity of sound by the human auditory system. This is true especially for realistic complex sounds that are often encountered in many applications such as modeling the audibility and annoyance of sound, voice recognition, noise cancelation, and the development of advanced hearing aid devices. The purpose of this study was to establish the response of low frequency (below 100 Hz) auditory filters. Two experiments were designed and executed; the first was to measure subjects’ hearing threshold for pure tones (at 25, 31.5, 40, 50, 63, and 80 Hz), and the second was to measure the Psychophysical Tuning Curves (PTCs) at two signal frequencies (Fs= 40 and 63 Hz). Experiment 1 involved 36 subjects while experiment 2 used 20 subjects selected from experiment 1. Both experiments were based on a 3-down 1-up 3AFC adaptive staircase test procedure using either a variable level tone or variable level, narrow-band, noise masker. A summary of the results incl...

A likelihood-based method for point and interval estimation on auditory filter data

A novel method for determining auditory filter (AF) shapes given a set of n-alternative forced choice (nAFC) responses from a single human subject or set of subjects is discussed. The method works by developing a function which maps individual nAFC responses into likelihood values—either supporting or conflicting with a proposed model AF shape. The aggregate of these likelihoods is then used as an objective function for optimization schemes for point estimation, or as the basis function for Metropolis Hastings-like algorithms for interval estimation, both of either parameters of the AF model or of the entire AF shape. The method is demonstrated on simulated up-down staircase data. The consistency of the method is discussed in the context of canonical methods for AF data analysis, some of which are shown to produce systematic errors. Other possible benefits of this approach are discussed including the ability of the method to: combine data from heterogeneous nAFC tasks (e.g., notched-noise maskers with ton...

Uncertainty estimates of psychoacoustic thresholds obtained from group test

A common goal in laboratory psychoacoustic testing is determining the level, or threshold, at which a test signal is judged subjectively equivalent to a reference signal. Adaptive methods, in which the next signal level depends on the response to the previous signal, are most efficient and precise for determining thresholds but only accommodate one subject at a time. At NASA, testing one subject at a time to achieve a sample size representative of a larger population would exhaust testing resources. Instead, three or four subjects are tested simultaneously using preselected signal levels. When a threshold is estimated from a curve fit through group test data, techniques are required for assessing the threshold uncertainty. In this presentation, we examine the Delta Method, the Generalized Linear Model (GLM), the Nonparametric Bootstrap, and Bayesian Posterior Estimation (BPE). Each technique is first exercised on a manufactured, theoretical dataset. When we are confident we are using the methods correctly...

Recent advances in aircraft source noise synthesis

For several decades, research and development has been conducted at the NASA Langley Research Center directed at understanding human response to aircraft flyover noise. More recently, a technology development effort has focused on the simulation of aircraft flyover noise associated with future, large commercial transports. Because recordings of future aircraft are not available, the approach taken utilizes source noise predictions of engine and airframe components which serve as a basis for source noise syntheses. Human subject response studies have been conducted aimed at determining the fidelity of synthesized source noise, and the annoyance and detectability once the noise is propagated (via simulation) to the ground. Driven by various factors, human response to less common noise sources are gaining interest. Some have been around for a long time (rotorcraft), some have come and gone, and are back again (open rotors), and some are entirely new (distributed electric driven propeller systems). Each has u...