Omnidirectional Noise Research Articles

Long tunnel noise propagation

The number of long tunnels in Sydney has increased in recent years with the development of underground road and rail projects. Understanding the propagation of noise along the tunnels and into the environment during the construction phase is important when assessing noise for 24/7 tunnelling operations. In-tunnel noise assessment of mechanical equipment is also required as part of the project's technical criteria and approvals. The prediction of in-tunnel noise levels is required from stationary sources such as ventilation fans and jet fans. The acoustic design of these tunnels is challenging as a feature of the acoustics of long spaces is that classical room acoustic theory is not applicable, since the assumption of a diffuse field does not hold with the extreme dimensional condition. This paper details the difference between in-tunnel noise measurements from an omni-directional noise source compared with the modelled results from 3D noise modelling software packages.

Passive, remote and omnidirectional suppression of sound source radiation via an acoustic superscatterer

Effective direct control of the sound source is the fundamental solution to the problem of noise. Herein, we propose a passive, non-closed and remote scheme for omnidirectional reduction of the sound power radiated from vibrating sources. The physical mechanism of this scheme is to design an acoustic superscatterer based upon the idea of transformation media so that the virtual boundary of the acoustic superscatterer can overlap with the radiation boundary of the sound source to construct drastic multiple scattering effects. Through theoretical analyses and numerical simulations, we confirm the effectiveness of adopting an acoustic superscatterer to significantly suppress the sound radiation power generated by some typical dipolar sources in air. Our study shows that by arranging no more than two acoustic superscatterers at designated positions away from a dipolar thin rod, about 90% of the sound radiation power, i.e. 10 dB, can be suppressed in all directions of the dipole axis. This preliminary work could aid research into the use of passive methods to achieve non-contact omnidirectional noise control of vibrating sources.

Ambient noise exposure induces long-term adaptations in adult brainstem neurons

To counterbalance long-term environmental changes, neuronal circuits adapt the processing of sensory information. In the auditory system, ongoing background noise drives long-lasting adaptive mechanism in binaural coincidence detector neurons in the superior olive. However, the compensatory cellular mechanisms of the binaural neurons in the medial superior olive (MSO) to long-term background changes are unexplored. Here we investigated the cellular properties of MSO neurons during long-lasting adaptations induced by moderate omnidirectional noise exposure. After noise exposure, the input resistance of MSO neurons of mature Mongolian gerbils was reduced, likely due to an upregulation of hyperpolarisation-activated cation and low voltage-activated potassium currents. Functionally, the long-lasting adaptations increased the action potential current threshold and facilitated high frequency output generation. Noise exposure accelerated the occurrence of spontaneous postsynaptic currents. Together, our data suggest that cellular adaptations in coincidence detector neurons of the MSO to continuous noise exposure likely increase the sensitivity to differences in sound pressure levels.

Numerical study of aerodynamic noise behaviors for a vertically-installed flat strut behind an asymmetrical airfoil

The aerodynamic noise generated from a cooling fan and its support frame is one of the major noise sources of many engineering machines, such as automobile engines and communication base-stations. A generic model composed of a vertically-installed flat strut behind an asymmetrical airfoil is used to investigate numerically the noise characteristics and generation mechanism of a cooling fan by using the SST-SAS turbulence model combining with FW-H formula. The airfoil–strut configuration is varied either by changing the perforation rate of the strut or by adjusting the streamwise gap between the airfoil trailing edge and the strut. The motivation is to investigate the effects of the two parameters on the aerodynamic and noise behaviors of the airfoil–strut​ model and evaluate the influences of noise control methods. The analysis of noise generation mechanism and far-field directivity indicates that in the vertical direction both the unsteady lift forces acting on the airfoil and strut contribute equally to the far-field noise, whereas in the upstream/downstream direction the noise is produced mainly by the unsteady drag on the strut. In addition, the whole model shows the directivity characteristic of an omnidirectional noise source. With the increase of the strut perforation rate, both the unsteady lift and drag on the airfoil and strut are reduced due to the attenuation of the Karman vortex street in the flow field, resulting in a far-field noise reduction. There exists a frequency lock-in (i.e. a resonance) between the airfoil and the strut in which the unsteady characteristic of the airfoil is induced by the downstream strut. However, this lock-in loses its strength with the increase of the streamwise separation between the airfoil and strut, accompanying with a change of far-field noise.

Seismic wavefield divergence at the free surface

This paper explores the concept of pressure measurements in a land seismic acquisition setting. We first review the theory for pressure measurements near the surface of the Earth and show the significance of the S-to-P conversion which results in the pressure being proportional to the sum of the slowness-scaled horizontal velocity fields. In the second part of this paper we test a land hydrophone prototype via a small-scale experiment to validate the pseudopressure measurement. Potential applications are briefly discussed. This study suggests that such a land hydrophone can potentially allow local and omni-directional noise attenuation, via adaptive subtraction using the pseudo-pressure data as noise model, and therefore can potentially allow sparser acquisition geometries with associated field effort and cost reduction.

Noisy voice or voice in noise: Evaluation of cognitive load on the speaker, work in progress

We are comparing the cognitive load induced by various types of noise in the processing of information from speech. We examine if there is a difference in cognitive load between external noise sources (background noise) and internal noise sources (dysphonic voice). Our hypothesis is that noisy voices could be more cognitively demanding than background noise because they are more similar to the target signal spatially and temporally and the perceived link with the target signal is stronger. 60 normal hearing subjects (18-30 years) listen to texts in ten different conditions: (1) Healthy voice in multitalker babble noise, (2-4) Three types of dysphonic voices in background noise, (5-7) Three dysphonic voices in silence, (8) Healthy voice in silence, (9) Healthy voice with omnidirectional noise equivalent to dysphonic noise, (10) Healthy voice with dysphonic noise from same direction. We evaluate cognitive load in 4 ways: (1) during the listening phase by subjects’ performance on a secondary graphic task, (2) after the listening phase by subjects’ answers to a multi choice questionnaire and (3) by a free recall of the text, (4) by subjects’ grading of their listening effort. Methodological considerations will be discussed and preliminary results will be presented.

Noise‐rearing disrupts the maturation of multisensory integration

It is commonly believed that the ability to integrate information from different senses develops according to associative learning principles as neurons acquire experience with co-active cross-modal inputs. However, previous studies have not distinguished between requirements for co-activation versus co-variation. To determine whether cross-modal co-activation is sufficient for this purpose in visual-auditory superior colliculus (SC) neurons, animals were reared in constant omnidirectional noise. By masking most spatiotemporally discrete auditory experiences, the noise created a sensory landscape that decoupled stimulus co-activation and co-variance. Although a near-normal complement of visual-auditory SC neurons developed, the vast majority could not engage in multisensory integration, revealing that visual-auditory co-activation was insufficient for this purpose. That experience with co-varying stimuli is required for multisensory maturation is consistent with the role of the SC in detecting and locating biologically significant events, but it also seems likely that this is a general requirement for multisensory maturation throughout the brain.

Fitting Data Under Omnidirectional Noise: A Probabilistic Method for Inferring Petrophysical and Hydrologic Relations

While geophysicists recognize that all measurements of media properties are subject to noise, methods for fitting petrophysical relations commonly employ regression-based formulas, which assume no error in one of the properties being related (the “independent" variable). To derive a more rigorous method for fitting such relations, we take a probabilistic viewpoint on the problem of fitting petrophysical relations to sample correlation data. Unlike prior approaches, we take into account the fact that noise is present in both measured properties. Under basic assumptions, we derive a new objective function for such problems which is proportional to the data likelihood and which can be used for both model parameter optimization and model selection. We present several numerical experiments outlining the utility of our method, and compare results of our method against results of other commonly used methods, such as kriging, regression, and total distance minimization. The results of our applications using the maximum likelihood technique appear visually accurate, and we also provide quantitative comparisons of performance that suggest the method produces more desirable results.

Experience‐dependent refinement of the inhibitory axons projecting to the medial superior olive

Neurons in the medial superior olive (MSO) analyze interaural time differences (ITDs) by comparing the arrival times of the two excitatory inputs from each ear using a coincidence detection mechanism. They also receive a prominent inhibitory, glycinergic projection from the ipsilateral medial nucleus of the trapezoid body (MNTB), which contributes to the fine-tuning of ITD analysis. Here, we investigated developmental changes of the axonal arborisation pattern of single Microruby-labeled MNTB neurons projecting to the MSO region. During the first 2 weeks after hearing onset, the axonal arborisation of MNTB neurons was significantly refined resulting in a narrowed projection area across the tonotopic axis of the MSO and a redistribution of the axonal endsegments to a mostly somatic location. Rearing the animals in omnidirectional noise prevented the structural changes of single MNTB projections. These results indicate that the functional elimination of inhibitory inputs on MSO neurons after hearing onset, as described previously, is paralleled by a structural, site-specific refinement of the inputs and is dependent on the normal acoustic experience of the animal.

Maturation of glycinergic inhibition in the gerbil medial superior olive after hearing onset

The neurones of the medial superior olive (MSO) are the most temporally sensitive neurones in the brain. They respond to the arrival time difference of sound at the two ears with a microsecond resolution; these interaural time differences are used to localize low-frequency sounds. In addition to the excitatory inputs from each ear, the MSO neurones also receive binaural glycinergic projections, which have a critical role in sound localization processing. Recently, it was shown that the glycinergic input to the MSO undergoes an experience-dependent structural reorganization after hearing onset. To explore the maturation of inhibition during the development of sound localization on a cellular level, glycinergic currents and potentials were measured in gerbil MSO principal cells from postnatal (P) day P12-P25 by whole-cell patch-clamp recordings. The synaptic glycinergic currents accelerated to rapid decay kinetics (approximately 2 ms) and rise times (approximately 0.4 ms) after hearing onset, reaching maturity around P17. Since the kinetics of miniature glycinergic currents did not change with age, it is likely that a higher degree of transmitter release synchrony is the underlying mechanism influencing the acceleration of the kinetics. During the same period, the synaptic glycinergic potentials accelerated four-fold, largely as a result of a prominent decrease in input resistance. In accordance with a reorganization of the glycinergic inputs, the evoked peak conductances decreased more than two-fold, together with a three-fold reduction in the frequency of miniature events after hearing onset. These age-dependent changes were absent in animals that had been reared in omni-directional noise, indicating that an experience-dependent pruning of synaptic inputs is important for the maturation of functional inhibition in the MSO. Taken together, these striking developmental adjustments of the glycinergic inhibition in the MSO most probably reflect an adaptation to improve the encoding of auditory cues with great temporal precision and fidelity during the maturation of sound localization behaviour.

Development of Sound Localization Mechanisms in the Mongolian Gerbil Is Shaped by Early Acoustic Experience

Sound localization is one of the most important tasks performed by the auditory system. Differences in the arrival time of sound at the two ears are the main cue to localize low-frequency sound in the azimuth. In the mammalian brain, such interaural time differences (ITDs) are encoded in the auditory brain stem; first by the medial superior olive (MSO) and then transferred to higher centers, such as the dorsal nucleus of the lateral lemniscus (DNLL), a brain stem nucleus that gets a direct input from the MSO. Here we demonstrate for the first time that ITD sensitivity in gerbils undergoes a developmental maturation after hearing onset. We further show that this development can be disrupted by altering the animal's acoustic experience during a critical period. In animals that had been exposed to omnidirectional white noise during a restricted time period right after hearing onset, ITD tuning did not develop normally. Instead, it was similar to that of juvenile animals 3 days after hearing onset, with the ITD functions not adjusted to the physiological range. Animals that had been exposed to omnidirectional noise as adults did not show equivalent abnormal ITD tuning. The development presented here is in contrast to that of the development of neuronal representation of ITDs in the midbrain of barn owls and interaural intensity differences in ferrets, where the representations are adjusted by an interaction of auditory and visual inputs. The development of ITD tuning presented here most likely depends on normal acoustic experience and may be related to the maturation of inhibitory inputs to the ITD detector itself.

Metallo-dielectric electromagnetic bandgap structures for suppression and isolation of the parallel-plate noise in high-speed circuits

A novel approach for the suppression of the parallel-plate waveguide (PPW) noise in high-speed printed circuit boards is presented. In this approach, one of the two conductors forming the PPW is replaced by an electromagnetic bandgap (EBG) surface. The main advantage of the proposed approach over the commonly practiced methods is the omnidirectional noise suppression it provides. For this purpose, two EBG structures are initially designed by utilizing an approximate circuit model. Subsequently, the corresponding band structures are characterized by analytical solutions using the transverse resonance method, as well as full-wave finite-element simulations. The designed EBG surfaces were fabricated and employed in a number of PPW test boards. The corresponding frequency-domain measurements exhibited bandgaps of approximately 2.21 and 3.35 GHz in the frequency range below 6 GHz. More importantly, suppression of the PPW noise by 53% was achieved based on time-domain reflectometry experiments, while maintaining the signal transmission quality within the required specifications for common signaling standards.

Time-reversing array retrofocusing in noisy environments

Acoustic time reversal is a robust means of retrofocusing acoustic energy, in both time and space, to the original sound-source location. However, noise may limit the performance of a time-reversing array (TRA) at long source-array ranges, or when the original-source or TRA-element power levels are low. The operation of a TRA requires two steps (reception and transmission) so both TRA-broadcast noise and ambient noise must be taken into account. In this paper, predictions are made for how a simple omnidirectional noise field influences the probability that the signal amplitude from a narrow-band TRA will exceed the noise at the TRA's retrofocus. A general formulation for the probability of TRA retrofocusing, which can be used for TRA design, is developed that includes: the variance of the noise field, the original source strength, the TRA's element output power, the number of TRA elements (N), and the propagation characteristics of the environment. This formulation predicts that a TRA's array gain (in dB) at the retrofocus may be as high as + 10log10(N) to + 20 log10(N) depending on the relative strengths of the original source and the TRA's elements. Monte Carlo simulations in both a free-space environment and a shallow-ocean sound-channel environment compare well to this probability formulation even when simple approximate parametric relationships for the appropriate Green's functions are used. The dominant deviation between theory and simulation in the sound channel is caused by acoustic absorption.

The developmental emergence of the representation of auditory azimuth in the external nucleus of the inferior colliculus of the guinea-pig: the effects of visual and auditory deprivation

A topographic representation of the auditory azimuth has been described in the external nucleus of the inferior colliculus (ICX) of the guinea-pig [3]. This representation is characterized by directional multi-unit responses, at threshold stimulation intensities, with directional preferences organized in such a way as to represent the auditory azimuth along the rostro-caudal axis of the ICX. The following paper considers the emergence of that map and the role of developmental experience in its elaboration. Multi-unit responses to free-field broad-band auditory stimuli were recorded in the ICX. At threshold stimulation intensities, multi-unit receptive fields (MURFs) obtained from younger animals showed the same discrete spatial tuning as found in MURFs from animals older than 35 DAB (days after birth). However, a normal adult topographic representation was not present until animals were at least 30–32 DAB. Visual deprivation, by dark-rearing from birth until mapping (at 35–43 DAB), had no obvious detrimental effects on auditory receptive field size or topographic order in the ICX. Auditory deprivation was achieved by rearing animals in an environment of continuous omnidirectional noise from birth until mapping (47–53 DAB). Following auditory deprivation, receptive fields remained relatively discrete, but no correlation between rostro-caudal position of the recording site and the angle of the best response was observed. Thus, the representation of auditory azimuth in the ICX appears to be unperturbed by developmental visual deprivation but is susceptible to developmental auditory deprivation.

Vertical noise versus wind speed

The outpost sunrise exercise was conducted at a deep, high bottom loss area in the Pacific Ocean. Vertical 24-element arrays, nested for 75 and 150 Hz, were moored at the seabottom and at critical depth. The exercise was conducted during the fall, when high wind speeds are likely; wind speed (w) was recorded from a nearby buoy. For w≥4.5 m/s, omnidirectional noise depended on receiver depth and frequency band. At 300 Hz, ambient noise recorded below the critical depth was well correlated with wind speed, but at lower frequencies, this was not so. Correlation of beam noise level [BNL(φ)] for w≥4.5 m/s depended on vertical angle φ. Horizontally (φ=0°), the field was dominated by distant noise. BNL(φ) for φ≤15° closely tracked w. BNL(φ) was estimated at 65 and 130 Hz. BNL(90°) increased from 55 to 70 dB//μPa2 Hz-sr as w increased from 4.5 to 11 m/s. Theoretical BNL(φ) with source directionalities of a dipole, plane of monopoles, and volume of monopole planes are compared with data. A source modeled as a of a plane of monopoles between λ/5 and λ/3 depth best fist the data. [Work supported by the Office of Naval Research.]

Post-crucial Period Effects of Auditory Experience and Deprivation on the Guinea-pig Superior Collicular Map of Auditory Space.

The auditory space map in the superior colliculus of the guinea-pig requires for its normal emergence at 32 days after birth (DAB) bimodal, auditory and visual experience during a 4-day crucial period (26 - 30 DAB) (Withington-Wray et al., Neurosci. Lett., 116, 280 - 286, 1990d). The need for auditory experience has been proposed to be linked to the rapid changes in the cues for auditory localization brought about by head growth. Head growth is particularly rapid during the first postnatal month but continues more slowly until approximately 70 DAB. This suggests that sensory experience may be required beyond the initial crucial period to accommodate later growth. In this paper the role of auditory experience beyond the crucial period has been investigated. Deprivation of ordinary auditory experience was effected by placement of the animals in an environment in which continuous omnidirectional noise obscured the cues of sound direction. Animals deprived of auditory experience during the crucial period and then allowed normal experience showed limited ability to construct an auditory space map and the resulting map was less accurate than that found in normally reared animals. Auditory deprivation following normal experience during the crucial period caused a profound degradation, of both spatial tuning and topography, of auditory multi-unit receptive fields in the superior colliculus. The spatial tuning and topography of auditory fields from older animals (100 DAB) deprived of ordinary auditory cues for a 4-week period were normal. Thus, in the guinea-pig, susceptibility of the superior collicular space map to deprivation of auditory cues is limited by the age of the animal. The timing of the cessation of vulnerability may, in part, be due to the stabilization of directional cues affected by head growth.

Deep sound channel noise from high-latitude winds

The contribution to underwater ambient noise from the persistent winds which blow at high latitude is discussed and estimated. Energy is ducted into low-loss paths in the deep sound channel by favorable horizontal sound-speed gradients. A concept of ‘‘wind-noise lanes’’ is developed into a simple model which is used to predict the resulting underwater ambient noise levels as a function of season and latitude. Three main oceans are considered—the Pacific, Atlantic, and Indian Oceans. In general terms, predicted omnidirectional levels at 50 Hz lie between 65 and 75 dB re: 1 μPa2/Hz and are comparable to those generally associated with light-to-moderate ship traffic. The spectral shape of high-latitude wind noise is also similar to that associated with ships. High-latitude wind noise arrives within ±15° of horizontal, and the predicted magnitude of this low-angle component also matches measurements well. It is concluded that the component of noise associated with high-latitude winds can be significant at low frequencies. Properties of omnidirectional noise, as well as vertical and horizontal directivity, may be dominated by this component under some conditions. It appears, however, to have been often overlooked in the interpretation of data.

The Measurement of Noise from Moving Vehicles

A new method for measuring vehicle noise is proposed that dynamically corrects the sound pressure for the source distance as the vehicle passes. The result is a sound pressure at unit distance from the vehicle. When logarithmic units are used, the quantity is defined as a noise source level (NSL). A system using a sound level meter, traffic radar, and means for a polar plot of NSL was built and evaluated. For an omnidirectional noise source on a vehicle, spherical spreading was verified and a polar plot, uniform with track angle, was produced. The directivity of a directive source on the vehicle was measured during a pass-by and under stationary conditions. Comparable results were obtained. The vehicle noise measurement method provides rapid data acquisition of source levels and directivity for noise research, a basic measure for noise certification of vehicles, and an improved means for noise ordinance enforcement.

Correlation of Ambient Sea Noise

Space-time correlations of ambient sea noise were measured with vertical pairs of hydrophones in the frequency range of 125–1131 cps and with horizontal pairs in the range of 22–63 cps at various sea states (SS) in the deep ocean. The experimental results are compared with those obtained from theoretical models of a uniform distribution of surface-noise sources, radiating with an intensity pattern of cos2nα, where α is the angle from the vertical. At SS5 and frequencies above 400 cps, a good fit to the experimental spatial correlations as well as to the principal peak of the space-time correlation is obtained for n = 1. At the same sea state, the 250-cps data correspond to n = 12. In the low-frequency range, omnidirectional noise sources explain the experimental data up to and including SS5. The model does not give a sufficient explanation for the space-time correlations at SS3 in the range 400–1131 cps.

Optimum filtering and multichannel receivers

We will examine the problem of optimum filtering when signals and noises are incoming on different channels. The filtering is defined by a rectangular matrix of filters and the non-linear operation in the signal processing is the general quadratic system. After the calculation of the output signal-to-noise ratio, we discuss the problem of optimum filtering. In this paper we consider only the solution in some particular cases as, for example, the correlator. In the second part, we discuss some properties of the optimum linear filters and we show in the third how the problem of directivity synthesis in omnidirectional noise is a particular case of optimum filtering.