Effects of a near-field rigid sphere scatterer on the performance of linear microphone array beamformers

© 2016 Acoustical Society of America. Beamformers enable a microphone array to capture acoustic signals from a sound source with high signal to noise ratio in a noisy environment, and the linear microphone array is of particular importance, in practice, due to its simplicity and easy implementation. A linear microphone array sometimes is used near some scattering objects, which affect its beamforming performance. This paper develops a numerical model with a linear microphone array near a rigid sphere for both far-field plane wave and near-field sources. The effects of the scatterer on two typical beamformers, i.e., the delay-and-sum beamformer and the superdirective beamformer, are investigated by both simulations and experiments. It is found that the directivity factor of both beamformers improves due to the increased equivalent array aperture when the size of the array is no larger than that of the scatter. With the increase of the array size, the directivity factor tends to deteriorate at high frequencies because of the rising side-lobes. When the array size is significantly larger than that of the scatterer, the scattering has hardly any influence on the beamforming performance

SoundCompass: a distributed MEMS microphone array-based sensor for sound source localization

Sound source localization is a well-researched subject with applications ranging from localizing sniper fire in urban battlefields to cataloging wildlife in rural areas. One critical application is the localization of noise pollution sources in urban environments, due to an increasing body of evidence linking noise pollution to adverse effects on human health. Current noise mapping techniques often fail to accurately identify noise pollution sources, because they rely on the interpolation of a limited number of scattered sound sensors. Aiming to produce accurate noise pollution maps, we developed the SoundCompass, a low-cost sound sensor capable of measuring local noise levels and sound field directionality. Our first prototype is composed of a sensor array of 52 Microelectromechanical systems (MEMS) microphones, an inertial measuring unit and a low-power field-programmable gate array (FPGA). This article presents the SoundCompass's hardware and firmware design together with a data fusion technique that exploits the sensing capabilities of the SoundCompass in a wireless sensor network to localize noise pollution sources. Live tests produced a sound source localization accuracy of a few centimeters in a 25-m2 anechoic chamber, while simulation results accurately located up to five broadband sound sources in a 10,000-m2 open field

On the design of a (H)EV steerable warning device using acoustic beam forming and advanced numerical acoustic simulation

This paper describes the simulation-based design methodology used in the eVADER project for the development of targeted acoustic warning devices for increased detectability of Hybrid and Electric Vehicles (HEVs) while, at the same time, reducing urban noise pollution. A key component of this system is an external warning signal generator capable of projecting the warning signals to a contained area in front of the vehicle where potential at-risk situations are detected. Using acoustic beam forming principles a suitable warning strategy and an initial layout for realizing such a system is defined. Starting from this information, acoustic Finite and Boundary Element models of the transducer array allow assessing more realistically the performance impact of the system integration and of the most critical changes in the acoustic environment in which the signal generator needs to operate

Simulation-based design of a steerable acoustic warning device to increase (H)EV detectability while reducing urban noise pollution

This paper describes the simulation-based design methodology used in the eVADER project for the development of targeted acoustic warning devices for increased detectability of Hybrid and Electric Vehicles (HEVs) while, at the same time, reducing urban noise pollution compared to conventional acoustic pedestrian warning systems. A key component of this system is an external warning signal generator capable of projecting the warning signals to a contained area in front of the vehicle where potential at-risk situations are detected. Using acoustic beam forming principles a suitable warning strategy and an initial layout for realizing such a system is defined. Starting from this information, acoustic Finite and Boundary Element models of the transducer array allow assessing more realistically the performance impact of the system integration and of the most critical changes in the acoustic environment in which the signal generator needs to operate

Discrete interferences optimum beamformer in correlated signal and interfering noise

This paper introduces a significant special situation where the noise is a collection of D-plane interference signals and the correlated noise of D+1 is less than the number of array components. An optimal beamforming processor based on the minimum variance distortionless response (MVDR) generates and combines appropriate statistics for the D+1 model. Instead of the original space of the N-dimensional problem, the interference signal subspace is reduced to D+1. Typical antenna arrays in many modern communication networks absorb waves generated from multiple point sources. An analytical formula was derived to improve the signal to interference and noise ratio (SINR) obtained from the steering errors of the two beamformers. The proposed MVDR processor-based beamforming does not enforce general constraints. Therefore, it can also be used in systems where the steering vector is compromised by gain. Simulation results show that the output of the proposed beamformer based on the MVDR processor is usually close to the ideal state within a wide range of signal-to-noise ratio and signal-to-interference ratio. The MVDR processor-based beamformer has been experimentally evaluated. The proposed processor-based MVDR system significantly improves performance for large interference white noise ratio (INR) in the sidelobe region and provide an appropriate beam pattern

Microphone Array Verification for Broadband Sodar

Monet seurantajärjestelmät kuten tutka (engl. radar) toimivat lähettämällä signaalia ja kuuntelemalla kaikuja tästä lähetetystä signaalista. Tutkan tapauksessa lähetetty signaali on radioaalto. Ääniaaltoja voidaan myös käyttää kuten radioaaltoja tutkassa ja väliaineen ollessa ilma kutsutaan laitetta äänitutkaksi (engl. sodar). Tutkassa vastaanotto ja lähetys tapahtuvat antenneilla, kun taas äänitutkassa mikrofoneilla ja kaiuttimilla. Kohteen etäisyys saadaan laskemalla aika signaalin lähetyksen ja kaiun vastaanoton välillä. Kohteen suunta taas saadaan selvitettyä kääntämällä vastaanottavia sensoreita. Tämä diplomityö on osa isompaa kokonaisuutta, joka tähtää halpaan tutkan toiminnallisuuden testausjärjestelmään. Työn tavoitteena oli selvittää riittääkö halpojen mikrofonien tarkkuus laajakaistaisen äänitutkan toteuttamiseen. Tämän selvittämistä varten tutustuttiin ensimmäisenä äänitutkan taustalla tapahtuviin ilmiöihin. Koska äänitutkaan liittyvää kirjallisuutta on tehty verrattain vähän esiteltiin se tutkan teorioiden avulla. Työssä käsitellään myös keilanmuodostusta. Keilanmuodostus on signaalinkäsittelymenetelmä sensoriryhmän spatiaalisen toiminallisuuden hallintaan. Keilanmuodostus parantaa suorituskykyä, mahdollistaa digitaalisen keilankääntämisen sekä laajan taajuuskaistan hallinan. Itse mikrofonien varmentaminen tapahtui tekemällä teoreettiset simulaatiot sekä käytännön mittaukset ja vertailemalla näitä keskenään. Ensimmäisten mittausten jälkeen tuli jo selväksi että ostetut mikrofonit riittävät äänitutkan toteuttamiseen. Tällaisenaan mikrofonien epäideaalisuudet näkyvät ryhmän keilakuviossa. Tämän takia mikrofonit pitää kalibroida ennen käyttöä ja siksi tämä työ esittelee myös mikrofonikalibrointimenetelmän, jonka jälkeen keilakuvio on verrattain lähellä ideaalitapausta. Työssä mitattiin ja simuloitiin myös digitaalista keilankääntöä sekä laajakaistaista keilanmuodostusta. Molemmissa tapauksissa mittaustulokset ovat lähellä ideaalisia simulaatioita. Täten työ todistaa, että ostetut mikrofonit ovat riittävät laajakaistaiseen äänitutkasovellukseen.Many surveillance systems, such as radar (radio detection and ranging), work in a way that a signal is transmitted and its echoes are detected. In the case of radar, the transmitted wave is a radio wave. When sound waves are used instead of radio waves and the medium is air, the system is called sodar (sonic detection and ranging). In radar, antennas are the receiving and transmitting sensors, while in sodar the sensors are microphones and speakers. The range of the target can be determined by calculating the time between the transmission of the signal and the reception of the echo. The bearing of the target can be determined by rotating the receiving sensors. This thesis is a part of a project that aimed to develop a cheap radar-emulation environment. The main objective of the thesis was to verify if inexpensive microphones are feasible for a microphone array implementation for the broadband sodar. First, the phenomena behind sodar were studied. Since literature on the topic is scarce, sodar is explained with the theories of radar. The concept of the beamforming was also studied. Beamforming is a signal processing method for controlling the spatial properties of a sensor array. Beamforming enhances the performance, and enables both digital beam steering and the control of the broad frequency band signals. The microphone veri cation was accomplished with theoretical simulations and practical measurements, and comparing these. After the rst measurement it already became clear that the microphones studied are capable for the sodar implementation. However, the microphone unidealities are apparent in the beam pattern of the measured array. For this reason, the microphones should be calibrated before use. Thus, this thesis introduces a microphone calibration method after which the beam pattern becomes nearly ideal. The digital beam streering and broadband beamforming were also simulated and measured. In both cases, the measurement result is close to the ideal models. Thus, this thesis proves that inexpensive microphones are capable for broadband sodar implementation

Robust Multichannel Microphone Beamforming

In this thesis, a method for the design and implementation of a spatially robust multichannel microphone beamforming system is presented. A set of spatial correlation functions are derived for 2D and 3D far-field/near-field scenarios based on von Mises(-Fisher), Gaussian, and uniform source location distributions. These correlation functions are used to design spatially robust beamformers and blocking beamformers (nullformers) designed to enhance or suppress a known source, where the target source location is not perfectly known due to either an incorrect location estimate or movement of the target while the beamformers are active. The spatially robust beam/null-formers form signal and interferer plus noise references which can be further processed via a blind source separation algorithm to remove mutual components - removing the interference and sensor noise from the signal path and vice versa. The noise reduction performance of the combined beamforming and blind source separation system approaches that of a perfect information MVDR beamformer under reverberant conditions. It is demonstrated that the proposed algorithm can be implemented on low-power hardware with good performance on hardware similar to current mobile platforms using a four-element microphone array