Hemispherical Microphone Array Research Articles

Drone noise directivity and psychoacoustic evaluation using a hemispherical microphone array.

This paper proposes an experimental setup for measuring the sound radiation of a quadrotor drone using a hemispherical microphone array. The measured sound field is decomposed into spherical harmonics, which enables the evaluation of the radiation pattern to non-probed positions. Additionally, the measurement setup allows the assessment of noise emission and psychoacoustic metrics at a wide range of angles. The obtained directivity patterns using a third-order spherical harmonic decomposition (SHD) are shown to exhibit low distortion with respect to the original measurements, therefore, validating the SHD as an adequate representation strategy. Furthermore, the noise emissions are evaluated, and the highest noise emission is observed in the 90 ° azimuth direction. The exterior spherical acoustic holography description is employed to evaluate psychoacoustic metrics at arbitrary far-field positions and validated on a reference microphone. The estimated psychoacoustic metrics are closely related to the target metrics, which allows for sound quality analysis at any point external to the drone.

Optimal beamformer design in spherical sector harmonics domain

Beamforming using spherical microphone arrays (SMAs) with processing in spherical harmonics (SH) domain is widely being studied. This is due to the ease of array processing in SH domain with no spatial ambiguity. Despite the widespread applications of SMA in beamforming, its size makes it inconvenient for practical or commercial use. Building of SMA over a rigid sphere is also a challenging task. Additionally, use of the entire sphere comes at the cost of more microphones and signals to process. Hence, it is uneconomic to use full SMA when sources are only present in restricted regions of the environment. Attempts have been made in literature to use hemispherical microphone array for beamforming. Acoustic image principle was utilized to enable application of SH but with greater computational complexity. In this paper, the use of a spherical sector microphone array is proposed for beamforming. An orthonormal spherical sector harmonics (S2H) basis function is used for data model development. An ideal direction-invariant beampattern using S2H basis function is utilized. Subsequently, to make the beamformer robust against sensor noise with a sharp directivity pattern at the direction of arrival and zero elsewhere, a new maximum directivity and white noise gain (MD-WNG) beamformer is designed for a spherical sector microphone array. Performance analysis of the proposed beamformer is presented using various experiments on directivity pattern and array gain. The proposed method is additionally compared with several existing beamforming techniques. The comparison reveals that the S2H based beamforming provides reasonably better results compared to the existing methods.

The reduction of gunshot noise and auditory risk through the use of firearm suppressors and low-velocity ammunition

Objective: This research assessed the reduction of peak levels, equivalent energy and sound power of firearm suppressors. Design: The first study evaluated the effect of three suppressors at four microphone positions around four firearms. The second study assessed the suppressor-related reduction of sound power with a 3 m hemispherical microphone array for two firearms. Results: The suppressors reduced exposures at the ear between 17 and 24 dB peak sound pressure level and reduced the 8 h equivalent A-weighted energy between 9 and 21 dB depending upon the firearm and ammunition. Noise reductions observed for the instructor’s position about a metre behind the shooter were between 20 and 28 dB peak sound pressure level and between 11 and 26 dB LAeq,8h. Firearm suppressors reduced the measured sound power levels between 2 and 23 dB. Sound power reductions were greater for the low-velocity ammunition than for the same firearms fired with high-velocity ammunition due to the effect of N-waves produced by a supersonic bullet. Conclusions: Firearm suppressors may reduce noise exposure, and the cumulative exposures of suppressed firearms can still present a significant hearing risk. Therefore, firearm users should always wear hearing protection whenever target shooting or hunting.

Incorporating image sources in the time-domain beamforming model for the localization of sound sources in reverberant rooms

In this study, we use a small hemispherical microphone array to locate impulsive or stationary sound sources in reverberant rooms. A modified version of the conventional time-domain beamforming model was used, which incorporates physical information from the early specular reflections of the sound source, as well as the geometric and absorption characteristics of the room. In other words, the propagation model takes into account the potential image sources up to a given image order. We show that the technique can be seen as a time-reversal process, the measured pressure field being simultaneously re-emitted from both the actual array position and its virtual image positions. We then present results applying this modified model to simulated data (generated using an image-source propagation model), as well as measurements in real rooms, using both impulsive and continuous broad-band noise. In all cases, the modified model shows an improvement in localization over conventional beamforming. The issue of appro...

Metamorphic hemispherical microphone array for three-dimensional acoustics

This article describes the realization of a metamorphic stretchable microphone array, which can be inflated by air to morph from a planar to a hemispherical shape. The array undergoes morphological changes to adjust their receive characteristic. To realize this device, a metamorphic printed circuit board technology (m-PCB) is described. The resulting products are millimeter-thin stretchable silicone embedded and electrically interconnected electronic structures with mechanical properties, which resemble a silicone membrane. The microphone array is used to localize a sound source in a 3D space. The results of the planar orientation (resting shape), and the 3D hemispherical orientation after air inflation are compared. The inflated hemispherical microphone array proofs to be better for 3D acoustic localization and/or beam-forming.

Signal processing for hemispherical measurement data

To realistically model the sound propagation in rooms, a detailed knowledge of the reflection properties of the surrounding surfaces is required. In this context, the reflection properties include both the sound absorption as well as scattering. In order to be able to measure the angle-dependent reflection properties of surfaces in-situ, a hemispherical microphone array was recently designed and built. For a reduction of the required hardware an efficient, rotationally symmetric sampling was chosen, so that 28 microphones on two concentric semicircles are employed to measure a total of over 3000 positions on a hemisphere in just over 15 minutes. This contribution will give an overview over the required signal processing steps to process the measurement data from such a microphone array. Special emphasis will be placed on the determination of the microphone positions and the special case of data available on a hemispherical surface. Also, the sound field model used to determine the impedance on the surface will be explained. Preliminary results will be presented.

Alternative closeness functions for eye microphone array

A new signal processing algorithm, accompanied by a novel hemispherical microphone array structure for sound source localization in three-dimensional spaces was presented [H. Alghassi et al., ‘‘Acoustic source localization with eye array,’’ JASA 120(5) (2006)]. This localization methodology, which has some analogy to the eye in localization of light rays, uses concepts of two-microphone (pinhole) or three-microphone (lens) cell structures alongside a special closeness function (CF) to approximate the proximity of the sound source direction to each of the hemisphere microphone directions based on particular similarity measures among signals. The CF plays a major role in the accuracy of the final source direction estimation. The earlier multiplicative CF (MCF) operates based on vector multiplication of spatial derivative and time derivative of microphone signals. This work presents two additional categories of CFs and compares them with MCF. The difference CF (DCF) is based on subtraction of delayed reference signal and shell microphone signals, while the correlative CF (CCF) is based on multiplication of delayed reference signal and shell microphone signals. Similar to MCF, both DCF and CCF perform demonstrated linear output versus deviation angle. Although DCF and MCF have not shown improved experimental accuracy compared to MCF, they attained lower computational complexity.

Spherical and hemispherical microphone arrays for capture and analysis of sound fields

The capture of the spatial structure of a sound field and analysis is important in many fields including creating virtual environments, source localization and detection, noise suppression, and beamforming. Spherical microphone arrays are a promising development to help achieve such capture and analysis, and have been studied by several groups. We develop a practical spherical microphone array and demonstrate its utility in applications for sound capture, room measurement and for beamforming and tracking. To accommodate equipment failure and manufacturing imprecision we extend their theory to handle arbitrary microphone placement. To handle speech capture and surveillance we describe the development of a new sensor, the hemispherical microphone array. For each array the practical performance follows that predicted by theory. Future applications and improvements are also discussed. [Work supported by NSF.]

Acoustic source localization with eye array

A novel signal-processing algorithm and array for sound source localization (SSL) in three-dimensional space is presented. This method, which has similarity to the eye in localization of light rays, consists of a novel hemispherical microphone array with 26 microphones on the shell and one microphone in the sphere center. The microphones on the shell map a geodesic hemisphere called two-frequency icosahedron; hence, each microphone has at least four other orthogonal microphones. A signal-processing scheme utilizes parallel creation of a special closeness function for each microphone direction on the shell in the time domain. Each closeness function cell (lens cell) consists of center microphone, shell microphone, and one of its orthogonals. The closeness function output values are linearly proportional to spatial angular difference between the sound source direction and each of the shell microphone directions. By choosing microphone directions corresponding to the highest closeness function values and implementing a linear weighted spatial averaging on them, the sound source direction is estimated. Contrary to traditional SSL techniques, this method is based on simple parallel mathematical calculations in the time domain with low computational costs. The laboratory implementation of the array and algorithm shows reasonable accuracy in a reverberant room.

Validation of the cylindrical microphone array for measuring sound power levels of noise sources

Recent papers have (i) introduced the concept of the cylindrical microphone array for the measurement of sound power levels of noise sources as an alternative to the parallelepiped and hemispherical arrays [M. A. Nobile and J. A. Shaw, Proc. Inter-Noise 99], (ii) set forth a detailed procedure for using the cylindrical array that could readily be incorporated into international standards [Nobile et al., Proc. Noise-Con 2000], and (iii) presented the results of measurements using that procedure on a 1.75-m-high noise source using a variety of microphone positions to draw conclusions regarding the number of microphones, equal versus unequal areas, and traversing versus fixed-point implementations [Nobile et al., Proc. Inter-Noise 2002]. The cylindrical array has since been incorporated into ISO 7779 and is being proposed for inclusion in the upcoming revision of ISO 3744. In an effort to gain experience with this new array and to help validate its use, a prototype rotating cylindrical array was constructed, and measurements were taken on a variety of noise sources. The results are compared to those taken with the well-established hemispherical microphone array, using 40 coaxial circular paths.