This article addresses the way in which we can generate an "acoustically bright zone" in a space. The bright zone is defined as the volume where we can have higher acoustic energy than in other space. A method is proposed to generate the bright zone by controlling multiple monopole sources. Two kinds of cost functions involved with acoustic brightness are defined. One is the ratio of the brightness of a zone to the input power, and the other expresses the "contrast" between the bright zone and the other space. Through eigenvalue analysis, the optimal volume velocity distribution of the monopoles has been obtained.

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A graphene thermoacoustic loudspeaker with a thin polymer mesh is fabricated using screen-printing. An experiment with substrates of various free-standing areas shows that a higher sound pressure level can be achieved as compared to previously reported graphene thermoacoustic loudspeakers. Moreover, a modified equation to predict the sound pressure level of the thermoacoustic loudspeaker with a thin and patterned substrate is proposed and verified by experimental results.

We built a thermoacoustic loudspeaker employing N-doped three-dimensional reduced graphene oxide aerogel (N-rGOA) based on a simple template-free fabrication method. A two-step fabrication process, which includes freeze-drying and reduction/doping, was used to realize a three-dimensional, freestanding, and porous graphene-based loudspeaker, whose macroscopic structure can be easily modulated. The simplified fabrication process also allows the control of structural properties of the N-rGOAs, including density and area. Taking advantage of the facile fabrication process, we fabricated and analyzed thermoacoustic loudspeakers with different structural properties. The anlayses showed that a N-rGOA with lower density and larger area can produce a higher sound pressure level (SPL). Furthermore, the resistance of the proposed loudspeaker can be easily controlled through heteroatom doping, thereby helping to generate higher SPL per unit driving voltage. Our success in constructing an array of optimized N-rGOAs able to withstand input power as high as 40 W demonstrates that a practical thermoacoustic loudspeaker can be fabricated using the proposed mass-producible solution-based process.

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The eigenbeam estimation of signal parameters via the rotational invariance technique (EB-ESPRIT) is a well-known subspace-based beamforming algorithm for a spherical microphone array. EB-ESPRIT uses a recurrence relation to directly estimate directional parameters expressing directions-of-arrival (DOAs) of sound sources without an exhaustive grid-search. In the conventional EB-ESPRIT, the directional parameter along the elevational direction is given by a tangent function, which inevitably produces two shortcomings. First, the tangent function becomes singular for sources near the equator in spherical coordinates. Furthermore, two sources lying in exactly opposite directions in the spherical coordinates are indistinguishable and a strong ambiguity problem arises. In this work, an EB-ESPRIT technique based on generalized eigenvalue decomposition (GEVD) is proposed to resolve the singularity and ambiguity problems. The proposed technique uses three independent recurrence relations for spherical harmonics, thus the singularity problem due to the tangent function can be completely avoided. A common transformation matrix for extracting DOAs from recurrence relations are found from the GEVD, and the use of cosine and sine functions makes it possible to find DOAs without ambiguity and without extra transforms or angle-pairing processes. It is demonstrated that the proposed method not only overcomes the singularity and ambiguity problems, but also outperforms conventional techniques in terms of DOA accuracy.