For the visualization of a sound field, a widely used method is the superimposition of the sound information onto a camera view. Although it effectively enables the understanding the relationship between space and sound, a planar display cannot resolve depth information in a straightforward manner. In contrast, a see-through head-mounted display (STHMD) is capable of representing three-dimensional (3D) vision and natural augmented reality (AR) or mixed reality (MR). In this paper, we propose a system for the measurement and visualization of a sound field with an STHMD. We created two visualization systems using different types of STHMDs and technologies for realizing AR/MR and a measurement system for a 3D sound intensity map, which can be used together with the visualization system. Through three visualization experiments, we empirically found that the stereoscopic viewing and the convenient viewpoint movement associated with the STHMD enables understanding of the sound field in a short time.
To measure environmental noises, many noise measurement and mapping systems have been studied. However, these systems are costly because many measurement points are required to construct a detailed and three-dimensional noise map. In recent years, with advances in unmanned aerial vehicle (UAV) technology, a multirotor aircraft has been frequently used as general use. Although it could be applied to acoustic measurements, it causes loud noise as it must always rotate its propellers during flying. Herein, a noise and spatial mapping system with a blimp robot is proposed. The proposed system achieved a silent, slow, and omnidirectional movement with a balloon filled with helium gas. Furthermore, the simultaneous localization and mapping (SLAM) technique is applied for the system's positional tracking and surrounding spatial mapping with a stereo camera. To evaluate our system, three experiments were conducted. First, the propeller rotational noises of the proposed system were compared to a general recreational-use multirotor. Next, the acoustical effects of a blimp, such as reflection and diffraction, were measured to decide the microphone position. Finally, a preliminary experiment was conducted to construct a simple three-dimensional noise map in a large experimental room. The results show that the proposed system could construct a three-dimensional indoor noise map by combining the sound information and the positional information.
In sound field reproduction with Higher-Order Ambisonics (HOA), a sweet spot is formed around a reproduction point that is generally a center of spherical loudspeaker array. The conventional HOA decoding with a unique solution yields a spherical sweet spot, in which the reproduction error is less than 4% and whose radius is theoretically defined in the literature. On the other hand, it is known that Max-rE decoding or a least norm solution derived when using a larger number of loudspeakers would lead to an enhancement of reproduction accuracy. However, it is yet to be revealed how Max-rE decoding and the least norm solution affects the reproduction accuracy and the size and shape of the sweet spot, when they are individually or simultaneously applied to HOA decoding. This paper numerically investigates the reproduction accuracy of HOA with such different HOA decoding methods. The numerical results suggest that, compared to the conventional decoding method, ones with Max-rE and/or the least norm solution result in deformed or expanded sweet spot and suppression of reproduction errors outside the sweet spot.
Two-demensional (2D) models of a flue organ pipe are studied with compressible fluid simulation, specifically compressible Large Eddy Simulation, focusing on the influence of the geometry of the flue and the foot on the jet motion and acoustic oscillation in the pipe. When the foot geometry is fixed, the models having a flue with chamfers show good performances in stabilizing the acoustic oscillation in the steady state. Furthermore, we find that the foot chamber works as a Helmholtz resonator. If the frequency of the acoustic oscillation in the pipe is higher than the resonance frequency of the Helmholtz resonator by almost the full-width at half-maximum, anti-phase synchronization between the acoustic oscillation in the pipe and that in the foot chamber occurs. In this case, the acoustic oscillation in the pipe grows rapidly in the attack transient and is stabilized in the steady state.
Wind turbine noise (WTN) generally has amplitude modulation (AM) components, which often cause psychological annoyance in residential areas around the power generation plant. To investigate the directivity of AM sound generated from a wind turbine, field measurements have been performed under various wind conditions. Some receiving points were set circularly around a single wind turbine, and meteorological and associated wind turbine operational data were collected along with corresponding acoustic data. The method for extracting the AM components from the analyzed sound pressure levels at 100 ms intervals is based on the ideas of the F-S method. The results revealed a distinguishable directivity pattern of the strength of the AM components contained in WTN. The magnitudes of AM sound become lower in the downwind directions and highest in the direction approximately 60° relative to the front of the nacelle. In addition, the strength distributions of the AM components at distances of up to 200 m from the turbine were examined. It was found that almost all the magnitudes of AM sound still remained above 2 dB at a distance of even 200 m, whereas the A-weighted sound pressure levels decreased with increasing propagation distance.