Comparing the end correction of a spherically baffled piston to infinitely baffled and unbaffled circular radiators (L).

Models of acoustical systems commonly employ end corrections to represent the radiation impedance of a vibrating element. Although several analytic solutions appear in the literature, the end corrections of an infinitely baffled circular piston and an unbaffled semi-infinite circular pipe remain popular in modeling applications. This Letter compares the end correction of these two configurations to that of a radially vibrating cap on a sphere. The results show that the spherically baffled end correction, when expressed as a function of spherical-cap radius, falls between these two extremal boundary conditions.

On the low-frequency acoustic center.

Acousticians typically consider the acoustic center of a source to be the point from which sound waves appear to diverge spherically. Many applications require the center's accurate determination, but its deeper significance and means of assessment have often remained ambiguous. This work revisits the acoustic center and shows how a low-frequency sound radiator with omnidirectional far-field directivity has a center defined by its dipole-to-monopole moment ratio. This definition yields conclusive results for several theoretical sources and highlights the limitations of characterizing the acoustic center only in terms of an equivalent point source.

Low-frequency radiation from a vibrating cap on a rigid spherical shell with a circular aperture.

Theoretical models based on spherical geometries have long provided essential insights into the directional behavior of sound sources such as loudspeakers and human speech. Because commonly applied models predict omnidirectional radiation at low frequencies and increasing directionality at higher frequencies, they fail to predict the directional characteristics of certain sources with different source geometries. These sources include violins and open-back guitar amplifiers that have openings or ports connecting a cavity or enclosure to the exterior domain. This work presents the low-frequency radiation from a vibrating cap on a rigid spherical shell with a circular aperture to study the directional characteristics of such sources. The proposed model predicts dipolar radiation at very low frequencies, monopolar radiation near the Helmholtz resonance, and increasing directionality at higher frequencies. Experimental results based on measuring the sound field of an open-back spherical loudspeaker validate the theoretical model and highlight its utility in predicting directional behavior.

Directional characteristics of two gamelan gongs.

The distinctive geometry and structural characteristics of Balinese gamelan gongs lead to the instrument's unique sound and musical style. This work presents high-resolution directivity measurements of two types of gamelan gongs to quantify and better understand their acoustic behavior. The measured instruments' structural modes clearly impact their far-field directivity patterns, with the number of directional lobes corresponding to the associated structural mode shapes. Many of the lowest modes produce dipole-like radiation, with the dipole moment determined by the positions of the nodal and antinodal regions. Higher modes exhibit more complex patterns with multiple lobes often correlated with the location and number of antinodal regions on the gong's edge. Directivity indices correspond to dipole radiation at low frequencies and quadrupole radiation at intermediate and higher frequencies. Symmetry analysis confirms that the gong's rim significantly impacts the resultant directivity.

High-resolution spherical directivity of live speech from a multiple-capture transfer function method.

Although human speech radiation has been a subject of considerable interest for decades, researchers have not previously measured its directivity over a complete sphere with high spatial and spectral resolution using live phonetically balanced passages. The research reported in this paper addresses this deficiency by employing a multiple-capture transfer function technique and spherical harmonic expansions. The work involved eight subjects and 2522 unique sampling positions over a 1.22 or 1.83 m sphere with 5° polar and azimuthal-angle increments. The paper explains the methods and directs readers to archived results for further exploration, modeling, and speech simulation in acoustical environments. Comparisons of the results to those of a KEMAR head-and-torso simulator, lower-resolution single-capture measurements, other authors' work, and basic symmetry expectations all substantiate their validity. The completeness and high resolution of the measurements offer insights into spherical speech directivity patterns that will aid researchers in the speech sciences, architectural acoustics, audio, and communications.