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.

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.

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.

Effects of added absorption on the vocal exertions of talkers in a reverberant room.

Occupational speech users such as schoolteachers develop voice disorders at higher rates than the general population. Previous research has suggested that room acoustics may influence these trends. The research reported in this paper utilized varying acoustical conditions in a reverberant room to assess the effects on vocal parameters of healthy talkers. Thirty-two participants were recorded while completing a battery of speech tasks under eight room conditions. Vocal parameters were derived from the recordings and the statistically significant effects of room acoustics were verified using mixed-model analysis of variance tests. Changes in reverberation time (T20), early decay time (EDT), clarity index (C50), speech transmission index (STI), and room gain (GRG) all showed highly correlated effects on certain vocal parameters, including speaking level standard deviation, speaking rate, and the acoustic vocal quality index. As T20, EDT, and GRG increased, and as C50 and STI decreased, vocal parameters showed tendencies toward dysphonic phonation. Empirically derived equations are proposed that describe the relationships between select room-acoustic parameters and vocal parameters. This study provides an increased understanding of the impact of room acoustics on voice production, which could assist acousticians in improving room designs to help mitigate unhealthy vocal exertion and, by extension, voice problems.

Teachers and Teaching: Speech Production Accommodations Due to Changes in the Acoustic Environment.

School teachers have an elevated risk of voice problems due to the vocal demands in the workplace. This manuscript presents the results of three studies investigating teachers' voice use at work. In the first study, 57 teachers were observed for 2 weeks (waking hours) to compare how they used their voice in the school environment and in non-school environments. In a second study, 45 participants performed a short vocal task in two different rooms: a variable acoustic room and an anechoic chamber. Subjects were taken back and forth between the two rooms. Each time they entered the variable acoustics room, the reverberation time and/or the background noise condition had been modified. In this latter study, subjects responded to questions about their vocal comfort and their perception of changes in the acoustic environment. In a third study, 20 untrained vocalists performed a simple vocal task in the following conditions: with and without background babble and with and without transparent plexiglass shields to increase the first reflection. Relationships were examined between [1] the results for the room acoustic parameters; [2] the subjects' perception of the room; and [3] the recorded speech acoustic. Several differences between male and female subjects were found; some of those differences held for each room condition (at school vs. not at school, reverberation level, noise level, and early reflection).

Classroom acoustics for vocal health of elementary school teachers.

School teachers have an elevated risk of voice problems due to vocal demands in the workplace. ANSI SI2.60-2002 provides a standard for classroom acoustics, but it focuses primarily on students and unoccupied classroom settings. This presentation explores a preliminary study of six elementary school teachers that included measurements of architectural acoustics parameters and noise-levels of their classrooms, as well as their speech levels and fundamental frequencies over the course of a school day. The measurement methods and speech trends are discussed for the various cases, demonstrating that classroom acoustics standards may benefit from greater attention to teacher vocal health.

Finite-difference simulations of transient radiation from a finite-length pipe.

The far-field radiation originating from a finite-length pipe is well studied, especially for steady-state conditions. However, because all physical systems do not begin in steady state, these radiation characteristics are only valid after the transient portion of the solution has decayed. Understanding transient radiation characteristics may be important (particularly for systems transmitting very short-duration signals), as they can differ quite significantly. A numerical complication to this problem involves dealing with a sharp corner in the domain of interest. While many numerical studies have attempted to couple solutions from the domains inside and outside a pipe, the analysis presented in this work treats the computational domain as a single region by expressing the entire physical domain as a map from a simple rectangular domain in generalized curvilinear coordinates. This method will be introduced in detail and general results of transient radiation will be presented for an infinitely baffled, finite-length pipe using the finite-difference method expressed in generalized curvilinear coordinates. Comparison will be made to previous results [P. Stepanishen and R. A. Tougas, J. Acoust. Soc. Am. 93, 3074-3084 (1993)] that used a semi-analytic approach with certain assumptions.

An extended lumped-element model and parameter estimation technique to predict loudspeaker responses with possible surround-dip effects.

Lumped-element models have long been used to estimate the basic vibration and radiation characteristics of moving-coil loudspeakers. The classical low-frequency model combines and simplifies several important driver elements, predicting only a single mechanical resonance wherein the diaphragm (e.g., cone and dust cap) and the inner portion of the surround move together as an effective piston. Even if the diaphragm maintains piston-like motion with increasing frequency, the flexible surround eventually vibrates out of phase, producing another resonance whereby a noticeable "surround dip" may occur in the radiated pressure spectrum. The classical model is unable to predict this behavior. This paper explores an extended lumped-element model that better characterizes the distinct diaphragm, surround, spider, and other properties of a loudspeaker in a plane rigid baffle. It extends effective modeling to mid frequencies and readily predicts a surround dip in the radiated response. The paper also introduces a method to estimate model parameters using a scanning laser Doppler vibrometer, a surround resonance indicator function, and a constrained optimization routine. The approach is validated by its ability to better predict on-axis pressure responses of several baffled loudspeakers in an anechoic environment.

Evaluation of moving-coil loudspeaker and passive radiator parameters using normal-incidence sound transmission measurements: theoretical developments.

The parameters of moving-coil loudspeaker drivers are typically determined using direct electrical excitation and measurement. However, as electro-mechano-acoustical devices, their parameters should also follow from suitable mechanical or acoustical evaluations. This paper presents the theory of an acoustical method of excitation and measurement using normal-incidence sound transmission through a baffled driver as a plane-wave tube partition. Analogous circuits enable key parameters to be extracted from measurement results in terms of open and closed-circuit driver conditions. Associated tools are presented that facilitate adjacent field decompositions and derivations of sound transmission coefficients (in terms of driver parameters) directly from the circuits. The paper also clarifies the impact of nonanechoic receiving tube terminations and the specific benefits of downstream field decompositions.

Creating an active-learning environment in an introductory acoustics course.

Research in physics education has indicated that the traditional lecture-style class is not the most efficient way to teach introductory physical science courses at the university level. Current best teaching practices focus on creating an active-learning environment and emphasize the students' role in the learning process. Several of the recommended techniques have recently been applied to Brigham Young University's introductory acoustics course, which has been taught for more than 40 years. Adjustments have been built on a foundation of establishing student-based learning outcomes and attempting to align these objectives with assessments and course activities. Improvements have been made to nearly every aspect of the course including use of class time, assessment materials, and time the students spend out of the classroom. A description of the progress made in improving the course offers suggestions for those seeking to modernize or create a similar course at their institution. In addition, many of the principles can be similarly applied to acoustics education at other academic levels.

The effects of non-cardioid directivity on incidence angle estimation using the polar energy time curve.

Assessment of desirable reflections and control of undesirable reflections in rooms are best accomplished if the reflecting surfaces are properly localized. Several measurement techniques exist to identify the incident direction of reflected sound, including the useful polar energy time curve (Polar ETC), which requires six cardioid impulse response measurements along the Cartesian axes. The purpose of this investigation is to quantify the incidence angle estimation error introduced into the Polar ETC by non-cardioid microphone directivities. The results demonstrate that errors may be minimized with a cardioid-family microphone possessing a certain range of directivities and by maximizing the measurement signal-to-noise ratio.

Generalized acoustic energy density.

The properties of acoustic kinetic energy density and total energy density of sound fields in lightly damped enclosures have been explored thoroughly in the literature. Their increased spatial uniformity makes them more favorable measurement quantities for various applications than acoustic potential energy density (or squared pressure), which is most often used. In this paper, a generalized acoustic energy density (GED), will be introduced. It is defined by introducing weighting factors into the formulation of total acoustic energy density. With an additional degree of freedom, the GED can conform to the traditional acoustic energy density quantities, or it can be optimized for different applications. The properties of the GED will be explored in this paper for individual room modes, a diffuse sound field, and a sound field below the Schroeder frequency.

Active sound transmission control of a double-panel module using decoupled analog feedback control: experimental results.

Low-frequency sound transmission through passive lightweight partitions often renders them ineffective as means of sound isolation. As a result, researchers have investigated actively controlled lightweight partitions in an effort to remedy this problem. One promising approach involves active segmented partitions (ASPs), in which partitions are segmented into several distinctly controlled modules. This paper provides an experimental analysis of a double-panel ASP module wherein the source- and transmitting-side panels are independently controlled by an analog feedback controller. Experimental results, including plant frequency response functions, acoustic coupling strengths, frequency response functions, and transmission losses (TLs) of single- and double-panel modules, are presented and compared to numerical predictions. Over the bandwidth of 20 Hz to 1 kHz, the average measured TL for an actively controlled single-panel module was 29 dB, compared to 14 dB for the passive case. The average measured TL over the same bandwidth for the actively controlled double-panel module was 57 dB, compared to 31 dB for the passive case.

A double-panel active segmented partition module using decoupled analog feedback controllers: numerical model.

Low-frequency sound transmission has long plagued the sound isolation performance of lightweight partitions. Over the past 2 decades, researchers have investigated actively controlled structures to prevent sound transmission from a source space into a receiving space. An approach using active segmented partitions (ASPs) seeks to improve low-frequency sound isolation capabilities. An ASP is a partition which has been mechanically and acoustically segmented into a number of small individually controlled modules. This paper provides a theoretical and numerical development of a single ASP module configuration, wherein each panel of the double-panel structure is independently actuated and controlled by an analog feedback controller. A numerical model is developed to estimate frequency response functions for the purpose of controller design, to understand the effects of acoustic coupling between the panels, to predict the transmission loss of the module in both passive and active states, and to demonstrate that the proposed ASP module will produce bidirectional sound isolation.

Measurement of sound power and absorption in reverberation chambers using energy density.

Reverberation chamber measurements typically rely upon spatially averaged squared pressure for the calculation of sound absorption, sound power, and other acoustic values. While a reverberation chamber can provide an approximately diffuse sound field, variations in sound pressure consistently produce uncertainty in measurement results. This paper explores the benefits of using total energy density or squared particle velocity magnitude (kinetic energy density) instead of squared pressure (potential energy density) for sound absorption and sound power measurements. The approaches are based on methods outlined in current ISO standards. The standards require a sufficient number of source-receiver locations to obtain suitable measurement results. The total and kinetic energy densities exhibit greater spatial uniformity at most frequencies than potential energy density, thus requiring fewer source-receiver positions to produce effective results. Because the total energy density is typically the most uniform of the three quantities at low frequencies, its use could also impact the usable low-frequency ranges of reverberation chambers. In order to employ total and kinetic energy densities for sound absorption measurements, relevant energy-based impulse responses were developed as part of the work for the assessment of sound field decays.

On the significance of reflection coefficients produced by active surfaces bounding one-dimensional sound fields.

Active boundary surfaces intended to control reverberation or other characteristics of enclosed sound fields have often been investigated using plane wave tubes. This paper presents an analysis of actively terminated semi-infinite and finite-length plane wave tubes to provide needed clarification of the effects of these surfaces. By considering relationships between complex pressure-amplitude reflection coefficients and acoustic energy quantities, the investigation reveals that increases in reflection coefficient moduli at terminations do not always produce corresponding increases in total energy or energy flux in adjacent fields. These relationships are shown to depend upon physical properties of the acoustic spaces, sources, and source positions. The investigation also demonstrates how the impact of reflection coefficients with moduli exceeding unity may be easily misinterpreted.