Volume 86, Issue S1, November 1989
Index of content:
- PROGRAM OF THE 118TH MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
- Tutorial on Architectural Acoustics
86(1989); http://dx.doi.org/10.1121/1.2027407View Description Hide Description
The basic considerations of architectural acoustics—isolation from unwanted noise and vibration, control of mechanical systemnoise, and room acoustics design—are all clearly exemplified in Sabine's design for Boston Symphony Hall. Opened in 1900, this hall is one of the outstanding successes in musical acoustics. Yet, as we approach the hundredth anniversary of Sabine's first experiments, acoustical characteristics remain one of the least considered aspects of building design. This is due, in part, to the difficulty of visualizing the acoustical outcome of design decisions, complicated by individual judgment as to what constitutes good acoustics. However, the lack of a comprehensive teaching program remains the dominant problem. Significant advances over the past 2 or 3 decades in measurement and evaluation have refined the ability to design predictability and to demonstrate acoustical concerns to others. New techniques such as sound intensity measurements, new descriptors for room acoustics phenomena, and the refinement of recording, analysis, and amplification techniques provide fresh insights into the behavior of sound in air and other media. These topics are reviewed with particular emphasis on the need for a comparable advance in translation of acoustic principles into building technologies.
- Session A. Architectural Acoustics I: Electronic Room Simulation for Production and Reproduction
- Invited Papers
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ARCHIMEDES is a psychoacoustics research project, funded under the European EUREKA scheme. Three partners share the work involved: The Acoustics Laboratory of The Technical University of Denmark; Bang and Olufsen of Denmark; and KEF Electronics of England. Its primary object is to quantify the influence of listening room acoustics on the timbre of reproduced sound. For simulation of the acoustics of a standard listening room, an electroacoustic setup has been built in an anechoic chamber. The setup is based on a computermodel of the listening room, and it consists of a number of loudspeakers positioned on an imaginary sphere surrounding the position of the test subject. The setup has been designed for the highest degree of flexibility. This includes the possibility of simulation of directivity characteristics of normal domestic loudspeakers and absorption coefficients of the surfaces of the listening room. This paper is a presentation of the system, with special emphasis on the psychoacoustical background of the design. This will include a discussion of choice of experimental procedure, test stimuli, and test subjects as well as purpose built loudspeakers and the DSP system.
86(1989); http://dx.doi.org/10.1121/1.2027448View Description Hide Description
The design of loudspeakers is gradually being put on a scientific basis. Art and intuition have given way to engineering guidelines as the relationships between perceptions and technical measurements have been elucidated. Indeed, within limited circumstances, loudspeakers can be designed to meet specific engineering design objectives, with considerable confidence in how they will be subjectively evaluated. In practice, however, all of the relevant conditions are not controlled, and several factors conspire to preclude universal satisfaction among listeners. Most of the uncertainty appears to be in the interactions between loudspeakers, rooms, listeners, and program material. This paper reviews the present state of knowledge, and outlines the areas most in need of further work. It is clear that additional psychoacoustical data and suitable technical innovations can alleviate some of the remaining problems. Others, though, may be better treated by standardization.
86(1989); http://dx.doi.org/10.1121/1.2027449View Description Hide Description
This paper will briefly review the recent history of reverberation enhancement and then examine the processes involved in the development of the RODS (Reverberation‐on‐Demand System) concept. The theory and implementation of the RODS concept will be explained, followed by examples and results of recent installations.
86(1989); http://dx.doi.org/10.1121/1.2027491View Description Hide Description
A source‐independent technique to measure accurately the amplitude and phase response of sound systems in concert halls is discussed. Measurements may be made during live performances or events, using music or voice as the test signals. Correlation is shown between the impulse response and the results obtained using music signals. An equalizer that corrects for many room resonances in both amplitude and phase simultaneously has been developed. The effect of this equalizer on concert systems in an existing venue is shown.
86(1989); http://dx.doi.org/10.1121/1.2027492View Description Hide Description
A spatial sound processor for stereo headphone and loudspeaker reproduction is described that can position sound elements within a three‐dimensional reverberant space surrounding the listener. Spatial motion of sound sources in three dimensions is created by dynamic filtering based on head‐related transfer functions. Additional filters and delay lines capture air absorption and Doppler shifting as the propagation time is manipulated for both direct and indirect sound. The spatiotemporal distribution of early reflections is captured for a given source/listener orientation: The gain, delay, and directional filtering of simulated reflections are responsive to changes in the specified position and orientation of the sound source and the listener's head in the simulated environment. The spatial processor can be used for headphone reproduction using a head‐tracking device, and can also be used in more typical reproduction settings such as living rooms with stereo loudspeakers. In the latter case, additional processing is employed to stabilize the stereo image and produce a spatially diffuse reverberant surround effect over a wide range of listening positions.
- Contributed Papers
86(1989); http://dx.doi.org/10.1121/1.2027493View Description Hide Description
Room simulation for the purpose of predicting acoustic behavior and quality has recently become a popular topic in room acoustics. At the Ruhr University, the experience of the binaural human listener in a room simulation using both physical and computer modeling has been authentically recreated. This paper reports on the latest stage of the present work in binaural room simulation using a scaled down physical model, where a sensitive miniature dummy head with accurately scaled pinnae (scale factor 1:10) serves as a receiver to pick up the modelsound field. A versatile PC‐based system measures the binaural impulse responses in model space according to the m‐sequence transform and also carries out the convolution of the room‐impulse responses with anechoic speech or music signals. An especially wide broadband ultrasonic transmitter system is necessary to provide the desired complex stimulation of the sound field. After the necessary signal processing, the resulting signals can be listened to binaurally via headphones. [Work supported in part by Deutsche Forschungsgemeinschaft.]
86(1989); http://dx.doi.org/10.1121/1.2027494View Description Hide Description
The pole/zero model and the finite impulse response (FIR) model are used as system models for the identification of unknown acoustical systems. The size of the model is particularly important in discrete‐time implementation as it determines the convergence rate of adaptation and capacity of real‐time processing. The order of the pole/zero model is related to modal distribution of systems while the order of the FIR model depends on its damping factor. Effective orders of both models are estimated from the statistical properties of acoustical systems. In a three‐dimensional enclosure, its volume and reverberation time are used for estimation. It is shown that, when modal density of the system is low, such as in a small enclosure, and frequency range is narrow, pole/zero modeling can greatly reduce the model order. [Work supported by NTT Human Interface Laboratories, Tokyo 180, Japan.]
- Session B. Physical Acoustics I: Scattering, Propagation, Diffraction, and Reflection
86(1989); http://dx.doi.org/10.1121/1.2027534View Description Hide Description
It has been suggested that a plane wave axially incident on a large aspect ratio scatterer couples to both the insonified end of the scatterer and to the end in the geometric shadow, at low kD/2 (Dis either the minor axis of a spheroid or the diameter of a cylinder) [Williams et al., J. Acoust. Soc. Am. 85, 2372–2377 (1989)]. This effect is concretely established, and its experimental consequences are discussed in some detail. A novel closed‐form expression is derived for the axisymmetric, elastic response of a large aspect ratio target that directly incorporates the bipolar coupling of the acoustic and elastic fields. The form of the final expression is, in some respects, similar to that obtained from the generalized geometric theory of diffraction in the high‐frequency limit for elastic spheres and cylinders, although the physical assumptions are quite different. Simple, approximate estimates of the parameters involved in this expression are obtained and compared with a T‐matrix calculation.
The acoustic scattering by a submerged, elastic spherical shell: The transition from thin to thick shells86(1989); http://dx.doi.org/10.1121/1.2027535View Description Hide Description
A fundamentally oriented analysis of the pole structure of the acoustic scattering matrix in the low‐ to high‐frequency region (0⩽ka⩽1000) as a function of mode number, density, and sound speed for several shell thicknesses has been previously presented [J. Acoust. Soc. Am. Suppl. 1 83, S94 (1988); Suppl. 1 84, S185 (1988); Suppl. 1 85, S95 (1989)]. The three most interesting results of this analysis were: (1) Due to fluid loading, the vacuum antisymmetric Lamb mode a 0 and the rigid Franz modes “switch tails” to produce a subsonic a 0_ mode that resembles the vacuum antisymmetric Lamb mode at low ka and an a 0+ mode that resembles a 0 at high ka; (2) the mid‐frequency enhancement of a thin shell is associated with this bifurcation of the antisymmetric Lamb wave [J. Acoust. Soc. Am. 85, 114–124 (1989)]; and (3) the existence of strong thickness resonances associated with the existence of regions of negative group velocity for the third antisymmetric Lamb wave on a thin spherical shell. In this presentation, the trajectories of the poles of the acoustic S matrix as a function of the thickness of the shell are followed. A unified picture of the resonance structure of a spherical shell as a function of shell thickness is presented, and the importance of tail switching of the elastic and diffractive degrees of freedom in the transition from a thick to a thin shell is illustrated.
86(1989); http://dx.doi.org/10.1121/1.2027536View Description Hide Description
The theoretical description of the scattering from objects near medium boundaries has been the subject of a number of recent articles [e.g., G. Kristensson and S. Ström, J. Acoust. Soc. Am. 64, 917–936 (1978); G. S. Sammelmann and R. H. Hackman, J. Acoust. Soc. Am. 82, 324–336 (1987)]. These articles consider scatterers in nonattenuating, liquid media. When objects buried in sediment are considered, however, sediment attenuation introduced fundamentally new features into the description of the scattering process. The interaction of an incident plane wave with the sediment‐seawater interface distorts the phase‐amplitude relation of the wave incident on a scatterer in the sediment. The description of the scattering of these inhomogeneous waves requires the introduction of new, “suitably modified” basis states for the solution of the scattering problem. The modification of the T‐matrix and waveguide theories developed at the Naval Coastal Systems Center is discussed and a numerical study of the scattering by a thin spherical shell buried in an attenuating liquid sediment is presented.
High‐frequency acoustic scattering from a doubly periodic ellipsoidal surface: Neumann boundary conditions86(1989); http://dx.doi.org/10.1121/1.2027537View Description Hide Description
An exact solution is obtained for the scatter of an acoustic plane wave from an infinite surface constructed from a doubly periodic array of infinite and parallel elliptical semicylinders. Neumann boundary conditions are imposed and the Helmholtz‐Kirchhoff integral used to calculate the scattered pressure field. Fredholm integral equations of the first and second kind are used to calculate the surface field. Numerical calculations are performed to determine the effects of geometric parameters on the scattered pressure field and the dependence of the surface field on the type of Fredholm integral equation used. Results are also compared with those for scattering from a sinusoidal surface.
Contributions to the form function for elastic spheres based on a product expansion of the S function: Numerical tests86(1989); http://dx.doi.org/10.1121/1.2027538View Description Hide Description
Associated with the scattering phase shift δ n , of the nth partial wave for a sphere of radius a is the functionSn (x) = exp[2iδ n ], where x = ka. A theorem from classicalscattering theory leads to a product expansion of Sn , which allows for multiple resonances [P. L. Marston, J. Acoust. Soc. Am. Suppl. 1 84, S185 (1988)]. This Sn remains manifestly unitary even for multiple resonances l and clarifies implicit assumptions of RST. The elastic contributions to the form function depend on complex ka pole locations and , where Xnl > 0, Γ nl > 0, and * denotes complex conjugation. In the case of only two resonances (l = 1,2), it may be shown that . The fnl have a Breit‐Wigner form and ξ n is the phase shift associated with the background factor of Sn . The term is a term associated with the pole at in the left half of the complex ka plane while is an interaction term. The effect of is small if Γ nl ≪ Xnl . This approximation of is confirmed by numerical comparison with the , based directly on the product expansion. The comparison also shows that omission of the term can introduce substantial errors for x between X n1 and X n2. [Work supported by ONR.]
86(1989); http://dx.doi.org/10.1121/1.2027571View Description Hide Description
Cusp diffraction catastrophes that open up roughly transverse to the propagation direction are known to exist for both light and sound [P. L. Marston, J. Acoust. Soc. Am. 81, 226–232 (1987)]. They may be produced by reflecting high‐frequency sound from curved surfaces. In the present research, a closely related caustic in which the cusp curves join to form a pair of lips is studied. This caustic was predicted to exist [J. F. Nye, Nature 312, 531–532 (1984)] in light backscattered from horizontally illuminated oblate drops of water, provided the axis ratio q = D/H was within certain ranges. The associated rays are diffracted at the drop's surface and have only one internal reflection. As q is increased above the critical value q 4≈1.31 associated with a hyperbolic umbilic focal section, the cusp points at the ends of the lips caustic were predicted to merge in the backward direction at a “lips event” when . For , no caustics for this class of ray are expected while a second lips event occurs at q L2. Observations of farfield scattering from levitated drops support Nye's analysis and illustrate a mechanism for producing lips caustics. The observed backscattering is weak for q between q L1 and q L2. [Work supported by ONR.]
86(1989); http://dx.doi.org/10.1121/1.2027572View Description Hide Description
Quantitative ray methods for leaky surface elastic waves are applied [P. L. Marston, J. Acoust. Soc. Am. 83, 25–37 (1988)] to approximate the quasiperiod of structures in backscattering and total scattering cross sections. The relevant amplitudes in this model contain a Fabry‐Perot resonance denominator, which for spheres is for the lth class of SEW; x = ka = ωa/c and cl (x) and β l (x) are the SEW phase velocity and radiation damping coefficient. The increment in x between resonances for a given l follows from the condition Δ(xc/cl ) = 1. From the Taylor expansion , the approximate resonance spacing Δx≈cgl/c for weakly dispersive SEW having a group velocity cg (x) is obtained. (This approximation has been noted by others.) The quasiperiod of a broad structure in the total scattering cross section is also approximated as , sin θ l = c/cl , when dispersion is neglected. This follows from a condition on the phase increment of the forward amplitude. Both types of variations due to Lamb wave contributions can be seen in exact and ray‐synthesis calculations for a spherical shell. [Work supported by ONR.]
New rigorous and ray‐acoustic traveling wave formulations for source‐excited thin elastic spherical shells immersed in fluids86(1989); http://dx.doi.org/10.1121/1.2027573View Description Hide Description
A continuous Legendré transform spanning the polar angle (θ) domain − ∞ < θ < ∞ is employed to derive a rigorous new integral representation for source‐excited‐time‐harmonic pressure fields in the presence of a thin elastic spherical shell immersed in different interior and exterior fluids. The new formulation identifies directly the traveling waves with their multiple encirclements of the shell by extending the θ domain from its conventional 0⩽θ⩽π range with periodicity constraints into the unbounded (multisheeted) domain without these constraints. The new formulation also systematizes the treatment of spherical caps and other truncated sections. Periodicity for the closed shell is recovered by summing contributions from an infinite array of image sources located in the “non‐physical” portion θ < 0 and θ > π of the angular space. The rigorous solution is obtained by synthesis over a complex spectral continuum, and various alternative representations are derived from it. Special attention is given to rigorous high‐frequency asymptotic forms that describe the wave phenomena in terms of incident and geometrically reflected ray fields, and also in terms of surface guided ray fields. The latter are excited by phase matching of the incident ray field to the traveling wave modes in the shell, and they reach the observer by phase matched detachment [see, also, P. L. Marston, J. Acoust. Soc. Am. 83, 25–37 (1988)]. The phase matching applies to directly excited leaky waves as well as to waves that decay initially into the fluid; the latter are excited from an exterior source by evanescent tunneling. [Work supported by the Office of Naval Research and David Taylor Research Center.]
86(1989); http://dx.doi.org/10.1121/1.2027574View Description Hide Description
The graphic depiction of sound is not a straightforward matter. Stationary representations using lines, waves, or circles leave the notion of movement (either particle or energy) and pressure fluctuations within the wave to the reader's imagination. The common sinuous wave shape evident in most all sources of study originates from Lissajous, and resulted from his transfer of tuning fork motions into a point of reflected light that traversed a screen. Lissajous performed his experiments with finely focused electric lamp light, tuning forks, and mirrors. Similar experiments can be replicated today using laser light and audio speakers. Thus one can more readily gain an appreciation for the familiar wavepattern representing sound, and see by the simple instruments used to create it, how this waveform is not a true depiction of the shape of sound waves in a medium. It is in reality a displacement versus time graph. By using multiple forks or speakers in opposing planes and tuned to various frequencies, the creation of the single‐ and multiple‐ellipse Lissajous patterns that are often produced on an oscilloscope screen can also be accomplished.
86(1989); http://dx.doi.org/10.1121/1.2027575View Description Hide Description
Acoustic surface shape resonances are vibrational excitations that are localized in the vicinity of an isolated protuberance or indentation on the otherwise planar, stress‐free surface of a semi‐infinite elastic medium. The protuberance may be fabricated from the same material as the substrate, or from a different material. In general, there is an infinite number of such resonances associated with a given surface perturbation. Their frequencies are discrete because of the loss of translational symmetry caused by the surface perturbation; they are complex because they overlap the range of frequencies allowed the vibrations of the substrate, into which they can decay; and they depend on the shape of the protuberance or indentation, and on the relation of the material properties of the protuberance (mass density, elastic moduli) to those of the substrate. Methods for calculating the frequencies of the acoustic surface shape resonances associated with protuberances or indentations of simple forms are described. Results are presented that suggest that acoustic surface shape resonances can be studied experimentally by the scattering of acoustic waves from the structure supporting them.
86(1989); http://dx.doi.org/10.1121/1.2027612View Description Hide Description
Computations of diffraction of an acoustic point source about a hard and soft disk are presented. The solution, given as a series of oblate spheroidal wavefunctions, is based on the classical analysis [J. J. Bowman, T. B. A. Senior, and P. L. E. Uslenghi, Electromagnetic and Acoustic Scattering by Simple Shapes (North‐Holland, Amsterdam, The Netherlands, 1969), Chap. 14]. In the limit as the source goes to infinity, the plane‐wave solution is recovered. Computations with the field point on the disk verify the boundary conditions. Parameter studies are presented, and convergence criteria are discussed.
86(1989); http://dx.doi.org/10.1121/1.2027613View Description Hide Description
By redistributing the energy flux of an incident plane wave, a monolayer of identical bubblelikescatterers at an interface may, at frequencies close to the monopole resonance ω0, drastically alter the reflectivity of the surface.Reflectivity calculations are given for a wave incident normally upon infinite square lattices (of basis l) for three models: (1) air bubbles in a liquid fullspace, (2) air bubbles at a water/hard surface or water/elastic plate interface, and (3) air‐filled cavities in a rubber layer at such an interface. These illustrate the essential role of multiple scatter and, most interestingly, the possibility of reflectivity nulls for incident frequencies ω≃ω0. For case (3), use of rubber constants given by earlier writers [Gaunaurd et al., J. Acoust. Soc. Am. 65, 573–594 (1979)] shows that it is theoretically possible to select values of kl such that a hard wall (or an elastic plate) becomes fully anechoic for given ω≃ω0. For the rubber types considered, the effective bandwidth of this effect (99% anechoicity) Δω/ω0 varies between 0.01 and 0.05. [Work supported by ONR.]