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Electroacoustical simulation of listening room acoustics for project ARCHIMEDES

Søren Bech

1989
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Journal of the Acoustical Society of America
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TUI. Architectural acoustics: The forgotten dimension. Ewart A. Wetherill (Wilson, lhrig, and Associates, lnc., 5776 Broadway, Oakland, CA 94618) The basic considerations of architeclural acouslics---isolation from unwanled noise and vibration, control of mechanical system noise, and room acoustics design---are all clearly exemplified in Sabinc's design for Boston Symphony Hall. Opened in ! 900, this hall is one of the outstanding successes in musical acoustics. Yet, as we approach the
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... 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 acouslical 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 demonsIrate acoustical concerns to others. New techniques such as sound intensity measurements, new descriptors for room acoustics phenomena, and the refinemen t 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. Sl Presented by: Ewart A. Wetherill ( Wilson, lhrig, and Associates, 5 776 Broadway, Oakland, California 94618) Invited Papers 8:15 A1. Electroacoustical simulation of listening room acoustics for project ARCHIMEDES. S6rcn Bcch (The 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 computer model of the listening room, and it consists ofa nmnber 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 loud- speakers 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. 8:45 A2. The psychoaeoustics of loudspeaker sound reproduction--Past achievements and present problems. Floyd EThe 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. 9:15 A3. RODS---An advanced approach to electronic reverberation enhancement. Peter W. Barnett (AMS Acoustics, Ltd., 52 Chase Side Southgate, London N14 SPA, United Kingdom) 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. S2 J. Acoust. Soc. Am. Suppl. 1, Vol. 86, Fall 1989 118th Meeting: Acoustical Society of America S2 9:45 A4. •plali•tion using voice and music •s the source. 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 •esponse 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. 10:15 A5. A spatial sound processor for headphone and Ioudsl•eaker reproduction. A spatial sound processor for stereo hcadpbone and loudspeaker reproduction is described that can position sound elements within a threc-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. 10:45-11.'00 Break Contributed Papers 11:00 11:15 A6. Binaural simulation technique for scale modeling. N. Xiang and J. Blauert (Lehrstuhl far Allgemeine Elektroteehnik und Akustik, Ruhr Universit•it, D-4630 Bochum 1, Federal Republic of Germany) 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 accuratcly scaled pinnae (scale factor 1:10) serves as a receiver to pick up the model sound field. A versatile PC-based system measures the binaural impulse responses in model space according to the m-sequence transform and also earfica out the convolution of the room-impulse responses with ancchoic 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 bc listened to binaurally via headphones. [ Work supported in part by Deutsche Forsehungsgemeinschaft. ] A7. On the model order for the identification of acoustical systems. 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 threedimensional 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.] 11:30-12:00 Panel Discussion S3 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 t•e 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. 8:15 B2. The acoustic scattering by a submerged, elastic spherical shell: The transition from thin to thick shells. A fundamentally oriented analysis of the pole structure of the acoustic scattering matrix in the low-to high-frequency region (00 , F•>O, and * denotes complex conjugation. In the case of only two resonances (l = 1,2), itmay be shown that f,• =f•t +f"2 d-f l, i"•l +f•. The f"• have a Breit-Wigner form f• --x--(i/2)F•t ] and •, is the phase shift associated with the background factor of S•. The term f• is a term associated with the pole at --x,*,t in the left half of the complex ka plane while f•i.•l is an interaction term. The effect of f• is small if F• rr 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 O•ce of Naval Research and David Taylor Research Center. ] 10:f10 B9. The optical illusion of sound. Bradford L. Swartz (Communication Disorders, Central Michigan University, Mt. Pleasant, MI 48859) 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 wave pattern 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. 10:15 B10. Acoustic surface shape resonances. 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. S5 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,qcoustic Scattering by $imple 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. 10:45 B12. Reflectivity of monolayers of bubblelike scatterers at water/solid interfaces. Ivan Tolstoy (Knockvennic, Castle Douglas DG7 3PA, SW Scotland) By redistributing the energy flux of an incident plane wave, a tooriolayer of identical bubblelike scatterers at an interface may, at frequencies close to the monopolc resonance tot" drastically alter the reflectivity of the surface. Reflectivity calculations arc 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 to-•too-For case (3), use of rubber constants given by earlier writers [Gaunaurd etal., 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 to•-too. For the rubber types considered, the effective bandwidth of this effect (99% anechoicity) ,xco/co o varies between 0.01 and 0.05. [Work supported by ONR.] 11:00 B13. Acoustical images of a submerged aluminum sphere. Char]cs F. Gaumond, Brian H. Houston (Codes 5132 and 5136, Naval Research Laboratory, Washington, DC 20375-5000), and Sheila Woo (Sachs Freeman Associates, Landover, MD 20785) Acoustical images using reflection tomography [P. B. Abraham and C. F. Gaumond, J. Acoust. Soc. Am. 82, 1303-1314 (1987)] wcrc made from 3-D acoustical scattering data. The data were taken from a 7.6-cm (3.0-in.) solid aluminum sphere using a semicircular hoop array with radius 0.85 m, which rotated around the sphere. The clement spacing in the hoop was 2 ø. The hoop was rotated in 2 ø increments from the backscattered direction to the forward direction. The frequency range of the data was 2.7-25 kHz. These acoustical images were compared with analytically produced acoustical images. 11:15 B14. Prediction of bending resonances on elastic spheroids due to oblique incident plane waves from exact beam theory and comparison to T-matrix calculations. M. F. Werby (NORDA, Numerical Modeling, Stennis Space Center, MS 39529) and G. C. Gaunaurd (Naval Surface Warfare Center, White Oak Laboratory, Silver Spring, MD 20903) Exact beam theory predicts the presence of "bending" or flexural resonances from oblique incident plane waves. In an earlier paper, Timoshenko theory, which assumes a constant moment of inertia along the beam, was employed to derive an approximate theory to predict flexural resonances. Good agreement was found (for the lower modes) with the exact T-matrix calculations based on exact 3-D elastodynamics. It is possible to solve the exact beam equation using phase matching techniques to predict "bending" resonance locations. Such calculations are performed for spheroids for aspect ratios from 1.5 to 10, with extremely good agreement between beam theory and the exact T-matrix calculations for all modes. 11:30 B15. Sound scattering from a submerged finite cylinder with ribs. Philip The effects of an almost-periodically spaced set of ribs on the scattering of sound from a submerged cylinder of finite length are investigated. The cylinder is either assumed soft (corresponding to a very thin air-filled shell) or rigid (corresponding to a heavy thick shell), and its scattering amplitude is obtained using the geometrical theory of diffraction. The rib scattering amplitude is found from curvature corrected results of the theory for a flat plate carrying a rib. Numerical results are shown for the case of soft cylinders; they exhibit the onset and emergence of rib effects over the background of scattering from the body of the cylinder. 11:45 B16. Diffraction tomographie interferometry, a new imaging technique. A new imaging technique, diffraction tomographic interferometry (DTI), is proposed. Double-exposure DTI and time-averaged DTI are considered. The Born approximation is used to obtain the scattered wave and the filtered-backpropagation algorithm in the reconstruction. For double-exposure DTI, summation of the two tomograms gives the diffraction tomographic interferogram. Reconstruction gives a cross-sectional image covered with interference fringes. For time-averaged DTI, the intensity of the reconstructed image is expressed in terms of the characteristic function. This gives a cross-sectional contoured image of the object modulated by a system of interference fringes. For the proposed experimental setup for the double-exposure DTI, the interferogram of the object, a turbine blade in its unstressed state and stressed state, is recorded with the object illuminated from various directions. For the proposed experimental setup for the time-averaged DTI, a diffraction tomogram is exposed for a period of time by rotating the illuminating source around the object several times, during which the object executes a motion. S6

doi:10.1121/1.2027447
fatcat:7muohki2i5gktltdx2dhsiumze