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Journal of the Acoustical Society of America

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Feb 1977

Volume 61, Issue 2, pp. 249-613

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Higher‐order effects of initial deformation on the vibrations of crystal plates

Xanthippi Markenscoff

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 436-438 (1977); (3 pages) | Cited 3 times

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A system of plate equations for the thickness‐shear and flexural vibrations superposed on large initial deflection due to bending is derived; in the stress–strain relations the terms associated with the fourth‐order elastic stiffness coefficients are retained. An explicit formula for the change in the fundamental cutoff thickness shear frequency is obtained and the effects of the terms associated with the fourth‐order constrants appear to be significant for large gradients of the rotation angles.
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43.40.Dx Vibrations of membranes and plates

Moving‐load stability of a circular plate on a floating central collar

C. D. Mote, Jr.

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 439-447 (1977); (9 pages) | Cited 9 times

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The eigenvalue problem and transverse response of a circular plate, that is free at the periphery and that slides freely along the axis of symmetry without bending rotation, are theoretically analyzed. The occurance of eigenvalues in the boundary conditions is accounted for with an extended operator definition in the equation of transverse motion. The stability of these plates under concentrated loads moving at uniform speed is analyzed for (i) harmonic transverse loading and (ii) loading proportional to transverse displacement and velocity. The harmonic loading case leads to a classical, critical‐speed analysis. The proportional loading case represents the excitation of the plate by transverse position guides. The number, orientation, and mechanical properties of the guides determine the transverse stability of the plate‐guide dynamic system.
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43.40.Dx Vibrations of membranes and plates

Introduction to session on the noise of large machines St. Louis, Missouri, 6 November 1974

O. L. Angevine

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 448-449 (1977); (2 pages)

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This is a special joint session sponsored by the Technical Committees on Noise and on Architectural Acoustics. There were six invited papers.
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43.50.Cb Noise spectra, determination of sound power
43.50.Yw Instrumentation and techniques for noise measurement and analysis
43.50.Jh Noise in buildings and general machinery noise

Sound‐power measurements on large machinery installed indoors: Two‐surface method

George M. Diehl

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 449-455 (1977); (7 pages)

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Sound‐power ratings of machinery are becoming more important for a number of reasons. Industrial installations must comply with recently enacted state and local noise control codes, and sound power ratings are needed to predict compliance. In addition to this, The Federal Noise Control Act of 1972 requires labeling of machinery to show maximum noise emission, and this, logically, may be in terms of sound power. There are several ways to determine the sound power of machinery under laboratory conditions, but these procedures are usually not applicable in industrial environments. The two‐surface method offers the best practical approach to the problem of calculating sound power ratings of large machinery installed indoors, under actual operating conditions.
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43.50.Cb Noise spectra, determination of sound power
43.50.Yw Instrumentation and techniques for noise measurement and analysis
43.50.Jh Noise in buildings and general machinery noise

Qualification procedures for free‐field conditions for sound‐power determination of sound sources and methods for the determination of the appropriate environmental correction

Gerhard Hübner

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 456-464 (1977); (9 pages)

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For determination of sound power of sources by the ’’method of enveloping measurement surfaces,’’ the test environment should provide a measurement surface which lies (1) outside the nearfield of the sound source under test and (2) inside a sound field free of undesired sound reflections from room boundaries or reflecting objects near the source. Methods to check the free‐field conditions and to qualify a given measurement surface for an actual source under test are (1) the absolute comparison test using a (small) calibrated reference sound source, (2) the relative comparison test using a small test sound source which radiates broadband noise that remains essentially constant during the measurement, and (3) the reverberant test, which requires measurement of reverberation time. Method 3 is only applicable in closed spaces (rooms). Methods 1 and 2 may be used in rooms and outdoors. Methods 1 and 2 require replacing the source under test by the reference sound source or test sound source in the test site. If the source under test cannot be removed, methods 1 and 2 still allow qualification for free‐field conditions, with less accuracy. This paper deals mainly with the relative comparison test (method 2) and gives information about the accuracy of the determination of the environmental corrections factor K under different field conditions.
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43.50.Cb Noise spectra, determination of sound power
43.50.Jh Noise in buildings and general machinery noise

Investigation of procedures for estimation of sound power in the free field above a reflecting plane

Curtis I. Holmer

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 465-475 (1977); (11 pages)

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This paper presents results from an experimental investigation of the accuracy and precision of various measurement procedures for determing sound‐power output of ’’large’’ machines in the free field over a reflecting plane out of doors. The purpose of the investigation was to place error bounds on several proposed nearfield measurement procedures, chiefly through the comparison of such properties with sound power levels determined from farfield measurements. The sources used in the study included 17 portable air compressors of various types and sizes powered by internal combustion engines. The data analysis centers on the comparison of sound power levels estimated from measured sound pressure levels on two measurement surfaces, one at 7‐m radius (farfield) and a second at 1 m from the surface of the machine (nearfield). Empirical estimates of precision and accuracy are derived for each of several proposed ISO procedures for determination of sound power level. The nearfield measurements were found to produce an overestimate of the farfield power level with the magnitude of the overestimate depending on the measurement surface shape. A measurement surface shape which ’’conformed’’ to the shape of the source was found to have the smallest associated overestimate. Microphone directivity was found to lead an underestimate of sound power level.
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43.50.Cb Noise spectra, determination of sound power
43.50.Jh Noise in buildings and general machinery noise

Measurement of sound absorption in rooms

J. B. Moreland

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 476-483 (1977); (8 pages)

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Existing methods for determining the sound absorption in rooms usually require some knowledge of the room dimensions. If sound‐pressure‐level measurements are made at various distances from a small source (such measurements herein referred to as a walk‐away test) the room sound absorption can be deduced provided the variation of sound pressure level with distance can be described by the simple equation SPL=PWL (sound power level)+10 log10(Q/4πr2+4/A′), which is commonly credited to Beranek. Unlike the reverberation time and estimated area methods (JSiαi), a walk‐away test can be used to determine the room absorption where either the room volume or wall areas and the absorption coefficients are uncertain or for rooms which are acoustically coupled to other rooms.
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43.50.Jh Noise in buildings and general machinery noise
43.55.Dt Sound absorption in enclosures: theory and measurement; use of absorption in offices, commercial and domestic spaces

Improving the acoustic environment for in situ noise measurements

O. L. Angevine

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 484-486 (1977); (3 pages)

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When using in situ measurements to characterize the noise of a machine in other installations, the measurement space must meet certain qualifications, as outlined by Hübner. In cases where nearby walls and other reflective surfaces interfere, such surfaces may be given a temporary sound‐absorptive treatment. If the machine is symmetrical, measurements on an unobstructed side can be accepted as typical of both sides. Temporary barriers may be used to shield measuring microphones from the direct noise of other nearby noise sources which cannot be turned off. While some installations are hopelessly unsuitable for measurements of typical machine noise, such methods can often make an installation acceptable. Similar approaches can be applied to special‐purpose facilities used for noise measurements of machines and engines. An effective facility can be developed at minimum cost for making free‐field measurements above a reflecting plane to distances of at least 5 ft. A recently developed facility of this type in a vacant automotive collision shop was made by covering less than 50% of the hard walls and ceiling with panels of acoustic absorption. Nearly spherical speading (4–6 dB per distance doubling) was obtained out to 24 ft for frequencies above 250 Hz.
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43.50.Jh Noise in buildings and general machinery noise
43.58.Dj Sound velocity
43.50.Cb Noise spectra, determination of sound power

Investigation of the surface acoustical intensity method for determining the noise sound power of a large machine in situ

Thomas H. Hodgson

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 487-493 (1977); (7 pages) | Cited 2 times

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Measurements of the surface acoustical intensity have been made on a large centrifugal chiller in order to determine the major noise‐radiating areas of the machine. In addition, an examination of the coherency of these major noise‐radiating areas has suggested that the total noise power can be obtained by addition of the noise power outputs of each component. The measurements were limited to A‐weighted sound‐power levels. The feasibility of the method has been checked by calculating the radiation efficiencies of the two dominant noise radiators. The values were found to be consistent with current knowledge of sound radiation from flexural waves in plates. Similar calculations based only on nearfield pressure measurements have demonstrated the very complex nature of the pressure field surrounding this type of large machine in situ.
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43.50.Cb Noise spectra, determination of sound power
43.50.Yw Instrumentation and techniques for noise measurement and analysis
43.50.Jh Noise in buildings and general machinery noise

Cross‐sensor beam forming with a sparse line array

Homer P. Bucker

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 494-498 (1977); (5 pages)

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By using the cross‐sensor field, a beam pattern (for a one‐wave, no‐noise acoustic field) can be generated for a sparse array which is the same as for a filled array. When real‐world effects, such as noise and multiwave acoustic fields, are considered, the performance of the sparse array degrades more than the preformance of the filled array. However, by time averaging the cross‐sensor field the performance of the sparse array is noticeably improved.
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43.60.Cg Statistical properties of signals and noise
43.30.Vh Active sonar systems

Implementation and validation of a model for surface reverberation

Robert C. Higgins, James T. Francis, and Ronald W. Hoy

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 499-507 (1977); (9 pages)

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A model for narrow‐band surface reverberation is formulated, implemented on a general‐purpose computer, and validated. At each instant of time, the model divides the ocean surface into an annulus of incremental sectors. The time and frequency characteristics of the reverberation from each sector are modeled by passing Gaussian random noise through narrow‐band digital filters. Spherical spreading, absorption, and nonisovelocity effects are included in the model. The validation is accomplished by statistically comparing the model’s output with experimental data collected in an open‐water environment.
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43.60.Cg Statistical properties of signals and noise
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.58.Ta Computers and computer programs in acoustics

Combined psychophysical and electrophysiological study on the role of combination tones in the perception of phase changes

T. J. F. Buunen, J. H. ten Kate, J. Raatgever, and F. A. Bilsen

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 508-519 (1977); (12 pages)

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It is a well‐known psychophysical fact that phase changes in a three‐component signal are audible provided that the frequency separation between the components does not exceed a critical value. A possible source of these effects is the phase‐dependent level of the ’’internal components’’ caused by an interaction within the ear of acoustic frequency components and ’’combination tones’’ of the form (n+1) f1−nf2. The experiments described in this paper are intended to verify this hypothesis by both psychophysical and electrophysiological experiments. The level of the internal component as a function of phase was psychophysically estimated in a forward‐masking experiment. Besides, single cell recordings in the cochlear nucleus of the cat were performed to investigate the phase dependence. It has been possible to demonstrate the interaction between acoustic components and combination tones for both experimental approaches. It can be described as a vector addition. Subsequent psychophysical and electrophysiological experiments on the detectability limits of combination tones have shown that these limits correspond to the maximum frequency separations for the audibility of phase changes.
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43.64.Qh Electrophysiology of the auditory central nervous system
43.66.Nm Phase effects
43.66.Dc Masking
43.66.Ki Subjective tones

Spatial differentiation as an auditory ’’second filter’’: Assessment on a nonlinear model of the basilar membrane

J. L. Hall

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 520-524 (1977); (5 pages)

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The mechanical tuning of the basilar membrane does not appear to be sharp enough to account for frequency selectivity of primary auditory‐nerve fibers. Various ’’second filters’’ have been proposed to provide the required sharpening. We have studied the properties of one such mechanism, in which the spatial derivative of membrane displacement is taken as the excitatory signal for primary fibers, on a nonlinear computational model of the basilar membrance. Because the wavelength in response to a tone decreases as the wave travels from base to characteristic place, both the slope and curvature of membrane response are enhanced in the neighborhood of the characteristic place. Thus spatial differentiation produces sharpening resembling the difference between mechanical and neural tuning. Phase (as a function of frequency) of the spatial derivative is similar to phase of the displacement, but frequency selectivity (amplitude as a function of frequency) for frequencies below the characteristic frequency is sharpened. In addition, spatial differentiation provides the distinction between mechanical nonlinearity (related in the model to membrane velocity) and neural excitatory signal required to account for suppression of response to a tone at the characteristic frequency f1 by a second tone at a lower frequency f2. Without this distinction, a tone at frequency f2 intense enough to suppress the f1 component of response at the f1 place would itself introduce a large component of response at frequency f2.
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43.64.Bt Models and theories of the auditory system
43.64.Kc Cochlear mechanics

Theory of binaural interaction based on auditory‐nerve data. II. Detection of tones in noise

H. Steven Colburn

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 525-533 (1977); (9 pages) | Cited 38 times

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A binaural interaction model is described in which the peripheral transduction from acoustical waveforms to firing patterns on the auditory nerves is included explicitly. Quantitative predictions are compared with available data on the binaural detection of low‐frequency tones masked by Gaussian noise. The model describes the parametric dependence of masking‐level differences in almost all available data.
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43.66.Ba Models and theories of auditory processes
43.66.Nm Phase effects
43.66.Dc Masking
43.66.Pn Binaural hearing

Reproduction of familiar melodies and the perception of tonal sequences

J. B. Davies and J. Jennings

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 534-541 (1977); (8 pages)

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Experimental studies of memory for tonal sequences have been extended to include well‐known melodies and long‐term memory. W. J. Dowling and D. S. Fujitani [J. Acoust. Soc. Am. 49, 524–531 (1971)] used real melodies in an investigation of interval (magnitude of pitch changes) and contour (direction of pitch changes). The study used the recognition paradigm, one of its advantages being that it enables subjects who have no skill in music production to carry out the tasks. However, it may be that the recognition paradigm is more useful for investigating stimulus variables rather than subject variables. The present study used a sample of professional classical musicians and a selected nonmusicians group. Both groups used an original production method to represent melodies and tonal sequences. The performance of the two groups was compared under conditions of dictation and production, using a contour task and a magnitude‐estimation task. Although the musicians were generally superior, there was little difference between musicians and nonmusicians in terms of perception of melodic contour. However, both groups performed at a much lower level when estimating interval magnitudes, suggesting that the size of pitch intervals is not normally coded in terms of magnitude. This finding is discussed in the light of Dowling and Fujitani’s interpretation; and an alternative ’’matching’’ model is proposed.
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43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music
43.66.Lj Perceptual effects of sound
43.75.-z Music and musical instruments

Role of dynamic cues in monaural and binaural signal detection

D. Wesley Grantham and Donald E. Robinson

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 542-551 (1977); (10 pages) | Cited 2 times

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Three experiments were carried out to investigate the importance of dynamic cues in monaural and binaural detection tasks. All experiments employed a two‐interval forced‐choice procedure, in which the signal, a 500‐Hz tone, 128 msec in duration, was added to the masker prior to gating in one of the two intervals. Both monaural (signal and masker diotic: M0–S0) and binaural (diotic masker, signal phase‐reversed in one ear relative to the other: M0–Sπ) conditions were investigated. The masker (70 dB SPL) was either a tone of 500 Hz (Experiment 1), a 500‐Hz carrier tone modulated by a 50‐Hz tone (Experiment 2), or a 500‐Hz carrier tone modulated by a band‐pass noise centered at 60 Hz (Experiment 3). The angle of addition of the signal to the masker (α) was either 0°, 45°, 90° or 135°. In general, for all values of α, S0 thresholds increased from Experiment 1 to Experiment 3, indicating that monaural signal detection is more difficult in a dynamic than in a static environment. Sπ thresholds did not change significantly from Experiment 1 to Experiment 2, thus giving no support to the notion that detection of regularly varying interaural cues (at a modulation frequency of 50 Hz) is easier than detection of static cues. However, Sπ thresholds in Experiment 3 were slightly higher than in Experiment 2, indicating that detection of randomly fluctuating interaural cues is more difficult than detection of regularly fluctuating cues.
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43.66.Pn Binaural hearing
43.66.Dc Masking
43.66.Nm Phase effects
43.66.Rq Dichotic listening

Detection of temporal gaps in noise as a measure of the decay of auditory sensation

M. J. Penner

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 552-557 (1977); (6 pages) | Cited 18 times

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The masking of silent intervals, or gaps, by surrounding noise bursts was investigated. The durations and the spectrum levels of the two noises were varied and the just detectable gap ΔT determined. The first experiment demonstrated the conditions in which duration judgments may have provided a cue. The remaining experiments were therefore designed so that this duration cue was not available. The second experiment indicated that changing the second noise duration from 200 to 2 msec did not greatly affect ΔT, suggesting a close relationship between forward masking and gap detection. Finally, Plomp [J. Acoust. Soc. Am. 36, 277–282 (1964)] has argued that a graph of the level of the second noise in decibels as a function of log ΔT traces the time course of decay of the first noise. If so, the data from the third experiment indicate that this decay rate depends on the spectrum level of the noise but not on its duration.
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43.66.Dc Masking
43.66.Lj Perceptual effects of sound
43.70.Dn Disordered speech
43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music

Threshold shift in the chinchilla from daily exposure to noise for six hours

James C. Saunders, John H. Mills, and James D. Miller

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 558-570 (1977); (13 pages)

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Trained chinchillas were exposed to 6 h of noise followed by 18 h of quiet for nine days. Thresholds (0.5–8.0 kHz) were measured immediately before and after each day’s exposure. The decay of threshold shift after the ninth day was followed until stable thresholds were again observed. This procedure was repeated for six levels (57–92 dB SPL) of an octave‐band noise centered at 4.0 kHz. The threshold shift measured after 4 min of quiet (TS4) appears to reach an asymptotic level (ATS4) after the first or second exposure. ATS4, measured at frequencies exhibiting greatest shift (5.7 kHz), increases with the level of the noise with the same slope (1.7 dB/dB) for the daily 6‐h exposures as for nearly continuous exposures. ATS4 is smaller for 6 h than nearly continuous exposures by about 5 dB and this difference can be explained by an equivalent‐power hypothesis. The decay of threshold shift was nearly complete after 18 h of quiet for the lowest levels of noise, while it was nearly complete only after 3–5 days for the intermediate levels of noise. The decay of threshold shift was never complete and small amounts of permanent threshold shift were observed for the highest levels of noise.
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43.66.Cb Loudness, absolute threshold
43.66.Sr Deafness, audiometry, aging effects
43.66.Ed Auditory fatigue, temporary threshold shift

Low‐frequency masking patterns

Jerry V. Tobias

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 571-575 (1977); (5 pages) | Cited 1 time

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Tones below 500 Hz show masking effects that are different from those of higher‐frequency tones. Where middle‐ and high‐frequency tones have their maximum effect at the masking frequency, and most of the rest of their effect at frequencies higher than themselves, low‐frequency tones are consistently most effective in the 400‐ to 600‐Hz range. Possible explanations are offered.
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43.66.Yw Instruments and methods related to hearing and its measurement
43.66.Dc Masking

Perception of one‐third octave‐band filtered speech

N. C. A. Chari, George Herman, and Jeffrey L. Danhauer

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 576-580 (1977); (5 pages)

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Intelligibility, redundancy (frequency domain), and phoneme confusions of speech were investigated through narrow‐band filtering. Filtered syllables (CV) and words (CVC) were presented to twelve paid, normally hearing subjects. Stimuli consisted of 17 English consonants paired with three vowels /i, u, a/, and filtered through five conventional one‐third octave bands centered at 0.5, 1, 2, 3.15, and 4 kHz, and one ’’critical’’ band (500 Hz wide) centered at 3.15 kHz. Results revealed (1) no significant differences between syllable and word intelligibility; (2) word or syllable intelligibility was maximum (50%) for the band centered at 2 kHz, and was greater for the bands above 2 kHz compared to those situated below this frequency; (3) speech intelligibility was reduced for the critical band compared to the one‐third octave band centered at 3.15 kHz; (4) about half of the syllables or words were perceived correctly in more than one band reflecting redundant perceptual cues along the frequency domain; and (5) consonantal‐confusion data revealed that manner and voicing features were least affected, and the place feature was most affected by filtering.
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43.70.Dn Disordered speech
43.71.Gv Measures of speech perception (intelligibility and quality)

On detecting nasals in continuous speech

Paul Mermelstein

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 581-587 (1977); (7 pages)

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The acoustic manifestation of nasal murmurs is significantly context dependent. To what extent can the class of nasals be automatically detected without prior detailed knowledge of the segmental context? This contribution reports on the characterization of the spectral change accompanying the transition between vowel and nasal for the purpose of automatic detection of nasal murmurs. The speech is first segmented into syllable‐sized units, the voiced sonorant region within the syllable is delimited, and the points of maximal spectral change on either side of the syllabic peak are hypothesized to be potential nasal transitions. Four simply extractible acoustic parameters, the relative energy change in the frequency bands 0–1, 1–2, and 2–5 kHz, and the frequency centroid of the 0–500‐Hz band at four points in time spaced 12.8 msec apart are used to represent the dynamic transition. Categorization of the transitions using multivariate statistics on some 524 transition segments from data of two speakers resulted in a 91% correct nasal/non‐nasal decision rate.
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43.72.Fx Talker identification and adaptation algorithms
43.72.Ar Speech analysis and analysis techniques; parametric representation of speech

Physics of the guitar at the Helmholtz and first top‐plate resonances

Ian M., Firth

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 588-593 (1977); (6 pages) | Cited 5 times

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The action of the guitar has been studied in detail in the vicinity of the Helmholtz air resonance and the first resonance of the top plate. Measurements of the input mechanical admittance, the output sound pressure, and their phases suggest an analogous acoustical circuit for the guitar identical with that used to describe the action of a loudspeaker in a bass‐reflex enclosure. Below the Helmholtz resonance sound radiated from the rose and from the top plate are out of phase. As a result sound radiated from the guitar is not enhanced by the rose. Above this resonance sound output from the rose and top plate are more nearly in phase with a resulting enhancement of radiated sound.
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43.75.De Bowed stringed instruments

Calculation of shading effect in the nearfield by ring‐function method

Takahi Hasegawa

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 594-595 (1977); (2 pages)

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This article presents a nonconventional way using the ring‐function method for the analysis of the nearfield of a circular piston in an infinite baffle for the case where the velocity distribution across the face of the piston has a parabolic profile.
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43.20.Rz Steady-state radiation from sources, impedance, radiation patterns, boundary element methods

Cylindrically symmetric coherence formulation for the ocean

J. J. McCoy and M. J. Beran

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 596-598 (1977); (3 pages)

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In a number of papers, equations have been derived which govern acoustic propagation in the ocean from an initial plane surface. In this paper we present the governing equation in cylindrical coordinates and then give the simplification introduced by cylindrical symmetry. The loss of coherence along a horizontal direction is then treated in three special cases.
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43.30.Bp Normal mode propagation of sound in water
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.20.Bi Mathematical theory of wave propagation

Nearfield axial levels of exponentially shaded end‐fire arrays

Robert H. Mellen

J. Acoust. Soc. Am. Volume 61, Issue 2, pp. 599-601 (1977); (3 pages)

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The contour integration method is used to compute nearfield axial levels for two exponentially shaded end‐fire arrays of finite cross section: a plane‐collimated cylindrical array and a spherically divergent conical array. The results are of interest in modeling parameteric virtual arrays.
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43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration
43.30.Qd Global scale acoustics; ocean basin thermometry, transbasin acoustics
43.25.Lj Parametric arrays, interaction of sound with sound, virtual sources
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