Volume 83, Issue 5, May 1988
Index of content:

Exercises in acoustical measurement and signal analysis: A real world approach
View Description Hide DescriptionA laboratory course in acoustics for senior engineering or physics students is described. Learning basic measurements and analysis, followed by significant engineering oriented projects, is the recommended format. The area of environmental acoustics is chosen to minimize expense, yet provide measurement experience applicable to other fields. A list of the minimum required facilities and equipment is presented. A brief description of the basic measurement part of the course is followed by a set of shopping lists of projects. It is intended that the students learn project planning, execution, and documentation in addition to the subject material.

Note on the wave propagation across the boundary separating two elastic diatomic media
View Description Hide DescriptionAfter establishing the equations of motion and the constitutive equations of the diatomic media, the solution of the resulting system of two coupled field equations is given. The relations between the wave amplitude coefficients are determined by appeal to the boundary conditions and an illustrative example involving relaxed bonds between the constitutive media is solved in some detail and illustrated by a graph.

Ultrasonic scattering by two concentric cylinders
View Description Hide DescriptionA series solution for scattering from two concentric, fluid cylinders of different speeds and densities has been employed to compute the scatteredpressure for a range of frequencies, material parameters, and emitter and detector functions. This range includes the nominal parameters of measurements that were made at a frequency of 1.0 MHz and a radius of 150 mm on a gel model with an outer radius of 25.00 mm and an inner radius of 9.52 mm. The nominal parameters correspond to a k a of 626.2 for the measurement radius, a k a of 106.0 for the outer cylinder, and a k a of 39.8 for the inner cylinder. Scattering was calculated for frequencies for 10 kHz to 10 MHz which resulted in k a values from 6.26 to 6262 for the measurement radius, from 1.06 to 1060 for the outer cylinder, and from 0.398 to 398 for the inner cylinder. The effects on scattering of changes in speed and density parameters of the concentric cylinders are presented. First, the speed of the outer cylinder was raised by a factor of 10 from the nominal while the density of the outer cylinder and the speeds and densities of the inner cylinder and the surrounding medium were held fixed. Next, the density of the outer cylinder was raised while its speed was fixed at nominal, as were the speeds and densities of the inner cylinder and the medium. Finally, the speed first and then the density of the inner cylinder was raised while the parameters of the outer cylinder and of the medium were held fixed at nominal.
Plane‐wave incidence as well as illumination from the one‐dimensional versions of an ideal disk with a radius of 2.11 wavelengths at 1 MHz in water and an equivalent Gaussian aperture were considered, both modeled as an angular spectrum of plane waves. Detection at a point receiver and by a small Gaussian aperture equivalent to a disk radius of 0.498 wavelengths at 1 MHz was also investigated, as were increases by a factor of 10 in emitter and detector apertures while all other parameters were fixed at their nominal values. The combination of the wide range of parameters in this entire study required the development of accurate computational algorithms for Bessel and Neumann functions of arbitrary argument and arbitrary integer order. Measured results for the gel model and calculated results for the model parameters corroborate one another. Although agreement between measured and calculated magnitudes is better than that between measured and calculated phases, the results show that our model includes important effects.

Propagation of guided waves in fluid‐coupled plates of fiber‐reinforced composite
View Description Hide DescriptionGuided wave propagation in fluid‐coupled plates of fiber‐reinforced composites has been investigated by studying ultrasonic reflection in these structures. From measurements of ultrasonic reflection on unidirectional graphite‐epoxy plates over a range of incident angles, experimental dispersion curves using a total‐transmission criterion for leaky plate waves have been constructed. Theoretical calculations with no adjustable parameters demonstrate excellent agreement with the experimental data. Unusual behavior observed in the fundamental total‐transmission curve led to a reexamination of the mode identification criteria. Both the total‐transmission curves and the normal modes of the fluid‐coupled plate differ significantly from the result expected on the basis of the widely used Cremer coincidence condition to identify propagating plate waves. It is found that these differences are particularly pronounced in cases where the ratio of fluid to solid densities is high, as for composite materials. The existence of such behavior is also demonstrated numerically in a fluid‐coupled aluminum plate by arbitrarily increasing the density of the fluid.

Acoustic wave propagation through porous media with arbitrary pore size distributions
View Description Hide DescriptionIn the Biot [J. Acoust. Soc. Am. 2 8, 168–178 (1956); 2 8, 179–191 (1956)] theory, the effect of frequency on the viscous forces within a porous medium is treated by replacing the kinematicviscosity ν by an oscillatory viscosity νF, in which F is a function of angular frequency ω, the kinematicviscosity ν, and the single pore size a. The mathematical expression of F for arbitrary distribution of pore sizes that can be used in the Biot theory without modification is presented. It is shown that porous media with a given permeability and porosity may be represented by an infinite number of pore size distributions. The velocities and attenuation of acoustic waves through such porous media are independent of the pore size distribution at the low‐frequency limit and at the high‐frequency limit, while they are strongly dependent on the pore size distribution in the intermediate frequency range. Comparisons between Hamilton’s [Geophysics3 7, 620–646 (1972)] data of attenuation coefficient of compressional waves through marine sediments and the prediction by our theory show good agreement.

A diffraction beam field expressed as the superposition of Gaussian beams
View Description Hide DescriptionThe diffraction field of a Gaussian planar velocity distribution is a Gaussian beam function under the condition (k a)^{2}≫1. This property makes a series of Gaussian functions attractive as a possible base function set. The new approach presented enables one to express any axisymmetric beam field in a simple analytical form—the superposition of Gaussian beams about the same axis but with beam waists of different sizes located at different positions along the axis. A computer optimization is used to evaluate the coefficients, as well as the beam waists and their positions. The extreme case of a piston radiator is used to test the approach. Good agreement between a ten‐term Gaussian beam solution and the results of numerical integration (or analytical solution on axis) is obtained throughout the beam field: in the farfield, the transition region, and the nearfield. Discrepancies exist only in the extreme nearfield (<0.1 times the Fresnel distance). For surface velocity distributions that are less discontinuous (smoother), the number of terms in the Gaussian beam solution is reduced. In the extreme case of a Gaussian radiator, only one term is needed. The approach, then, reduces the study of any axisymmetric beam field to the study of the much simpler Gaussian beam.

The response of planar elastic vibrators in a uniform flow field with broadband mechanical excitations
View Description Hide DescriptionFlow‐induced forces on structures can significantly affect their dynamic response. A new method is presented to evaluate the dynamic response of planar elastic vibrators in uniform fluid flow fields to broadband mechanical excitations. The approach is based on the use of an i n v a c u o eigenfunction expansion to solve the fluid‐loaded problem. The time‐dependent coefficients of the modal expansion for the velocity are shown to satisfy a set of coupled convolution integral equations that involve mode and Mach‐number‐dependent impulse responses. Pressures in the field are simply expressed as a summation of uncoupled convolution integrals. Numerical results are presented to illustrate the effects of Mach number on the transient response of a plate subjected to a broadband mechanical excitation.

Equivalent circuits for conical waveguides
View Description Hide DescriptionThe acoustical properties of a cone of length L and small‐end radius a _{0} are shown to be representable via an equivalent circuit involving a pair of inertances, a transformer, and a nontapered duct that has the same length and small‐end radius as the cone to be represented. This is shunted at the small end by an inertance proportional to x _{0}, which is the distance from this end to the (projected) apex of the cone. At the large end, there is a similar inertance proportional to the apical distance x _{ e }=x _{0}+L. This inertance has a negative sign. The nature and correctness of the representation are demonstrated and a number of illustrative examples are presented, including a transformation of a clarinet (which is cylindrical) by means of a vent hole that gives it the basic playing properties of a saxophone (which is conical). The equivalent circuits are easy to calculate with and speak well to the intuition to suggest properties of the cone that are not otherwise very apparent.

Acoustic radiation force experienced by a solid cylinder in a plane progressive sound field
View Description Hide DescriptionThe acoustic radiation force experienced by a solid cylinder suspended freely in a plane progressive sound field is calculated, taking into account the elasticity of the cylinder. The results of numerical calculations are presented, indicating the ways in which the form of the frequency dependence of the radiation force function Y _{ P } for cylindrical targets is affected by variations in the material parameters of the cylinder. The results are compared with those for solid spherical targets.

Long‐range scattering in a deep oceanic waveguide
View Description Hide DescriptionA general formalism is developed for the scattering from an arbitrarily shaped elastic shell in a range‐independent, inhomogeneous‐layered waveguide with an arbitrary sound‐speed profile. This formalism is exact and valid through all orders of multiple scattering between the scatterer and the waveguide. The waveguide is terminated with a rigid bottom to avoid the branch cut associated with an infinite half‐space, however there is sufficient flexibility to include multilayered, bottom sediment structure. This formalism is applied to the long‐range, low‐frequency acoustic scattering of a broadband acoustic pulse by an elastic spherical shell in a range‐independent, deep oceanwaveguide.

Low‐frequency backscatter from Arctic leads
View Description Hide DescriptionScale model experiments are used to demonstrate the significance of underwater low‐frequency backscatter from Arctic leads, its relation to dipole radiation, and the difference between backscatter from a leading edge and a trailing edge. Application of the theory of impulse diffraction from a pressure‐release wedge and its image is shown to successfully predict backscattering strength for a leading edge but to be inadequate for a trailing edge. The latter incorporates reradiation contributions from mode‐converted waves in the plate, and head wave radiation, as well as diffraction. The dominant backscatter from a lineal element is due to mirrored diffraction from a single wavelength extent of the wedgelike scattering element at the least‐time intercept. Backscattering strengths for point sources are given for a 3‐octave range of frequencies over k I=0.5, where k is the wavenumber in water and I is the depth of immersion of the ice model.

Ray statistics in reverberation
View Description Hide DescriptionFour simple approximate formulas (Sabine, Eyring, Millington, Kuttruff) give the reverberation time of a room. Just the common assumptions, that all four formulas share, are used here to derive a new approximate reverberation time by a queueing theory argument. The new approximation is exact for diffuse spheres but is inconvenient to use in general. The standard simpler formulas are obtained as approximations to the new one by making further assumptions. To compare these formulas, they are applied to diffusely reflecting ellipsoidal rooms. Kuttruff’s approximation requires the least drastic extra assumptions and is usually closest to the new one. However, the common assumptions, which the new approximation uses, can be so far from true that even Kuttruff’s formula is inaccurate in some examples.

Suppression tuning curves for spontaneous otoacoustic emissions in infants and adults
View Description Hide DescriptionSuppression tuning curves (STCs) for spontaneous otoacoustic emissions (SOAEs) were longitudinally obtained in seven infants (at 3 weeks, 2 months, and 3, 4, or 6 months of age) and in five adults. Excellent reproducibility was obtained for adult STCs. Reproducibility for infant STCs was poorer, but the curves at each age resemble those of adults both qualitatively and quantitatively as measured by slopes of the lower and upper segments of the STCs and by Q _{1} _{0}’s. Evidence from two subjects suggests that developmental changes in the fine tuning of the system may occur postnatally. Results are discussed with respect to the development of cochlear frequency selectivity.

Average discharge rate representation of voice onset time in the chinchilla auditory nerve
View Description Hide DescriptionResponses of chinchilla auditory‐nerve fibers to synthesized stop consonants differing in voice onset time (VOT) were obtained. The syllables, heard as /ga/–/ka/ or /da/–/ta/, were similar to those previously used by others in psychophysical experiments with human and with chinchilla subjects. Average discharge rates of neurons tuned to the frequency region near the first formant generally increased at the onset of voicing, for VOTs longer than 20 ms. These rate increases were closely related to spectral amplitude changes associated with the onset of voicing and with the activation of the first formant; as a result, they provided accurate information about VOT. Neurons tuned to frequency regions near the second and third formants did not encode VOT in their average discharge rates. Modulations in the average rates of these neurons reflected spectral variations that were independent of VOT. The results are compared to other measurements of the peripheral encoding of speech sounds and to psychophysical observations suggesting that syllables with large variations in VOT are heard as belonging to one of only two phonemic categories.

Signal transmission in noisy environments: Auditory masking in the tympanic nerve of the bushcricket M e t a b a l l u s l i t u s (Orthoptera: Tettigoniinae)
View Description Hide DescriptionPhysiological responses of the auditory leg nerve were recorded in the tettigoniid M e t a b a l l u s l i t u s to suprathreshold tone pulses of 12.45 kHz, which is close to the carrier frequency of the male’s call. This stimulus tone frequency was determined by characterizing the polar response of the foreleg. Physiological threshold of the receptors was calculated from intensity input/output curves, and the experimental stimulus was set at 40 dB above this threshold value. There was low variance in threshold values between preparations. Continuous octave filtered white noise centered on the stimulus frequency was presented at the same time as the tone pulse at increasing intensities. The summed action potentials (SAPs) of the whole leg nerve were averaged over 256 stimulus presentations and the magnitude of the response was calibrated to dB values. The range of noise levels was set between that inducing no decrease in the SAP response to the tone pulse stimulus, up to a masking intensity where the response to the tone pulse was only just observable. Decrement in SAP magnitude was linear, and complete masking occurred when the noise level was 20–25 dB above the initial level of zero masking. This final level was comparable in magnitude to the sound‐pressure level of the tone pulse and within the natural range of the insect’s auditory behavior. Following the cessation of the noise signal, the SAPs were monitored over intervals of 2 min until the SAP asymptoted to the preexperimental condition. The reduction in SAP magnitude during noise presentation was attributed to a loss in synchrony from the individual tympanic receptors.

Effective properties of multicomponent simultaneous maskers under conditions of uncertainty
View Description Hide DescriptionWhen more than one sinusoid is used as a masker, more masking is observed than would be predicted by a simple combination of their individual effects. This masking is dramatically increased when the masker components vary in frequency and intensity with each presentation. These studies manipulated several masker parameters under conditions of high masker uncertainty, examining the effect of excluding critical‐band components, fixing or randomizing component amplitudes and frequencies, and narrowing the frequency range of the components. The signal was always a 200‐ms, 1000‐Hz sinusoid, presented simultaneously with the 200‐ms masker. Removing critical‐band components reduced the amount of masking, but considerable masking remained that appears to be nonperipheral in origin. Fixing masker frequencies across the two intervals of a trial greatly reduced the masking observed, whereas fixing masker amplitudes had no effect. Reducing the frequency range from 5000 to 2700 Hz generally increased the masking observed, but appeared to depend on other masker parameters. Summaries across ten listeners show individual differences that are resistant to extensive training. It is difficult to account for most of the masking observed in terms of masker energy falling near the region of the signal.

Combined monaural and binaural masking release
View Description Hide DescriptionStimulus conditions were examined where both across‐frequency [comodulation masking release (CMR)] and across‐ear [binaural masking‐level difference (BMLD)] cues were available, as well as conditions where only one of these cue types was available. The main goal of the study was to determine how the two types of cues combine. The effects of comodulation were assessed either by modulating a masking noise and manipulating its bandwidth (experiment 1) or by using two comodulated narrow bands of noise separated in frequency (experiment 2). The masker was always No, and the 500‐Hz pure‐tone signal was either So or Sπ. The effect of the frequency of modulation was examined either by changing the frequency of the modulating stimulus (experiment 1) or by changing the bandwidth of the comodulated narrow‐band noise (experiment 2). Four of six subjects showed greater masking release when both BMLD and CMR cues were available than for either type of cue alone, whereas the other two subjects did not show an ability to combine the two cues for additional advantage. For the subjects who were able to combine the two types of cue, the additional advantage was greater for low frequencies of modulation. The results indicate that one component of CMR may be based upon across‐frequency envelope comparisons at a stage of processing after binaural analysis.

Discrimination of interaural differences of level as a function of frequency
View Description Hide DescriptionDiscrimination of interaural differences of level (IDLs) was measured for pure tones as a function of frequency and as a function of the interaural difference of phase or level of a standard. Varying the interaural difference of the standard was assumed to change the lateral position of its intracranial image. Threshold IDLs were approximately constant over a frequency range from 200–5000 Hz, except in a region near 1000 Hz where they were slightly elevated. Thresholds increased as the value of the standard interaural differences of phase or level increased, implying that interaural resolution declines as the lateral image moves away from midline. The results are generally consistent with the predictions of current models of lateralization, but additions to these models are required in order for them to account for the slight frequency dependence of threshold IDLs.

Adaptive staircase techniques in psychoacoustics: A comparison of human data and a mathematical model
View Description Hide DescriptionData from a simple tone‐in‐noise simultaneous masking task were used to evaluate each of two common adaptive staircase rules (a ‘‘1 up 2 down’’ rule and a ‘‘1 up 3 down’’ rule) and the parameter estimation by sequential testing (PEST) technique in combination with each of two psychophysical procedures [a two‐alternative forced‐choice (2AFC) and a three‐alternative forced‐choice (3AFC) procedure]. These human data were compared to predictions generated by a mathematical model based on Markov theory. The model predicts that threshold estimates obtained with the adaptive techniques should be equal to those derived with equivalent ‘‘fixed signal level’’ techniques. However, the human data indicate that the adaptive techniques tend to yield lower thresholds. The model predicts that the standard error of a threshold estimate obtained from an adaptive technique will decrease and approach zero as the number of trials used to compute the estimate increases. The human data show greater variability than predicted and approach a nonzero value as the number of trials increases. The predictions of the model suggest that the commonly used combination of the 2AFC procedure and the 1 up 2 down rule is the least efficient method of estimating a threshold and that the 3AFC procedure in combination with the 1 up 3 down rule is the most efficient method. The human data are less consistent, but generally show the combination of the 2AFC procedure and the 1 up 2 down rule to be one of the least efficient methods. Possible explanations for the differences between the model’s predictions and the human data, as well as suggestions for laboratory practice, are discussed.

Articulatory correlates of stress and speaking rate in Swedish VCV utterances
View Description Hide DescriptionArticulatory activity underlying changes in stress and speaking rate was studied by means of x‐ray cinefilm and acoustic speech records. Two Swedish subjects produced vowel–consonant–vowel (VCV) utterances under controlled rate–stress conditions. The vowels were tense /i a u/, and the consonants were the voiceless stops, notably /p/. The spectral characteristics of the vowels were not significantly influenced by changes in the speaking rate. They were, however, significantly emphasized under stress. At the articulatory level, stressed vowels displayed narrower oral tract constrictions than unstressed vowels at the two speaking rates studied. At the faster speaking rate, vowel‐ and consonant‐related gestures were coproduced to a greater extent than at the slower rate. The data, failing to produce evidence for an ‘‘undershoot’’ mechanism, support the view that dialect‐specific correlates of stress are actively safeguarded by means of articulatory reorganization.