摘要:

Tuning or varying the resonance frequency of a transducer by adding an additional layer of ceramic, called the control ceramic, is a method for designing a wide bandwidth transducer. By loading the control ceramic with an inductive electrical load the resonance frequency of the whole structure is variable or tunable over a wide range of frequency. Two designs of tunable transducer are discussed in this paper; a two‐plate thickness‐driven structure with a fundamental frequency of 240 kHz and a sandwich structure with a fundamental frequency of 30 kHz. Tunable ranges of 1.5 and 1.8 oct are obtained, respectively. It is established that the limit of the tunable range for the two‐plate transducer is between the fundamental harmonic and the second overtone of the whole structure, which are defined with the control ceramic short circuited. For the sandwich transducer the tunable range is between the fundamental harmonic and the first overtone of the whole structure, which are defined when the control ceramic short circuited. An analysis based on the Mason transmission‐line model shows that the fundamental limit of the tunable range is determined by the fundamental resonance frequency and the dynamic coupling coefficient of the drive ceramic and is affected by the presence of any bonds and shims between the ceramics. To obtain a continuously tunable frequency range the position of the drive ceramic must be between the radiating front‐end of the structure and the control ceramic.

ISSN:0001-4966    

DOI:10.1121/1.416244

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Microphone arrays with fixed optimum weights are known to suppress the background noise and reverberation that severely reduce the effectiveness of conventional hearing aids. By means of a general technique for digital frequency‐domain implementation of optimum broadband arrays that was developed recently [C. Liu and S. Sideman, J. Acoust. Soc. Am.98, 241–247 (1995)], a practically promising system is proposed to realize the arrays with the well‐known sensitivity‐constrained superdirective beamforming weights, and with five identical omnidirectional, cardioid, hypercardioid, or dipole microphones, respectively, in the endfire or broadside configurations, which were theoretically proposed by Stadler and Rabinowitz [J. Acoust. Soc. Am.94, 1332–1342 (1993)]. The digital broadband frequency‐domain beamforming system allows the broadband superdirective beamforming weights to be faithfully and independently applied to each frequency component of the signal. The practical application of this technique is demonstrated through computer simulation of the system in anechoic situations. Furthermore, its performance in simulated reverberant environments is evaluated.

ISSN:0001-4966    

DOI:10.1121/1.416245

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The structural power flow through the junction between two flat plates, coupled in an L‐shaped configuration, is considered using the mobility power flow (MPF) approach, for both in‐plane (longitudinal and shear) and out‐of‐plane (bending) waves’ propagation. Power flow by both types of waves is included by considering the junction edge between the two plates, when uncoupled, to be free. Mobility expressions are then derived for both in‐plane and out‐of‐plane degrees of freedom forces and responses. The results of the analysis show that the in‐plane waves do not significantly contribute to the structural power flow at relatively low frequencies, that is, for frequencies below a bending wave number and plate thickness product of approximately 0.1. In this case, the power flow results are not different from those obtained if the junction is assumed to be pinned, and power is transmitted by only the out‐of‐plane waves. However, as the frequency increases, the significance of the in‐plane waves’ contribution increases, and for a bending wave number and plate thickness product greater than approximately 1.0, the contribution from the in‐plane waves dominates. This condition is different from the well‐known result that if the thickness is greater than approximately 10% of the bending wavelength, simple bending theory does not apply. This condition deals with the significance of the in‐plane waves. In the frequency region where the in‐plane waves dominate, the in‐plane longitudinal waves are more significant than the in‐plane shear waves, although this has some dependence on the selected L‐shaped configuration. The in‐plane longitudinal waves couple directly to the out‐of‐plane waves because of the 90 degree junction.

ISSN:0001-4966    

DOI:10.1121/1.416246

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A reduction‐to‐parts (RTP) method has been developed to approximate the acoustic impedance of a cylindrical shell with hemispherical endcaps. The approximation greatly reduces the computational effort required to calculate the acoustic impedance as compared to discretization methods and a variational formulation. The shell is subdivided into three parts, two hemispheres and a finite‐length cylinder, which can be considered as subregions of separable geometric shapes. Surface displacements from the original shell are applied to these separable geometries, and the corresponding calculated surface pressure responses are mapped back to the original shell. An approximation to the acoustic impedance of the original shell is calculated using the prescribed displacements and the mapped surface pressures. The accuracy of the approximation is improved in the low‐frequency range when pressure generated on neighboring parts is taken into account. To verify the approximation, the surface and far‐field responses due to a point load are compared with the responses calculated using a variational formulation. The RTP method accurately predicts the near‐ and far‐field acoustic pressures.

ISSN:0001-4966    

DOI:10.1121/1.416247

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Conventional pipe laggings incorporate a porous jacket and an impervious outer cladding sheet. It has been observed during investigations of such pipe laggings that a simple porous jacket applied by itself to a pipe can actually increase the sound radiated. Reasons for this phenomenon are discussed. The effect of a rigid frame porous jacket around a pipe is examined theoretically for the breathing, bending, and ovalling modes of pipe vibration. The predicted insertion loss associated with the bending mode of pipe vibration is compared with the corresponding experimental result and some of the results of a parametric study are given.

ISSN:0001-4966    

DOI:10.1121/1.416248

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

This paper presents room acoustic measurements of the effects of adding or modifying an orchestra shell in several halls. The average effects, at audience seats, of adding an orchestra shell in three large multipurpose halls were small but within predicted ranges. At mid‐ and high frequencies, partial shells produced intermediate results. However, at lower frequencies more complex effects were observed, because the addition of an orchestra shell modified the grazing incidence seat dip attenuation. These effects depended on the geometry of the shell and on the receiver position in the hall. On‐stage measurements of support were increased by approximately 5 dB with the addition of an orchestra shell. Although orchestra shells had much larger effects on‐stage, they were also found to produce significant audible effects at audience seats.

ISSN:0001-4966    

DOI:10.1121/1.416213

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A new general algorithm for room acoustic prediction is presented. The algorithm, based on approximate cone tracing, handles diffuse reflection by a splitup of cones incident on diffusing surfaces. The splitup of cones treats the interaction between specular and diffuse reflection in a physically sensible manner. A ‘‘brute‐force’’ implementation of such an algorithm inevitably creates an exponential dependence of the calculation time on the number of reflection combinations between diffusely reflecting surfaces, and therefore results in extremely long processing times. By exploiting the properties of diffuse reflection, the described algorithm displays an essentially linear dependence resulting in processing times short enough also for personal computers. The algorithm adapts its behavior to the properties of each specific environment, relies on very few nonverifiable assumptions, and is designed to meet the specific requirements of auralization. The splitup of cones creates a myriad of weak diffuse reflections resulting in a very smooth late decay. The algorithm is described in detail and sample predictions are shown.

ISSN:0001-4966    

DOI:10.1121/1.416249

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Cross‐correlating the output of two widely separated receivers may fail to provide a means to estimate the arrival time difference of the first arriving signals when the cross‐correlation of accompanying multipaths have amplitudes that may be confused with that between the first arrivals. Based on information in two auto‐correlation functions, a new, robust, and efficient method is described for identifying which peak in the cross‐correlation function corresponds to the arrival time difference between the first arrivals. The information in auto‐correlation functions is often sufficient to identify the relative arrival times of all the multipaths at each receiver. The technique relies on (1) a high signal‐to‐noise ratio following auto‐ and cross‐correlation, and (2) time series which have been preprocessed to suppress all signals except those emanating from a single source. The technique works even when (1) the number of multipaths is unknown, (2) the received signal is not visible above the noise, (3) the transmission time is unknown, (4) the transmitted waveform is unknown, (5) the spatial coordinates of the multipaths are unknown or are impractical to estimate, and (6) the differential Doppler between multipaths is not too large. These techniques may be useful for localizing transient signals and mapping the environment with tomography.

ISSN:0001-4966    

DOI:10.1121/1.416250

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

It is demonstrated that the Cramer–Rao lower bound on direction‐of‐arrival estimation with two closely spaced, narrow‐band plane waves, with an arbitrary correlation coefficient, impinging on a uniform line array, is well‐approximated by an empirically derived, nonmatrix expression combining two previously published, special‐case approximations. The approximation is useful for a number of sensors down to about 6, signal‐to‐noise ratios from about −30 to +30 dB, and source separations down to about one‐sixth of a Rayleigh beamwidth. The root mean square error is about 11%.

ISSN:0001-4966    

DOI:10.1121/1.416251

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The mechanical behavior of the eardrum has previously been shown to depend critically on its shape, but accurate shape measurements have been difficult to make. Phase‐shift moiré topography provides a valuable technique for measuring such shapes, and measurement in the presence of large static pressures facilitates the determination of the boundaries of the pars tensa, pars flaccida, and manubrium. New measurements of the shape of the cat eardrum are presented. The presence of hysteresis in the pressure‐displacement response is demonstrated. The shapes are incorporated in individualized finite‐element models for four different ears, and the variability between and within animals is examined. Fixed‐manubrium low‐frequency displacements are simulated and compared for the different models.

ISSN:0001-4966    

DOI:10.1121/1.416252

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP