Pure Tone Excitation Research Articles

Nonlinear Acoustic Behavior Prediction and Analysis of Straight Through Perforated Tube Silencer With Flow

Straight through perforated tube silencers are widely used in intake and exhaust noise control, and their acoustic behaviors are influenced by the gas flow and high amplitude sound remarkably. To investigate the acoustic attenuation performance of straight through perforated tube silencers in the presence of flow and high amplitude sound, an approach based on the three-dimensional (3D) time-domain computational fluid dynamics (CFD) simulation is employed and validated by comparing predictions and measurements. Transmission loss of the straight through perforated tube silencer under pure tone and multi-tone sound excitations is predicted. Results show that, under the pure tone sound excitations, the acoustic attenuations are affected remarkably by the high amplitude sound, and the nonlinear effects turn to weaken as flow velocity increases. With the sound pressure level (SPL) decreasing along the axis of the silencer, the acoustic nonlinearity in the orifice weakens gradually. The shedding vortices are restricted to the region near perforations because of the transport of flow. When the multi-tone sound excitation is enforced on the inlet, transmission loss of the silencer is different from the pure tone excitation at the same SPL of every compositional frequency.

Investigation of damping effects on low-frequency steady-state acoustical behaviour of coupled spaces.

In the low-frequency range, the acoustical behaviour of enclosed spaces is strongly influenced by excited acoustic modes resulting in a spatial irregularity of a steady-state sound field. In the paper, this problem has been examined theoretically and numerically for a system of coupled spaces with complex-valued conditions on boundary surfaces. Using a modal expansion method, an analytic formula for Green’s function was derived allowing to predict the interior sound field for a pure-tone excitation. To quantify the spatial irregularity of steady-state sound field, the parameter referred to as the mean spatial deviation was introduced. A numerical simulation was carried out for the system consisting of two coupled rectangular subspaces. Eigenfunctions and eigenfrequencies for this system were determined using the high-accuracy eigenvalue solver. As was evidenced by computational data, for small sound damping on absorptive walls the mean spatial deviation peaks at frequencies corresponding to eigenfrequencies of strongly localized modes. However, if the sound damping is much higher, the main cause of spatial irregularity of the interior sound field is the appearance of sharp valleys in a spatial distribution of a sound pressure level.

A Transmission-Line-Based Cochlear Standing Wave Model To Elucidate Mechanism of Human Auditory System.

How do people hear sounds? As a counterpart of Prof. G. V. Békésy's traveling wave theory, we have proposed resonance theory of outer hair cells and cochlear standing wave theory, respectively. Based on these proposals, this paper develops a transmission-line-based cochlear standing wave model. Since the macroscopic cochlear model is designed as it looks like, various auditory physiology can be explained. Transient analyses with pure-tone excitation and Gaussian pulse excitation are carried out, and Prof. D. Kemp's otoacoustic emission (OAE) is demonstrated successfully.Clinical relevance-Our new model has a great potential to explain auditory physiology including structural inner disorders, hearing loss, and even tinnitus.

Decoupling analysis on nonlinear system based on the modified generalized frequency response functions

This paper presents a nonlinear decoupling approach based on the Modified Generalized Frequency Response Functions (MGFRFs) and the nonlinear feature of phase invariance, for the pure nonlinearity-input nonlinear system. The MGFRFs are defined by combining the ‘homotopy’ GFRFs and phase information of the system input. The nonlinear feature of phase invariance is extracted based on MGFRFs. The decoupling approach is proposed based on MGFRFs and extended from the pure tone excitation to the multi-tone excitations by considering phase invariance. Numerical simulation and experimental investigation were carried out, whose results have shown that nonlinear feature of phase invariance is correct and reasonable and the proposed decoupling approach is valid and feasible. The proposed decoupling approach can be employed to identify the excitation sources and to estimate nonlinear system parameters for the pure nonlinearity-input nonlinear vibration system.

Evaluation of Decay Times from Noisy Room Responses with Pure-Tone Excitation

Abstract Reverberant responses are widely used to characterize acoustic properties of rooms, such as the early decay time (EDT) and the reverberation times T20 and T30. However, in real conditions a sound decay is often deformed by background noise, thus a precise evaluation of decay times from noisy room responses is the main problem. In this paper this issue is examined by means of numerical method where the decay times are estimated from the decay function that has been determined by nonlinear polynomial regression from a pressure envelope obtained via the discrete Hilbert transform. In numerical experiment the room responses were obtained from simulations of a sound decay for two-room coupled system. Calculation results have shown that background noise slightly affects the evaluation of reverberation times T20 and T30 as long as the signal-to-noise ratio (SNR) is not smaller than about 25 and 35 dB, respectively. However, when the SNR is close to about 20 and 30 dB, high overestimation of these times may occur as a result of bending up of the decay curve during the late decay.

Energy Harvesting From Vibrations With a Nonlinear Oscillator

In this paper, we present a nonlinear electromagnetic energy harvesting device that has a broadly resonant response. The nonlinearity is generated by a particular arrangement of magnets in conjunction with an iron-cored stator. We show the resonant response of the system to both pure-tone excitation and narrow-band random excitation. In addition to the primary resonance, the superharmonic resonances of the harvester are also investigated and we show that the corresponding mechanical upconversion of the excitation frequency may be useful for energy harvesting. The harvester is modeled using a Duffing-type equation and the results are compared with the experimental data.

Characterization of Semi-active Control System Dynamics with Magneto-rheological Suspensions

This paper characterizes two semi-active control algorithms through strategy analysis and parametric simulation study. A brief literature review discloses two kinds of semi-active controls: non-model based skyhook-like control policy (or skyhook variations), and model-based clipped semi-active control strategies. Both can be formulated as a bi-state control algorithm. In this paper, the no-jerk skyhook control policy is chosen to represent bi-state strategies, while a nonlinear adaptive semi-active control algorithm without need of clipping control signals is selected for comparison study. Then the studied MR suspension and its model setup are presented. In the paper we explain dynamic analysis approach and controller tuning for this simulation study. While analytic analysis of strategies is based on linear base-excited vibration systems by using transmissibility plots, simulation study adopts sine-sweep excitations so that comparison of transient dynamics could be stressed out and simultaneously the vibration isolation capability compared in the frequency domain by directly observing the time domain data. In addition, one pure-tone excitation case is simulated to explain higher harmonics inherent with bi-state skyhook controls. From the simulation study, the effectiveness of both semi-active controls can be observed for vibration isolation. In addition, the adaptability of both selected controls is compared regarding plant variations and different excitation profiles, both of which are typical in vehicle suspension systems. Finally the characteristics of these two semi-active controls are summarized from the simulation findings.

Acoustical Behavior of the Large Anechoic Chamber at the Laboratoire de Mécanique et d'Acoustique in the Low Frequency Range

It is of vital importance to have detailed information about the characteristics of an anechoic chamber, since any imperfections lead to deviations from free-field conditions and will perturb measurements. In the present paper the behavior of the large anechoic chamber at the LMA is studied in the 20 to 200 Hz frequency range. The response to pure-tone excitation, generated by an open vented box, was recorded along two lines parallel to the walls. A numerical model for the radiating sound source was used to determine the strength of the source and its center. The region in the chamber where the deviation from free-field conditions was less than the allowed 1.5 dB was determined, based on the experimental data. Its was established that even above the cut-off frequency, deviations greater than the allowed 1.5 dB occurred. A door in the anechoic chamber was identified as at least one of the sources of these imperfections.

Experiences with anechoic chamber qualification per ISO 3745 and ASA/ANSI S12.35

ISO 3745 and ASA/ANSI S12.35 contain recommended procedures for the performance qualification of anechoic and semianechoic rooms. This paper presents experience with a number of different methodologies and analyses used in a series of tests configured to comply with these specifications. The methods include discrete measurements at fixed separation distances with multiple microphones, measurements taken with a single microphone repositioned between each measurement, and a computer-controlled continuous traverse system. Experience with different instrumentation chains will be briefly addressed. Also, a comparison of test results using pure tone and pink noise excitation will be presented. Finally, an algorithm will be described which, for a given continuous traverse test data set, determines the best acoustic source center and source offset distance to minimize the deviation from the permissible bounds contained within the specifications.

Experiments on Mach-Wave Interactions in a Compressible Shear Layer

A potential technique for improving the growth rate of compressible shear layers is studied, in which a wavy-wall geometry is configured in a confined supersonic shear-layer facility that generates Mach waves in the flowfield. The major objective of this work is to evaluate a numerical model that predicts the growth properties of a three-way resonant interaction of these spatial Mach waves with duct acoustic waves and Kelvin-Helmholtz waves excited artificially or naturally within the shear layer. Measurements show that a tuned pure tone excitation of Kelvin-Helmholtz waves couples with the wavy-wall-induced disturbances and duct acoustic waves to produce local shear-layer growth rates that are approximately 50% higher than the natural, smooth-walled baseline case. The conditions for optimum growth rates are in general concurrence with numerical predictions. In this study the flow physics of a compressible shear layer in a wavy-wall environment was investigated with mean and fluctuating flowfield measurements, as well as schlieren visualizations

Measurements of normal incidence surface impedance using the single transfer function method with multitone periodic random noise signal

Commonly used signals for determining the transfer function of a linear filter are pure tone excitation, white noise, or a pseudorandom noise (MLS) signal. However, it is known that the accuracy of this measurement depends strongly on the kind of signal used. In order to try to improve the quality of the measurement, multitone periodic random noise signal properties are investigated in the case of the normal incidence surface impedance measurement. Kundt’s tubes and free-field impedance results using a single microphone transfer function method are given in a large frequency range (20–4000 Hz) for a multitone periodic random noise signal. These results are compared with those obtained with pure tone, white noise, and MLS signal excitation. Also, the influence of noise immunity is investigated for each type of signal.

Reduction of plate vibration and acoustic radiation using adaptively controlled boundaries

Reduction of plate vibration and acoustic radiation at low frequencies using adaptively controlled boundaries is experimentally demonstrated. The control is based on changing the clamping pressure at the boundaries of the plate to continuously detune the plate resonance frequencies from the frequency of pure-tone excitation. Results are presented for the control of plate vibration with constant and varying frequencies of excitation and for the control of acoustic radiation with constant frequency excitation.

Optical detection of the response of a diffusion flame to excitation

The quantitative, dynamic laser schlieren sensing system has a spatial averaging characteristic response that makes it particularly sensitive to spatially coherent structures in a turbulent mixing region. This feature of the system forms the basis of its application in the present investigation to the detection of the response of a turbulent diffusion flame to broad band random excitation. The techniques of minimum mean square identification and homomorphic deconvolution are used to extract the system impulse response from measured correlation functions. It is found that a coaxial turbulent diffusion flame has maximum response at two distinct Strouhal numbers associated with mixing at inner and outer shear layers. The flame responds to pseudo random binary excitation applied acoustically from upstream by the formation of coherent mixing structures which form at the nozzle exit plane and convect downstream with the flow. The response is approximately linear up to an excitation level that is about 0.8% of the nozzle exit velocity. Higher levels of excitation produce saturated amplitude limited responses. It is inferred from the low level at which saturation occurs that the excitation acts by influencing the manner in which the shear layers break up, rather than by direct contributions of fluctuating vorticity in the shear layer. Although random excitation produces responses at more or less constant Strouhal number of all amplitudes, the application of pure tone excitation caused a substantial reduction in the lower Strouhal number of the outer flow structures for levels above the saturation level of excitation.

Nonlinear wave propagation through rigid porous materials. I: Nonlinear parametrization and numerical solutions

It is well known that the flow resistance, as defined by Darcy’s law, becomes a function of the fluid velocity when the fluid velocity becomes sufficiently large. For porous materials in air, there appears to be two separate nonlinear flow regimes: For low velocities, the resistance coefficient increases with the square of the fluid velocity; and, for high velocities, the resistance coefficient increases linearly with the magnitude of the fluid velocity. Similar regimes also exist for the mass coefficient; however, the mass coefficient is observed to decrease with increasing fluid velocity. (For pure-tone excitation, it is shown that these relationships then hold for the complex magnitude of the particle velocity.) In this paper, the two nonlinear flow regimes are parametrized for several porous materials. Nonlinear corrections are made to linear wave theory, and numerical solutions for pure tones are obtained that are then used to predict surface admittance and internal pressures of finite length sample. A simple surface pressure criterion is put fourth based upon the materials nonlinear parameters that gives an approximate maximum surface pressure that a porous material could be exposed to before nonlinear phenomena become important in describing the material’s characteristics.

Nonlinear wave propagation through rigid porous materials. II: Approximate analytical solutions

A wave perturbation method of analyzing the internal propagation and surface admittance characteristic of highly porous acoustic materials with rigid frames at high particle velocities is presented. It is an iterative method whose implementation is based on experimental observations. Numerical results for an open-cell foam material are compared with both experiments and exact numerical solutions of the fundamental equations of nonlinear motion. Most of the salient features of the surface admittance of a bulk material are predicted analytically with the accuracy increasing as the frequency of excitation is increased. The method is restricted to pure-tone excitation.

Finite amplitude acoustic propagation in a periodic structure

Linear wave propagation in periodically inhomogeneous media is characterized by the division of the frequency spectrum into regions known as passbands and stop bands, the waves (called Bloch waves) associated with which are propagated and attenuated, respectively. A dispersion relation is derived for zeroth-order propagation in a rectangular waveguide, which is periodically loaded with rigidly terminated side branches. This dispersion relation exhibits both the characteristic band structure and, in the low-frequency limit, Korteveg-DeVries dispersion. For the case of finite amplitude pure tone excitation, a quasilinear analysis shows that parametric upconversion is effectively blocked regardless of whether the second harmonic frequency resides in a passband or a stop band, though the blocking mechanisms are fundamentally different. A 25.4-mm×38.1-mm×6-m waveguide was built with 38.1-mm-deep side-branches at 0.1 m intervals. Early measurements show dispersion, band structure, and second harmonic behavior qualitatively similar to analytic and numerical results. Possible applications in the study of traveling wave amplification and cnoidal wave/soliton propagation are investigated. [Work supported by Office of Naval Research.]

Pulse-number distribution for the neural spike train in the cat's auditory nerve.

Pulse-number distributions (PNDs) were recorded from primary afferent fibers in the auditory nerve of the cat, using standard extracellular microelectrode recording techniques. Pure-tone and broadband-noise stimuli were used. The number of neural spikes (pulses) n was measured in a set of contiguous intervals, each of duration T seconds. The quantity n varies from one interval to another. These data were then used to determine the PND, which is the probability p(n,T) of occurrence of n spikes in the time T, versus the number n. The estimated mean and variance of p(n,T) were obtained. Two different values of T were used. An unexpected observation was that the count mean-to-variance ratio R is relatively constant and independent of the stimulus intensity. Use of the PND as a statistical measure of the underlying neural point process has a number of virtues. For example, the PND readily exhibits the existence of spike clusters (e.g., pairs) for some units. The PND is essentially unaffected by time jitter and time quantization and provides a statistically significant measure for units firing at low rates. A study of the scaled and unscaled pulse-interval distributions (PIDs), under conditions of spontaneous firing, demonstrates that the occurrences of neural events are generally not describable by a renewal process. Our investigation shows that none of the point processes customarily used to model the auditory neural spike train is consistent with all of the data. It appears that the encoding of acoustic information into nerve spikes in the peripheral auditory system takes the form of a cluster point process similar to the Neyman-Scott type. For pure-tone excitation, the PND will be well represented as a multinomial distribution in this case.

Standing waves in thin plates tensioned by in-plane loads subject to high frequency excitation

The problem investigated is that of the response of thin plates subjected to pure tone or wide band frequency excitation. A point load is applied perpendicular to the plates. In the case of isotropic or orthotropic plates excited at high frequencies ring shaped standing wave patterns are observed. No classical explanation exists covering this phenomenon. The present analysis indicates that there are two waves radiating from the excitation point with the same frequency, but at different speeds. One wave, a bending wave, is separated from the other one, a membrane wave.

Structural response to low‐frequency acoustic excitation

Many noise sources at Army installations are of a low‐frequency (25 to 125 Hz) impulsive nature (e.g., artillery and explosives training, or helicopters). Mitigation of the impacts due to these sources, both on‐ and off‐post, require an understanding of the response of a structure to low‐frequency excitation. Little information is available in the literature for response below about 63 Hz. In order to study the response below this band, a structure of typical residential construction has been erected inside the CERL Bi‐Axial Shock Test Machine (BSTM) building, and extensively instrumented for vibrational and acoustic response. The BSTM has been utilized to produce impulsive acoustic excitation (114 to 130 dB, 25‐ to 40‐ms duration) of the structure. A set of specially designed speakers has been utilized to produce nearly pure tone excitation (90–100 dB, 20 to 125 Hz). Production of the excitation signals and preliminary structural response results are presented.

Multiple sources in statistical room acoustics

The problem of the radiation due to pure tone excitation from two simple sources in a reverberant room is considered. The sources are separated by a fixed distance d. The room-averaged power and the spatial cross correlation have been computed for frequencies above the Schroeder frequency of the room using a modal expansion. The sources are located at (r0 + 12d) and (r0 − 12d). The room average is taken over all source locations by averaging over all r0 and over all angular orientations of d. It is assumed that d is much smaller than a typical room dimension L, so that terms of order (d/L) are neglected. The interference terms due to the presence of room boundaries measured at the observation point r are similar to those computed in the single source case, and become negligible sufficiently far from the room boundaries. If the magnitudes of the source strengths are Q1 and Q2 and if φ is the phase difference between the two source excitations, then the room-averaged power in the reverberant field far from the boundaries is proportional to Power ∼ Q12 + Q22 + 2Q1Q2 cos φ (sin kd/kd), where k is the wavenumber. The cross term is similar to the free-space mutual impedance term and vanishes when the spacing d is large compared to a wavelength. The spatial cross-correlation function R at two points in the reverberant field separated by a distance r has also been calculated. This function is R = (sin kr/kr), the same as for a single source, as long as R is averaged over all angular orientations of d. These results demonstrate that the sound power radiated from multiple sources or an extended structure is not simply additive. On the other hand, the statistical character of the sound field produced by multiple sources is very similar to the single source case.