Electroacoustic Transfer Function Research Articles

An Efficient Feedback Active Noise Control Algorithm Based on Reduced-Order Linear Predictive Modeling of fMRI Acoustic Noise

Functional magnetic resonance imaging (fMRI) acoustic noise exhibits an almost periodic nature (quasi-periodicity) due to the repetitive nature of currents in the gradient coils. Small changes occur in the waveform in consecutive periods due to the background noise and slow drifts in the electroacoustic transfer functions that map the gradient coil waveforms to the measured acoustic waveforms. The period depends on the number of slices per second, when echo planar imaging (EPI) sequencing is used. Linear predictability of fMRI acoustic noise has a direct effect on the performance of active noise control (ANC) systems targeted to cancel the acoustic noise. It is shown that by incorporating some samples from the previous period, very high linear prediction accuracy can be reached with a very low order predictor. This has direct implications on feedback ANC systems since their performance is governed by the predictability of the acoustic noise to be cancelled. The low complexity linear prediction of fMRI acoustic noise developed in this paper is used to derive an effective and low-cost feedback ANC system.

Crosstalk cancellation in virtual acoustic imaging systems for multiple listeners

The perception of a virtual sound source is achieved by ensuring that the sound pressures at the ears of the listener are equivalent to those produced by the source at the virtual position. Theoretically, with only two loudspeakers for a single listener, virtual sources positioned anywhere in space can be presented provided that “crosstalk cancellation” can be achieved. The crosstalk cancellation problem is central to the problem of sound reproduction since an efficient crosstalk canceller gives one complete control over the sound field at a number of target positions. However, all crosstalk cancellation systems implemented so far have in practice produced virtual sources for only a single listener at a time. The design of crosstalk cancellers for multiple listeners involves a detailed study of the relative orientation of both sources and listeners. It is vital in any multiple listener system to first establish the conditioning of the potential geometrical arrangements of transducers and listeners by using simple free field models of the electro-acoustic transfer functions between transducers and ears. This gives an important link between the conditioning of the electro-acoustic transfer function matrix and the inverse filters for crosstalk cancellation. Optimal transducer arrangements for the efficient crosstalk canceller have been identified for the case of two listeners and these are evaluated here with time domain simulations.

Digital filters used in the feedback loop of an ANR earplug

Commercially available active hearing protectors are usually implemented as ear muffs with an analog driven feedback loop. The active attenuation bandwidth of these devices is limited to about 800 Hz. Moreover, it is not possible to adapt the analog system for specific noise events or to the user’s morphology. Active earplugs allow one to extend the bandwidth of the active protection to frequencies higher than 2 kHz. A digital feedback system has been implemented for active earplugs, in order to allow an adaptation to different noises and users. As the acoustic delays are too short to obtain a causal system when using FIR filters, the filters have been implemented as IIR type filters. The paper presents numerical simulations of different possible algorithms, used for the cascaded IIR filter of the ANR system. It shows how numerical errors are propagated in cascaded Bi-Quad implementations, and how an optimum signal-to-noise ratio may be obtained. The numerical predictions and experimental results will be presented and compared. As these filters will be used in a closed-loop feedback system, a numerical simulation of the behavior of the closed-loop system, including the electroacoustic transfer function of the ear plug, will be presented.

Ear defenders employing active noise control

An ear defender having one or more microphones (1) for detecting the sound level in the proximity of the wearer's ear, one or more speakers (4) for generating a noise reduction signal within the earshell, and a digital feedback controller (W) for generating a feedback signal derived from the output of the microphones and for applying said feedback signal to the speaker inputs, and estimation means (F') for providing an estimation of the earshell transfer function and subtracting from the input to the feedback controller a signal representing the estimated electroacoustic transfer function of the system. The operation of the digital feedback controller may be controllable by the output of a computational circuit (C) operable on the difference between the microphone output and the feedback controller input modified by a model (F") of the electroacoustic transfer function. A second, analogue or digital, feedback controller (W') may be included to provide an approximate feedback signal to the speakers and to improve the impulse response of the system.

Temporal prediction of acoustic backscatter from sediments at 10–250 kHz

A temporal model of high-frequency (10–250 kHz) seafloor acoustic backscatter has been developed which simulates the average squared echo envelope from a monostatic echo sounder. Tests comparing the model to measured backscatter echoes from pelagic and terrigineous sediments are presented, and the diminishing contribution of the sediment volume with increasing acoustic frequency is demonstrated. The temporal model was developed to assist in the identification of sediments by statistically comparing its output to the echo amplitude time series measured with bottom mapping sonars. The model incorporates the sonar geometry, electroacoustic transfer function and transmitted signal characteristics, the ocean volume parameters, the statistical geoacoustic parameters which describe the water/bottom interface and the sediment volume, and the multiscale roughness statistics of the interface for the bottom types being investigated. [Work supported by ONR.]

Local sound field reproduction using digital signal processing

This work shows how an acoustic wavefront can be reconstructed locally by using only a few loudspeakers. The loudspeaker inputs are calculated by passing a set of signals recorded by only a few microphones through a matrix of causal digital filters having finite impulse responses. These filters, referred to as the inverse filters, are calculated by inverting (in the least-squares sense) a matrix which contains the electroacoustic transfer functions from the loudspeakers to the microphones. In practice, it is crucial to use a modeling delay and a regularization factor in order to achieve an accurate inversion. The technique is illustrated with an example that shows how well four loudspeakers can reproduce a sound field that has been recorded with three microphones. When the recorded field does not contain energy at frequencies whose acoustical wavelengths are shorter than the distance between adjacent microphones, the original field is reproduced remarkably accurately in the vicinity of the microphones regardless of the positions of the loudspeakers.

Application of parametric and non-parametric techniques to electroacoustic transfer function analysis

Electroacoustic devices or systems transfer functions are usually described by means of amplitude and phase Bode plots. They are obtained from usually large data series of excitation and response values. Acoustic set-up requirements vary considerably, from normal room to anechoic conditions, from complex instrumentation to simple sweeping ensembles. Filter frequency analysis, cross-spectral estimation by means of the discrete Fourier transform, time-delay spectrometry and the cepstral analysis methods are non-parametric examples. Model based techniques can be classified as identification techniques. They usually employ a limited set of parameters and achieve high information compression ratios and non-redundancy, flexibility and adequacy to convert to other types of models. In the present paper the most important established techniques for electroacoustic transfer function measurement are surveyed in their principles, possibilities, instrumentation and set-up requirements. A comparison is made with an input-output rational model based parametric technique, developed in the F.E.U.P. Practical results are reported, showing the accuracy obtained through this technique when applied to the measurement of a real full-range loudspeaker system. Nevertheless some difficult aspects still persist. In our point of view, a promising new way is ahead for measurement techniques in the electroacoustic field

Ultrasonic transducers as a black-box: equivalent circuit synthesis and matching network design

A new modeling technique for ultrasonic transducers is developed in order to build an analytical model in the Laplace s-domain. The model is intended for use in analog circuit CAD system for the front-end electronic design and to visualize the acoustic pulse modifications under different excitation conditions. The transducer is characterized by two analytical functions representing the driving point impedance and the electroacoustic transfer function. The transfer function is obtained as the ratio of the transducer axial response and the excitation voltage. The reference responses of the impedance and transfer function are derived by the Fourier transform of the measured signals. The model is derived by the measurements of the driving point current and voltage, and the field axial response is sensed by a hydrophone. The procedure for the model identification is described. The results of testing 5-MHz transducer for medical applications are presented. An approach for the design of broadband matching networks using a constant resistance network is reported.

Scattering Matrix Analysis of Surface Acoustic Wave Reflectors and Transducers

Absmcr-The phased periodic array scattering parameters [l] have been modified and corrected for use as a single electrode mixed units scattering matrix consisting of one electrical and two acoustic ports. These scattering parameter analytical expressions are functions of material constants, electrode geometry, and frequency. The threeport single-electrode scattering matrices are acoustically cascaded to produce a three-port device scattering mat&x which takes into account all finger interactions (acoustic and electric) for the combined effects of piezoelectric and mechanical scattering. The analysis agrees well with experimental measurements of input admittance, electroacoustic transfer function, and acoustic transmission and reflection coefficients as functions of frequency. Analysis results for the complete modeling of solid and split electrode transducers and transducers with floating electrodes are also presented.