Time Delay Spectrometry Research Articles

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

Fiber optic ultrasonic sensor using Raman-Nath light diffraction

A novel fiber optic ultrasonic sensor using the principle of Raman-Nath light diffraction has been developed. The sensor does not perturb the acoustic field and exhibits a wideband frequency response. In addition to the remote sensing of the field, it is suitable for measurements of both continuous and pulsed ultrasonic waves. The experimental results obtained with the sensor were compared to those measured using a calibrated PVDF needle hydrophone, showing excellent agreement. The sensor's frequency response in the range from 3 to 15 MHz, typical of that used in medical ultrasound imaging, was determined using the time delay spectrometry (TDS) technique. It appears that the fiber optic sensor provides a useful alternative to the widely used PVDF ultrasonic probes in specific applications where any perturbation between acoustic field and sensor is undesirable. Also, since the active element diameter of the sensor can be made comparable to the core diameter of an optical fiber, the fiber optic sensor minimizes the spatial averaging effects and offers significant improvement in comparison with the present state-of-the-art hydrophones which have a minimum diameter on the order of 300 /spl mu/m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Determination of the relative position of a source and receiver by time-delay spectrometry

A new method of measuring the transit time of acoustic signals is discussed; it is based on the use of wideband swept-frequency signals and exhibits high accuracy, noise immunity, and simple hardware implementation. The accuracy of the results can be increased several orders of magnitude according to a narrowband deconvolution procedure developed by the authors. The gain can be close to the channel signal-to-noise ratio, as is indeed confirmed by the results of numerical calculations and full-scale experiments.

Method for measuring the characteristics of underwater acoustic arrays in real environments

A method is described for measuring the amplitude—frequency and spatial (directional) characteristics of underwater acoustic arrays in real environments, i.e., in the presence of interfering reflections. The method is based on the application of time-delay spectrometry and wideband swept-frequency signals. The results of full-scale experiments are reported, demonstrating the high accuracy, noise immunity, and efficiency of the proposed method.

A binary method for measuring the electroacoustic characteristics of hands‐free telephones

Hands‐free telephones often are designed with performance features that make it difficult to obtain meaningful electroacoustic measurements using simple methods. Compressors, limiters, threshold detectors, noise guards, and other more complex speech‐sensitive functions change the telephone’s electroacoustic characteristics depending on the type and level of signal applied to it. To realistically account for such variables, electroacoustic characteristics of several hands‐free telephones were measured using a binary method, in which two signals were applied to the device simultaneously. The first signal was a bias, which temporarily caused the telephone to operate in a well‐defined manner. Bias signals used in this work included noise, noise bursts, and speech fragments. The second signal was used for the actual measurement. It was applied at a low level relative to the bias, so the effect of the bias was not disturbed. The measurement signal used was a series of very fast sine sweeps, processed using an enhanced time delay spectrometry algorithm. This method was chosen to enable simulated free‐field measurement of the telephone in an ordinary room. The measurement setup will be described, and measurement results will be shown and explained. Benefits and limitations of the method will be discussed.

Photopyroelectric measurement of the thermal diffusivity of YBa2Cu3O7-xAND Bi2Sr2CaCu2Ox

Abstract Thermal diffusivity measurements for superconducting YBa2Cu3O7-x and Bi2Sr2CaCu2Ox are reported over the temperature range from 50 to 300 K. The results were obtained using frequency-modulated time-delay photopyroelectric spectrometry (FM-TDPS), which consists of chirped laster excitation of the sample and detection of the associated impulse by a thin-film pyroelectric detector.

Simultaneous measurement of thermal diffusivity, thermal conductivity and specific heat by impulse-response photopyroelectric spectrometry

A novel photothermal technique is developed, which enables the simultaneous measurement of the thermal diffusivity α, thermal conductivity κ, and the specific heat C of a sample. The technique is based on frequency-modulated time-delay photopyroelectric spectrometry (FM-TDPS), which consists of chirped laser excitation of the sample and detection of the thermal impulse response by a thin-film pyroelectric detector. No calibration is required for the measurements; absolute values for α, κ, and C may be obtained without having to employ a reference sample. Results on superconducting YBa2Cu3O7−x are reported for the temperature range 50–300 K; the values obtained compare favorably with reported measurements of α, κ, and C for YBa2Cu3O7−x , which previously required separate experiments for their determination.

Experimental evaluation of the backscattered polar response of sound absorbing, reflecting, and diffusing materials

The sound that we hear in a room is a combination of the direct sound and indirect reflected sounds from room boundaries. Because the amplitude, directionality, and temporal distribution of the indirect reflections affect how one perceives the actual sound source, control of room reflections using absorption, reflection, and diffusion is a central consideration in acoustical design. Effective application of these ingredients is based on an understanding of the backscattered directional response for a given angle of incidence, angle of observation and frequency, i.e., the backscattered polar response. To evaluate the experimental polar response a testing technique using time delay spectrometry was developed [P. D'Antonio and J. Konnert, AES Preprint 2295, 79th AES, New York (October 1985)]. The method allows the evaluation of the backscattered frequency response of a sound modifying material mounted on the boundary surface at any scattering angle, for a given angle of incidence. For sound absorbing materials, for example, these “directional” scattering coefficients augment the traditional random-incidence ASTM reverberation chamber data. The technique will be described and polar response data for sound absorbing, reflecting, and QRD® diffusing panels will be presented. An alternative method using sound intensity will also be discussed and preliminary data presented.

A multiple-frequency hydrophone calibration technique

A method is described for comparing the sensitivity of two hydrophones over the frequency range 1-15 MHz. This technique forms the basis for the dissemination of national ultrasonic standards in the U.K. over this frequency range. A reference hydrophone is placed in an ultrasonic field and then the device being calibrated is substituted and the two output voltages are compared. This substitution method utilizes a broadband ultrasonic field produced by nonlinear propagation. Thus it is possible to cover the whole frequency range with a single measurement on each hydrophone. The overall uncertainty in the intercomparison of two hydrophones increases from +/- 4.2% at 1 MHz to +/- 8.2% at 15 MHz (95% confidence level). The method has been compared with discrete-frequency substitution, time-delay spectrometry, and absolute calibrations using the National Physical Laboratory (NPL) Primary Standard Laser Interferometer. Various designs and sizes of hydrophones were compared, and agreement was within the combined random uncertainties for all the comparisons.

Medical ultrasound imager based on time delay spectrometry

A reflection mode proof-of-concept medical ultrasound imager based on time delay spectrometry has been developed and tested. The system uses a broad band swept-frequency signal operating up to 10 MHz. Signal processing using an fast Fourier transform (FFT) permits extraction of range information. The imager has a higher signal-to-noise ratio than pulse-echo systems which allows high resolution at greater depths. The time delay spectrometry (TDS) spread spectrum operates at lower peak intensities than pulse-echo and permits more control of the spectral content and amplitude of the signal. At present, the system is non-real time which degrades in vivo imaging because of averaging over several cardiac cycles and tissue movement.

Comparison between design and installed acoustic characteristics of the NASA Lewis 9‐ × 15‐ft low‐speed wind‐tunnel acoustic treatment

The test section of the NASA Lewis 9- by 15-Foot Low-Speed Wind Tunnel was acoustically treated to allow the measurement of sound under simulated free-field conditions. The treatment was designed for high sound absorption at frequencies above 250 Hz and for withstanding the environmental conditions in the test section. In order to achieve the design requirements, a fibrous, bulk-absorber material was packed into removable panel sections. Each section was divided into two equal-depth layers packed with material to different bulk densities. The lower density was next to the facing of the treatment. The facing consisted of a perforated plate and screening material layered together. Sample tests for normal-incidence acoustic absorption were also conducted in an impedance tube to provide data to aid in the treatment design. Tests with no airflow, involving the measurement of the absorptive properties of the treatment installed in the 9- by 15-foot wind tunnel test section, combined the use of time-delay spectrometry with a previously established free-field measurement method. This new application of time-delay spectrometry enabled these free-field measurements to be made in nonanechoic conditions. The results showed that the installed acoustic treatment had absorption coefficients greater than 0.95 over the frequency range 250 Hz to 4 kHz. The measurements in the wind tunnel were in good agreement with both the analytical prediction and the impedance tube test data.

Application of time-delay spectrometry for calibration of ultrasonic transducers

Time-delay spectrometry (TDS) can conveniently be used for calibration and performance evaluation of piezoelectric electroacoustic transducers. The main emphasis of the work reported here is an experimental evaluation of the TDS technique. The TDS concept is introduced through a theoretical analysis. The experimental evaluation is carried out using specially designed measurement methods and instrumentation which uses a spectrum analyzer as the central analog signal processing unit. The optimal performance of the TDS measurement systems is analyzed in terms of relevant instrumentation parameters. The advantages and disadvantages of TDS, including practical performance limitations, are discussed, along with the measurement uncertainties of the method. It is shown that TDS in the frequence range covering both underwater acoustics and medical ultrasonics applications offers a viable alternative to other calibration techniques, such as those based on a gated burst measurement system.

Calibration of hydrophones based on reciprocity and time delay spectrometry

A brief survey is given of the calibration methods for hydrophones in the ultrasonic frequency range. The methods presently used in the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, West Germany, for hydrophone calibrations in the frequency range from 1 to 15 MHz are the primary subject of concern. These methods are the two-transducer reciprocity method for the calibration at discrete frequencies, the time-delay-spectrometry substitution method for quasifrequency continuous calibrations, and the two-transducer reciprocity method with time-delay spectrometry, also for quasi-frequency continuous calibration. Compared with the calibration at discrete frequencies, the expenditure of time for a calibration is considerably reduced in the case of the last-mentioned method. The influencing parameters which affect the evaluation of the measurement uncertainty are briefly discussed for the calibration methods applied at the PTB.

Interlaboratory comparison of hydrophone calibrations

The results of an interlaboratory comparison of hydrophone calibration techniques in the frequency range 1-10 MHz are reported. Two membrane hydrophones were calculated to six laboratories, and each laboratory determined the end-of-cable loaded sensitivities using their normal calibration methods; these included optical interferometry, planar scanning, reciprocity combined with time-delay spectrometry, and suspended-sphere radiometry. After converting the results to end-of-cable open-circuit sensitivities, in most cases agreement between the various values was within +/-10% at all frequencies.

Photopyroelectric thin-film instrumentation and impulse-response detection. Part III: Performance and signal recovery techniques

Frequency-modulation time-delay spectrometry (FM-TDS) was compared with stochastic excitation in photopyroelectric measurements on thin solid samples. These methods involve broadband signal excitation with simultaneous detection of all frequency components within the response bandwidth of the photothermal system. Fast signal recovery was achieved by FFT methods. Both FM-TDS and wideband random-noise excitation were found to yield high-quality, band-limited impulse-response information. Random noise measurements were found to be much less susceptible to distortions and nonlinearities in the excitation wave train. FM-TDS showed superior coherence and signal-to-noise ratio (SNR) to random methods. The results of this work demonstrate a photopyroelectric effect spectrometer capable of yielding fast, time-resolved information equivalent to the response of a pulsed laser, with low peak power. This important feature shows excellent potential for nondestructive thickness, thermal diffusivity, and thermal conductivity measurements on materials which are susceptible to optical damage.

Photopyroelectric thin-film instrumentation and impulse-response detection. Part II: Methodology

Frequency-modulation time-delay spectrometry (FM-TDS) has been implemented in photopyroelectric measurements of thermal diffusivity, on a series of well-characterized samples. The strategy of FM-TDS is sample excitation by a fast linear frequency sweep, whose autocorrelation function is mathematically equivalent to a Dirac delta function. The method permits the fast recovery of high-quality frequency and impulse-response information. Impulse responses recovered in the time-delay domain showed good agreement with a Green’s function model of transient heat conduction. The present work demonstrates that the FM-TDS measurement strategy yields photothermal information equivalent to that obtainable from a pulsed laser system, with much lower excitation power.

Evaluation of room speech transmission index and modulation transfer function by the use of time delay spectrometry

The literature shows that the modulation transfer function (MTF) and speech transmission index (STI) can be computed from the squared impulse response of a linear passive system. This paper describes an extension of this method to measurements of systems using time delay spectrometry (TDS). The new method makes use of both the real and imaginary parts of the complex analytic impulse response of the system (the energy‐time response). This allows more accurate determinations of STI and MTF because the calculations are based on a measurement that more closely follows the true energy decay in the room. The method requires fewer samples of the room's sound field and less spatial averaging to yield a given accuracy.

Time-delay spectrometry for ultrasonic transducer characterisation

The use of the technique of time-delay spectrometry in ultrasonic transducer characterisation is described for both transmitters and receivers. The principles of the technique, the choice of experimental parameters, and methods of instrumentation are outlined. Applications for measurement of the frequency response, linearity and directionality of miniature hydrophones, for transmitted field mapping, for finite receiver diffraction correction measurement, and alignment assessments are described and illustrated. Particular features of the technique are the continuous frequency response obtained, the high signal-to-noise ratio, and the ease of comparative measurements (e.g. for use in transducer batch production).

Optimizing control rooms for stereo imagery

Control rooms designers typically measure and specify rooms according to their physical structure and acoustic properties. They are unable, however, to measure or predict how well the room will support the subjective qualities of stereo imagery produced over loudspeakers. As the quality and salience of stereo imagery improve through the use of more sophisticated recording and processing techniques, control room requirements become more stringent. Beyond speaker placement, there are three primary factors that influence the perception of stereo images: time‐energy‐frequency characteristics of the speakers, spatio‐temporal distribution of early reflections, and the inclusion of acoustic diffraction. These are easily measured through the use of time delay spectrometry (TDS), but at present an adequate model for predicting subjective response from these physical measurements is lacking. Ensuring the perception of optimal stereo imagery requires the application of standardized subjective evaluation techniques. Currently under development at Northwestern Computer Music (NCM) is an evaluation technique using the Listening Environment Diagnostic Recording (LEDR™), which enables the immediate assessment of changes in stereo imagery that result from progressive changes in control room acoustical treatment. Field tests indicate that LEDR™ is valuable in the design and modification of control rooms for optimizing stereo imagery.

Piezoelectric Composite Materials for Ultrasonic Transducer Applications. Part II: Evaluation of Ultrasonic Medical Applications

Abstmct-The electro-acoustic properties of Lead zirconate titanate (PZT) rod-polymer composites relevant for ultrasonic transducer applications are reported. Acoustic impedance of the composite materials was measured by three different techniques in the frequency range 0.33.5 MHz. Dependence of the acoustic impedance as a function of volume fraction of PZT and frequency was also modeled theoretically. Time-delay spectrometry was employed to calibrate the free-field transmitting and receiving voltage responses of the composite materials. The acoustic impedance of the composite materials was in the range of 3-10 M rayl. The figure of merit in the receiving mode of composite materials was three times that of PZT. The figure of merit for a 20percent PZT composite (2 = 7.3 M rayl) was further enhanced by 50 percent using a single-layer impedance transformer of lucite (2 =3.3 M rayl). These composite materials were molded into curved shapes by simple thermal process to fabricate focused transducers. The axial and lateral beam profiles of focused composite transducers are presented.