An approach for time delay estimation, based on phase difference detection, is presented. A multiple-frequency short continuous wave pulse is used to solve the well-known phase ambiguity problem when the maximum distance exceeds a full wavelength. Within an unambiguous range defined with the lowest frequency difference between components, the corresponding phase difference is unique and any distance within this range can be determined. Phase differences between higher frequency components are used to achieve a finer resolution. The concept will be presented and the effectiveness of the approach will be investigated through theoretical and practical examples. The method will be validated using underwater acoustic measurements, simulating noisy environments, demonstrating resolutions better than a 50th of a wavelength, even in the presence of high levels ͑−5 dB͒ of additive Gaussian noise. Furthermore, the algorithm is simple to use and can be easily implemented, being based on phase detection using the discrete Fourier transform.

Abstract-Acoustic ultrasonic measurements are widespread and commonly performed using sensitive piezoelectric sensors. An accurate transducer system response to investigate pressure fluctuations in water and their subsequent detection remains a challenge. Typically, these sensors exploit the resonant behaviour of the piezoelectric active element, being designed to give maximum sensitivity in the bandwidth of interest. Calibration of such transducers can provide both magnitude and phase information describing the way in which the sensor responds to a surface displacement over its frequency range. Such resonant sensors are widely used for ultrasonic applications. The resonant nature of the sensors leads to the use of narrowband signals with central frequencies close to the resonant frequency of the piezoelectric element. Consequently, such devices work efficiently and linearly over only a narrow band of their overall frequency range. This causes phase and magnitude distortion of any linear broadband signal being transmitted through such a transmitter-receiver acoustic system. In the present work, we describe a software calibration technique to correct for distortion in a wideband piezoelectric transducer system. We consider only the input and the final output signals of the whole system. Compensating for the distortion of the magnitude and phase responses, we ensured the signal seen at the receiver represents a good replica of the desired signal. A Gaussian, linear, chirp signal was used to demonstrate our approach. This method may be applied to correct system distortion in a wide variety of ultrasonic applications.

This paper shows the use of phase measurement to estimate the distance to a target. Inspiration for this work comes from the observation that bats have been shown to have exceptional resolution with regard to target detection when searching during flight. Au and Simmons [“Echolocation in dolphins and bats,” Phys. Today 45(7), 40–45 (2007)] concluded bats with a center frequency of about 80 kHz (i.e., 4-mm wavelength), and 40-kHz bandwidth can have a resolution of distance in air approaching 20 μm. For this frequency, we see that the resolution achieved by the bat is about 200 times better than λ/2 (i.e., 2 mm at this frequency), which is usually used as a guide for resolution for analog systems. Moreover, Au and Simmons show, using time-frequency analysis, that there are essentially two frequencies present at any particular time within a single bat pulse. Considering this use of two frequencies we may infer a distance. A new bat-inspired algorithm is presented. This is based on phase measurement and, when applied to underwater acoustics, shows that a resolution of 1/50 of the wavelength can be achieved in practice.

Biologically inspired ultrasound has been investigated for measuring properties of materials in an underwater environment. Broadband transducers have been deployed which operate in the 40–200-kHz frequency range, with similar frequencies to those used by some echolocating mammals. Signals have been designed, which optimize the available bandwidth of the transducer, and analysis procedures have been developed to extract the desired information from acquired data. Measurements on single-layer targets, comprising plastic, metal, and glass, in solid and porous forms have established the performance of the system on well-controlled synthetic samples. The target thicknesses ranged from 0.5 to 40 mm, thus spanning the wavelength range of the signals used. Adequate penetration of the ultrasound into the samples, at the frequencies used, was demonstrated. Material and thickness discrimination was possible using frequency domain results, and modeling of the traces was performed in order to extract velocity and attenuation information. Scanning electron microscopy measurements on the porous targets revealed structural information that informs the interpretation of the ultrasonic results. The work on synthetic targets forms a basis to inform experimentation on natural geological materials exhibiting a wide range of structural characteristics. Initial results for sandstone samples are presented.

New-coded signals, transmitted by high-sensitivity broadband transducers in the 40–200 kHz range, allow subwavelength material discrimination and thickness determination of polypropylene, polyvinylchloride, and brass samples. Frequency domain spectra enable simultaneous measurement of material properties including longitudinal sound velocity and the attenuation constant as well as thickness measurements. Laboratory test measurements agree well with model results, with sound velocity prediction errors of less than 1%, and thickness discrimination of at least wavelength/15. The resolution of these measurements has only been matched in the past through methods that utilise higher frequencies. The ability to obtain the same resolution using low frequencies has many advantages, particularly when dealing with highly attenuating materials. This approach differs significantly from past biomimetic approaches where actual or simulated animal signals have been used and consequently has the potential for application in a range of fields where both improved penetration and high resolution are required, such as nondestructive testing and evaluation, geophysics, and medical physics.