Bojan Ivančević

Comparison of measured acoustic power results gained by using three different methods on an ultrasonic low-frequency device

The theme of this work is characterization of an ultrasonic low-frequency device, driven at an excitation frequency of around 25 kHz at different electrical excitation levels by using three different methods as proposed in IEC 61847 and IEC 61088 standards. The first method is based on the electromechanical characterization of the device. It consists of measuring the input electrical impedance around the excitation frequency in the unloaded and loaded conditions at a low level excitation voltage of 1 V. The equivalent RLC electrical circuit parameters of an unloaded and loaded device are determined in an anechoic tank and in a vessel at different immersion depths and tip positions in a complex geometry. The electroacoustic efficiency factor of the method is determined by knowing the real part of the radiation resistance and mechanical loss resistance which are transformed into an equivalent RLC electrical circuit of the transducer. The second method consists of measuring the spatial pressure distribution of an ultrasonic device near pressure release boundary in an anechoic tank. The acoustic reciprocity principle is used to determine the derived acoustic power of an equivalent point source in the form of radially oscillating sphere at the excitation frequency. The third method is based on the measurement of power dissipated in a restricted volume of water by using a calorimetric method. Some of the suggested methods are complicated to apply in the high energy ultrasonic devices whose size is much lower than the wavelength in the loading medium due to the occurrence of strong cavitation activity and influence of the sonotrode tip position in the complex standing wave field. However, the measured acoustic power found by using the three suggested methods is compared by means of the electroacoustic efficiency factor defined for each considered method. In the electromechanical characterization, which is made at low electrical excitation levels (applied electrical power of 1 mW at the series resonance frequency), the calculated maximum electroacoustic efficiency factor is around 48% when the influence of standing waves pattern on the radiation resistance is small. It is approximately the same as the one obtained by measuring the derived acoustic power in an anechoic tank (43%) without cavitation activity in front of the tip. When a strong cavitation activity is present in the loading medium, the bubble cloud has a significant influence on the derived acoustic power which is then dispersed in a broad frequency range and the electroacoustic efficiency factor of the method decreases down to 2%. A significant growth of the input electrical impedance magnitude at the excitation frequency is observed when the cavitation activity is present in front of the tip and when it is compared with the impedance magnitude measured at lower excitation levels without cavitation. The power dissipated in the loading medium almost linearly depends on the applied electrical power, with saturation at higher excitation levels. In the linear operating mode the electroacoustic efficiency factor of the calorimetric method (48%) is comparable with the efficiency factors of two other methods. In the nonlinear operating mode, it is larger (71%) due to a significant amount of heat energy released during the cavitation process.

Measuring derived acoustic power of an ultrasound surgical device in the linear and nonlinear operating modes

OBJECTIVE AND MOTIVATION The method for measuring derived acoustic power of an ultrasound point source in the form of a sonotrode tip has been considered in the free acoustic field, according to the IEC 61847 standard. The main objective of this work is measuring averaged pressure magnitude spatial distribution of an sonotrode tip in the free acoustic field conditions at different electrical excitation levels and calculation of the derived acoustic power at excitation frequency (f0 approximately 25 kHz). Finding the derived acoustic power of an ultrasonic surgical device in the strong cavitation regime of working, even in the considered laboratory conditions (anechoic pool), will enable better understanding of the biological effects on the tissue produced during operation with the considered device. EXPERIMENTAL METHOD The pressure magnitude spatial distribution is measured using B&K 8103 hydrophone connected with a B&K 2626 conditioning amplifier, digital storage oscilloscope LeCroy Waverunner 474, where pressure waveforms in the field points are recorded. Using MATLAB with DSP processing toolbox, averaged power spectrum density of recorded pressure signals in different field positions is calculated. The measured pressure magnitude spatial distributions are fitted with the appropriate theoretical models. THEORETICAL APPROACHES In the linear operating mode, using the acoustic reciprocity principle, the sonotrode tip is theoretically described as radially oscillating sphere (ROS) and transversely oscillating sphere (TOS) in the vicinity of pressure release boundary. The measured pressure magnitude spatial distribution is fitted with theoretical curves, describing the pressure field of the considered theoretical models. The velocity and displacement magnitudes with derived acoustic power of equivalent theoretical sources are found, and the electroacoustic efficiency factor is calculated. When the transmitter is excited at higher electrical power levels, the displacement magnitude of sonotrode tip is increased, and nonlinear behaviour in loading medium appears, with strong cavitation activity produced hydrodynamically. The presence of harmonics, subharmonics and ultraharmonics as a consequence of stable cavitation is evident in the averaged power spectral density. The cavitation noise with continuous frequency components is present as a consequence of transient cavitation. The averaged pressure magnitude at the frequency components of interest (discrete and continuous) in the field points is found by calculating average power spectral density of the recorded pressure waveform signal using the welch method. The frequency band of interest where average power spectral density is calculated is in the range from 15 Hz up to 120 kHz due to measurement system restrictions. The novelty in the approach is the application of the acoustic reciprocity principle on the nonlinear system (sonotrode tip and bubble cloud) to find necessary acoustic power of the equivalent acoustic source to produce the measured pressure magnitude in the field points at the frequency components of interest. RESULTS In the nonlinear operating mode, the ROS model for the considered sonotrode tip is chosen due to the better agreement between measurement results and theoretical considerations. At higher excitation levels, it is shown that the averaged pressure magnitude spatial distribution of discrete frequency components, produced due to stable cavitation, can be fitted in the far field with the inverse distance law. The reduced electroacoustic efficiency factor, calculated at excitation frequency component as ratio of derived acoustic power with applied electrical power, is reduced from 40% in the linear to 3% in the strong nonlinear operating mode. The derived acoustic power at other frequency components (subharmonic, harmonic and ultraharmonic) is negligible in comparison with the derived acoustic power at excitation frequency. DISCUSSION AND CONCLUSIONS The sonotrode tip and loading medium are shown in the strong cavitation regime as the coupled nonlinear dynamical system radiating acoustic power at frequency components appearing in the spectrum. The bubble cloud in the strong nonlinear operating mode decreases the derived acoustic power significantly at the excitation frequency.

The velocity of ultrasound propagation through brain tissue at low ultrasound frequencies

The acoustic properties of the brain tissue have been in the focus of much scientific research. These properties determine the possibilities of the application and development of the ultrasound methods, as well as the development of the equipment used by the neurosurgeon during the resection of the brain tissue. The results of the measurements of these acoustic properties done at megahertz frequency range have been published. Although the velocity of ultrasound wave propagation does not directly influence the distribution of the ultrasound field within the cranium, it enables the determination of the specific resistance, which has an important role in the distribution of the ultrasound field within the cranium. Using the published results and the conditions of the measurements that have been done, the velocity of ultrasound propagation for the frequency range at which cavitation ultrasonic surgical aspirators work has been estimated. After that the measurement of the velocity of the propagation has been d...

Influence of the calorimetric system setup on the output acoustic power measurement results

The problem of measuring an output acoustic power using calorimetric method has been considered in the nonlinear regimes of ultrasound transmitter. The point ultrasound source with sonotrode tip has been driven at different excitation electrical power levels and the influence of the tip position in the different calorimetric systems on the measurement results on output acoustic power is considered. Two calorimetric systems with different volumes of loading liquid (water) and different geometries (having influence on thermodynamical losses) have been used in the experiments. The exponential experimental temperature-time curves have been fitted with theoretical solutions and losses of the systems, output acoustic power and electroacoustic efficiency factor have been found assuming that all ultrasound energy is absorbed in the system. The radiation impedance dependence on complex pressure field distribution in the calorimetric system has been considered measuring the input electrical impedance of loaded and unloaded transmitter.

Acoustic characterization of an ultrasound surgical transmitter in the linear and nonlinear regime of working

The method for measurement of a derived acoustic power of an ultrasound surgical transducer has been suggested in the free field conditions. The pressure field of the transmitter, immersed in depth of quater wavelength and vibrating at the fundamental frequency (≈25kHz), has been measured with calibrated hydrophone at different excitation levels. In the linear regime, the transmitter has been theoretically described as an acoustic dipole, the source parameters have been found and good agreement between theoretical and experimental results is obtained. When transmitter is excitated at higher excitation levels, the nonlinear behaviour in loading medium appears, with strong cavitation activity. In the averaged power spectrum of the recorded acoustic pressure signal, is evident the presence of harmonics (n⋅f), subharmonics (f/q), ultraharmonics (n⋅f/q) of excitation frequency. The spatial pressure distribution of each discrete frequency component in the free acoustic field has been measured and its contribution to total acoustic power has been calculated. The total acoustic power in the cavitation noise signal is estimated integrating the averaged pressure power spectrum with appropriate contributions of each frequency component in the signal.

Noise Influence on Quality of Communication in a Tram

This paper is about testing the intelligibility of speech during tram ride (tram model TMK 2200). Masking signal (noise) was recorded while driving in the tram. Logatomes signal was recorded in the laboratory. Test signal was produced by mixing of noise and logatomes signals. Subjects were tested in the laboratory. Measurement results analysis shows us that in realistic conditions, during the ride, the quality, of communication is extremely low. We conducted additional tests with reduced noise level. The intelligibility results were much better

The design of narrow-band water loaded ultrasound transmitters

The performance of thin-disk transducers made of PZT-5A ceramic using acoustic matching and air-backing is described using simplified 1-D model and experimental results. Several ultrasound transducers have been designed for studying influence of high power ultrasound energy on tissue. PZT-5A ceramic has been used as active element with thickness approximately lambda/2, depending on working frequency (500 kHz, 1 MHz, 2 MHz) and with three different diameter to thickness ratio (D/t) for obtaining different directivity patterns in radiated medium. Theoretical input impedance, conductance and bandwidth of transducer systems has been compared with experimental results. Theoretical results have been derived using Masons equivalent circuit

Binaural hearing computer models in multisource environments

Computer models of the binaural hearing mechanism are constantly developed and improved in order to simulate the hearing process as good as possible. This is necessary because of increasing need to localize and hear a certain speaker among others, simultaneously speaking sound sources, especially in teleconferences or for hearing-impaired people. The basics of almost all computer models is the cross-correlation function of the binaural signal, adequate filtered and processed. The final goal is to filter out from the binaural signal a certain speaker without any noise or interference. This paper uses a complex short-time cross-correlation model to find its limitation in identifying the right number of sound sources in the frontal horizontal plane, as well as their position (azimuth). The tests were done using HRTF functions to simulate a virtual acoustic environment without reflections. The results were compared with tests made in real rooms using dummy head for binaural signal recording

Noise levels in dental practice

Noise can cause unwanted masking of sounds, interference with speech, even pain and injury, and temporary or permanent loss of hearing. Concerning exposure of noise levels at the workplace, industrial plants and huge machines are usually considered. However, there are ‘‘clean’’ jobs where people are exposed to significant noise. One example, which will be treated in this paper, is dental practice. In this work it is intended to show the results of SPL measurements that were done at the Dept. of Pedodontics in the School of Dental Medicine Zagreb. One‐ and two‐channel measurements were made using a sound level meter and an artificial head (head‐and‐torso simulator). Noise levels generated by various dental devices, their spectra, and differences between noise levels in left and right ear (in the dentist’s usual working position) were shown. The sound power and subjective sound levels regarding sound source frequency and intensity were determined upon results of these measurements. Special attention was pai...

The acoustical properties of large studios

New studios for Croatian Radio‐Television were planned with the starting user conditions. The background noise level must fulfill N15 or N20 criteria. Demanded reverberation time for a 400 m2 radio studio is Tr=0.6 s, for TV studios of the same area, Tr=0.7 s, and for the 1000 m2 TV studio, reverberation time is 1 s. Since all three studios with built‐in control rooms were built in an urban area with excessive noise, it was necessary to obtain very good protection against noise, and at the same time, using the same elements, achieve the desired reverberation time. Special attention was given to flutter echoes and achievement of diffuse sound fields. The inner shell construction was built up using special acoustical building blocks. Besides using double walls, the broad analysis was done in order to avoid any possible problem in sound insulation. The achieved coincident frequency for walls and floors is 7.6 Hz, and this is quite satisfactory. The computer simulation was used during the design of internal s...