Total Acoustic Output Research Articles

Three-Dimensional Finite Element Modeling of the Singer’s Formant Cluster Optimization by Epilaryngeal Narrowing With and Without Velopharyngeal Opening

This study aimed to find the optimal geometrical configuration of the vocal tract to increase the total acoustic energy output of human voice in the frequency interval 2-3.5 kHz “singer’s formant cluster”, (SFC) for vowels [a:] and [i:] considering epilaryngeal changes and the velopharyngeal opening (VPO). The study applied 3D volume models of the vocal and nasal tract based on computer tomography (CT) images of a female speaker. The epilaryngeal narrowing (EN) increased the total sound pressure level (SPL) and SPL of the SFC by diminishing the frequency difference between acoustic resonances F3 and F4 for [a:] and between F2 and F3 for [i:]. The effect reached its maximum at the low pharynx/epilarynx cross-sectional area ratio 11.4:1 for [a:] and 25:1 for [i:]. The acoustic results obtained with the model optimization are in good agreement with the results of an internationally recognized operatic alto singer. With the EN and the VPO, the vocal tract input reactance was positive over the entire fo singing range (ca. 75-1500 Hz). The VPO increased the strength of the SFC and diminished the SPL of F1 for both vowels, but with EN the SPL decrease was compensated. The effect of EN is not linear and depends on the vowel. Both the EN and the VPO alone and together can support (singing) voice production.

Analysis of spring softening effect on the collapse voltage of capacitive MEMS ultrasonic transducers

This paper explained the dependency of collapse voltage on semiconductor device structural features (membrane diameter, membrane thickness and the vertical distance between the electrodes) and physical characteristics (mechanical residual stress of the silicon nitride membrane) considering the electro-mechanical model of MEMS based Capacitive micromachined ultrasonic transducer (CMUT). To have sensitivity comparable to that of piezoelectric ultrasonic transducers (UTs), CMUTs need to be biased close to the collapse voltage. Maximum efficiency is achieved in the conventional mode of operation by biasing the device close to the collapse voltage. The total acoustic output pressure is determined by the efficiency of the device. Hence a careful investigation of the same is decidedly required. Finite element method (FEM) model by PZFlex and analytical model of single element CMUT with 0.75 µm thick silicon nitride membranes suspended on 0.5 µm thick cavity were developed showing resonance frequency at 5 MHz. Through these analyses, it is observed that membrane and vacuum gap thickness are both directly proportional to collapse voltage, while radius of the membrane and also its area are inversely proportional to collapse voltage. Initially the spring softening effect of the membrane has been neglected. Later the effect has been included in the proposed model and analyzed. It has been shown that the spring softening effect cannot be neglected for accurate CMUT modeling. A capacitive micromachined ultrasound transducer can be realized with the model described in this paper.

Relationship between Acoustic Power and Acoustic Radiation Force on Absorbing and Reflecting Targets for Spherically Focusing Radiators

Total acoustic output power is an important parameter required by standards for most ultrasonic medical equipment including high-intensity focused ultrasound (HIFU) systems. Radiation force balances are routinely used; however, radiation force is not strictly dependent on the ultrasound power but, rather, on the wave momentum resolved in one direction. Consequently, measurements based on radiation force become progressively less accurate as the ultrasound wave deviates further from a true plane wave. HIFU transducers can be very strongly focused with F-numbers less than one: under these conditions, the uncertainty associated with use of the radiation force method becomes very significant. International Standards IEC 61161 and IEC 62555 suggest plane-wave correction factors for unfocused transducers radiating onto an ideal absorbing target and focusing corrections for focused transducers radiating onto ideal absorbing targets and onto conical reflecting targets (IEC 61161). Previous models have relied on calculations based on the Rayleigh integral, which is not strictly correct for curved sources. In the work described here, an approach combining finite element methods with a discretization of the Helmholtz equation was developed, making it possible to model the boundary condition at the structure/fluid interface more correctly. This has been used to calculate the relationship between radiation force and total power for both absorbing and conical reflecting targets for transducers ranging from planar to an F-number of 0.5 (hemispherical) and to compare with the recommendations of IEC 61161 and IEC 62555.

Measurement of time averaged power in HITU fields—effects of duty cycle and target distance

The reliable description of the ultrasonic fields of high-intensity therapeutic ultrasound (HITU) devices is a prerequisite concerning the safe application of the method in the daily clinical routine. Since ultrasonic sensors used for the characterization of diagnostic fields are at high risk of being damaged in the strong therapeutic fields, the measurements are carried out in burst mode to reduce the acting temporal-average intensities. For the thorough investigation of possible differences between the excitation in continuous wave (cw) and burst mode, the temporal-average total acoustic output powers of two types of HITU transducers with f-numbers of approximately 1 and with working frequencies between 1.1 MHz and 3.3 MHz were investigated by means of a radiation force balance. The maximum cw equivalent power level was 300 W; the duty cycles varied between 1% and 90%. In addition, the possible effect of the transducer–target distance was investigated. It was found that the different turn-on and turn-off behaviour of the transducers caused variations of the effective duty cycle, depending on the power level and the power amplifier used. The temporal-average power declined with increasing distance as expected, and no focal anomaly was detected.

A Buoyancy Method for the Measurement of Total Ultrasound Power Generated by HIFU Transducers

Total acoustic output power is a key parameter for most ultrasonic medical equipment and especially for high intensity focused ultrasound (HIFU) systems, which treat certain cancers and other conditions by the noninvasive thermal ablation of the affected tissue. In planar unfocused fields, the use of a radiation force balance has been considered the most accurate method of measuring ultrasound power. However, radiation force is not strictly dependent on the ultrasound power but, rather, on the wave momentum resolved in one direction. Consequently, measurements based on radiation force become progressively less accurate as the ultrasound wave deviates further from a true plane-wave. HIFU transducers can be very strongly focused with F-numbers less than one: under these conditions, the uncertainty associated with use of the radiation force method becomes very significant. In this article, a new method for determining power is described in detail. Instead of radiation force, the new method relies on measuring the change in buoyancy caused by thermal expansion of castor oil inside a target suspended in a water bath. The change in volume is proportional to the incident energy and is independent of focusing or the angle of incidence of the ultrasound. The principles and theory behind the new method are laid out and the characteristics and construction of an appropriate target are examined and the results of validation tests are presented. The uncertainties of the method are calculated to be approximately ±3.4% in the current implementation, with the potential to reduce these further. The new technique has several important advantages over the radiation force method and offers the potential to be an alternative primary standard method. (E-mail: adam.shaw@npl.co.uk)

Pulsed Cavitational Ultrasound: A Noninvasive Technology for Controlled Tissue Ablation (Histotripsy) in the Rabbit Kidney

The optimal minimally invasive treatment for small renal masses continues to evolve. Current ablative technologies rely on thermal mechanisms for tissue destruction. However, the creation of precise lesions is limited by inhomogeneous heating/cooling due to tissue variability, perfusion effects and tissue charring. We hypothesized that nonthermal mechanical effects of ultrasound (cavitation) can be used to progressively homogenize tissue in controlled fashion with predictable results. We developed a focused annular array ultrasound system capable of delivering high intensity (greater than 20 kW/cm) short pulses (20 microseconds) of energy to a target volume. This system operates at a repetition frequency of 100 Hz, resulting in a low time averaged power output (approximately 5 W total acoustic output). Following approval from the institutional animal care committee a series of transcutaneous ablations were performed in the normal kidneys of 10 rabbits. Lesions created with a small number of pulses (10 or 100) produced scattered areas of damage characterized by focal hemorrhage and small areas of cellular injury in the targeted volume. Lesions created with greater numbers of pulses (1,000 or 10,000) demonstrated complete destruction of the targeted volume. Gross examination revealed that lesions contained a liquefied core with smooth walls and sharply demarcated boundaries. Histological examination demonstrated extensive areas of acellular debris surrounded by a narrow margin of cellular injury. This pulsed cavitational ultrasound system is capable of transcutaneous nonthermal destruction of renal tissue. Refinement of this technology for noninvasive ablation of small renal masses is currently under way.

Capacitive micromachined ultrasonic transducer design for high power transmission

Capacitive micromachined ultrasonic transducers (cMUTs) were developed to meet the demands of the ultrasonic industry. To achieve maximum efficiency, the conventional operation of the cMUT requires a bias voltage close to the collapse voltage. Total acoustic output pressure is limited by the efficiency of the cMUT and the maximum-allowed pulse voltage on the membrane. In this paper, we propose the collapse-snapback operation of the cMUT: the membrane is collapsed onto the substrate in the collapsing cycle, and released in the snapback cycle. The collapse-snapback operation overcomes the above-mentioned limitations of the conventional operation. The collapse-snapback operation utilizes a larger range of membrane deflection profiles (both collapsed and released profiles) and generates higher acoustic output pressures. The static finite element calculations were performed to design cMUTs with specific collapse and snapback voltages by changing the electrode parameters (radius (re), position (de), and thickness (te)). These designs were refined for optimum average displacement per cycle. An electrode radius greater than 60% of the membrane radius significantly improved the displacement per volt. Moderately thick membranes (te approximately 0.2 microm) were preferred, as thicker membranes reduced the displacement per volt. Under proper bias conditions, the collapse-snapback operation, designed for high-power transmission, allowed the application of pulse voltages larger than the difference of collapse and snapback voltages. Dynamic finite element calculations of an infinite cMUT array on the substrate loaded with acoustic fluid medium were performed to determine the dynamic response of the cMUT. Commercially available FEM packages ANSYS and LS-DYNA were used for static and dynamic calculations, respectively. The cMUTs were fabricated for optimal performance in the collapse-snapback operation. The transmit experiments were performed on a 2-D cMUT array using a calibrated hydrophone. Taking into account the attenuation and diffraction losses, the pressure on the cMUT surface was extracted. The cMUT generated 0.47 MPa (6 kPa/V) and 1.04 MPa (11 kPa/V) in the conventional and collapse-snapback operations, respectively. Therefore, collapse-snapback operation of the cMUTs was superior for high-power transmission.

Probing acoustic fields of clinically relevant transducers: the effect of hydrophone probes' finite apertures and bandwidths

The influence of finite aperture and frequency response of piezoelectric ultrasonic hydrophone probes on the free-field pulse intensity integral (PII) and mechanical index (MI) was investigated using a comprehensive acoustic wave propagation model. The model developed was capable of predicting the true pressure-time waveforms at virtually any point in the field. The input to the model used pressure amplitude data measured in the immediate vicinity of the acoustic source or transducer considered. The experimental verification of the model was obtained using a commercially available, 8 MHz, dynamically focused linear array and a single element, 5 MHz, focused rectangular source. The verification was performed at low and high excitation levels, corresponding to linear and nonlinear acoustic wave propagation, respectively. The pressure-time waveforms were recorded using piezoelectric polymer hydrophone probes that had different sensitivities, frequency responses, bandwidths, and active element diameters. The nominal diameters of the probes ranged from 50 to 500 microm, and their useable bandwidths varied between 55 and 100 MHz. The PII, used to calculate the thermal index (TI), was found to increase with increasing bandwidth and decreasing effective aperture of the probes. The MI, another safety indicator, also was affected, but to a lesser extent. The corrections predicted using the model were used to reduce discrepancies as large as 30% in the determination of PII. The results of this work indicate that, by accounting for hydrophones' finite aperture and correcting the value of PII, all intensities derived from the PII can be corrected for spatial averaging error. The results also point out that caution should be exercised when comparing acoustic output data. In particular, hydrophone's frequency characteristics of the effective diameter and sensitivity are needed to correctly determine the MI, TI, and the total acoustic output power produced by an imaging transducer.

The influence of finite aperture and frequency response of ultrasonic hydrophone probes on the determination of acoustic output

The influence of finite aperture and frequency response of piezoelectric ultrasonic hydrophone probes on the Thermal and Mechanical Indices was investigated using a comprehensive acoustic wave propagation model. The experimental verification of the model was obtained using a commercially available, 8 MHz, dynamically focused linear array and a single element, 5 MHz, focused rectangular source. The pressure–time waveforms were recorded using piezoelectric polymer hydrophone probes of different active element diameters and bandwidths. The nominal diameters of the probes ranged from 50 to 500 μm and their usable bandwidths varied between 55 and 100 MHz. The Pulse Intensity Integral (PII), used to calculate the Thermal Index (TI), was found to increase with increasing bandwidth and decreasing effective aperture of the probes. The Mechanical Index (MI), another safety indicator, was also affected, but to a lesser extent. The corrections needed were predicted using the model and successfully reduced the discrepancy as large as 30% in the determination of PII. The results of this work indicate that by accounting for hydrophones' finite aperture and correcting the value of PII, all intensities derived from the PII can be corrected for spatial averaging error. The results also point out that a caution should be exercised when comparing acoustic output data. In particular, hydrophone's frequency characteristics of the effective diameter and sensitivity are needed to correctly determine the MI, TI, and the total acoustic output power produced by an imaging transducer.

Multidriver loudspeaker apparatus with improved crossover filter circuits

A loudspeaker system includes at least two loudspeaker drivers, together with an electrical crossover network having filter circuits with at least two separate mutually exclusive frequency passbands. The filter circuits comprising the crossover network each possess brick-wall amplitude responses, i.e., passbands with very high band-edge amplitude vs. frequency response slops, on the order of 100 dB/octave in the better embodiments. The high passband band-edge slopes, which are realized by the inclusion of transmission zeros in the separate crossover filter transfer functions, takes together with further appropriate crossover filter transfer function synthesis causes the separate loudspeaker drivers comprising the loudspeaker system to function independently of one another in their contribution to total system acoustic output. It is shown that the loudspeaker system permits an accurate approximation to the ideal delay function in acoustic space, while minimizing acoustic wave interference among drivers operating in adjacent frequency band, and also reducing overall system nonlinear distortion.

A Survey of Output Characteristics of Some New Therapeutic Ultrasound Instruments Manufactured in Australia

Seven newly purchased therapeutic ultrasound transducers were tested using a radiation pressure balance and steel baffles in compliance with Australian Standard Specification T40-1969. Total acoustic output at maximum meter settings, and effective transducer radiating areas were measured. Only one transducer demonstrated the output values nominated by the manufacturer. Two had an appropriate relationship between effective area and total output, though these values were not those stated by the manufacturer. The remaining four had effective radiating areas below the Australian Standards Association tolerance. Three of these had an otherwise acceptable power output and one had a very low output. These data substantiate the need for adequate testing facilities to be available for physiotherapists using ultrasound clinically.

Techniques for displaying noise source identification data

Considerable efforts are being made to develop new techniques for noise source identification of complex machinery, such as internal combustion engines. Acoustic intensity measurements with closely spaced parallel microphones, microphone array measurements, and nearfield sound pressure contour measurements are three such techniques receiving interest at the present time. The measurement goal of such techniques is to identify and rank the contribution of individual sources, as well as to determine the total acoustic output of the system. Each of these techniques produces a relatively great number of individual measurements, and it is essential that convenient means of displaying the data be developed in order to evaluate the techniques and to make the results useful and easy to relate to. This paper describes several ways of displaying data from such measurements so that the strengths and weaknesses of the techniques can be assessed. Data are presented from simple but useful models using the three techniques. The data are presented in several formats including CRT, color displays, and contour plots.

Split-brain preparation by ultrasonic lesions in the rat

Ultrasound has been used to selectively lesion the corpus callosum in 18 rats. A focused beam, at 3.3 MHz of 3.5 W total acoustic output with 2 sec continuous wave exposure, was used to create approximately spherical lesions of 1 mm diameter. A series of overlapping lesions enabled severance of the entire corpus callosum at the midline. To make the technique more reliable an ultrasonic method was developed to accurately locate the corpus callosum. Verification of the lesions was made histologically and also electrophysiologically by observing the disappearance of trans-callosal evoked potentials. Damage to fibres other than the corpus callosum was minimal. Possible applications of this lesioning technique are mentioned.

Severing the corpus callosum in rats using ultrasound: Theoretical and experimental correlations

A single, focused, ultrasound beam of 3.5 W total acoustic output for 2 s at 3.3 MHz was used to create a series of overlapping lesions to sever the corpus callosum of the rat on the midline. A theoretical and experimental determination of the ultrasound field is described and histological evidence of the lesions is provided. Good correlation occurred between the theoretical and experimental results only for the intensity contour lines examined. The experimentally derived focal intensity of 162 W cm−2 (at 25 °C) was 24% of the intensity predicted from theoretical calculations. A difference of 2 mm was found for the axial position of maximal intensity, thermocouple plotting showing the focal region to be nearer the transducer than expected from the theory. Histological results indicated that, under certain conditions, from a single dose of ultrasound two lesions could be obtained corresponding to both the theoretically and experimentally derived locations. At lower intensities it was possible to ulitize the upper location to sever the corpus callosum, the lower intensity site then being subthreshold for the rat brain. The trackless localized damage caused by ultrasound makes this method of obtaining split-brain rats suitable for behavioral and biochemical studies, where any nonspecific damage may have undesirable effects.

Processing of airborne acoustic noise sources using passive range determination techniques

This paper presents a continuation of the passive range measurement efforts reported at the 90th conference. Previous work included the modeling of passive range determination using a single point receiver based on moving source observables. The outcome as reported last year was a parameterization of a hypothetical system that had the capability to determine range to source passively. This year, we have expanded the work to include building a fully operational system and field testing it against various moving sources. In addition, a pseudo-Fourier analysis section has been incorporated to accurately and correctly sort out and classify moving sources. This, coupled with a priori selection information coded into the system, enables it to perform those required functions against preselected sources while ignoring nondesired signals. We have concentrated this year on enlarging our data base of acoustic signatures with an eye to determining the points of commonality between major source classes. This has led to vastly simplified signal processing techniques compared with previously tried methods. Specifically, we have found that within a source class, certain fundamental signature components cannot be masked by purely physical means without materially degrading the operation of the source. Masking efforts employing jamming techniques are not as effective as desired, since jamming means that the total acoustic output has been raised yielding a more visible source. Baffling of noise sources, while effective for high-frequency noise components, tends to be less than effective when used to suppress the lower components without an attendant loss in source mobility performance.

A note on the acoustic output from round and interfering jets

A model is presented for estimating the acoustic output per unit slice in the adjustment region of a round subsonic jet, and the deduced contributions of the annular, adjustment and fully-developed regions are found to be approximately in the proportions 25:24:1. The method is extended to the case when two similar adjacent parallel round jets interfere, the estimated reduction in total acoustic output being up to 2 dB. An approach to the calculation of the attenuation of suppressor nozzles is also given.

On the Noise of Crowds

The behavior of crowds of people as acoustical-noise sources is a matter concerning which there is little definitive information, in spite of an increasing need for such knowledge. Although quantitative data are available for individual talkers and listeners, analysis of an aggregate of individuals—i.e., a crowd—is complicated by the fact that the total acoustic output is influenced by the effect upon communication of the resultant noise level. A particular problem, discussed in several papers a few years ago under the heading “cocktail-party acoustics,” is the spontaneous increase of crowd noise to a rather high level under certain conditions. A simple model is given that attempts to define these conditions. This model assumes that (1) both direct and reverberant sound must be considered, although either may be dominant in a given case, and (2) if, in a specific situation, the noise level due to normal conversation is such as to inhibit communication, the level will increase to a certain maximum value, at which point talkers are exerting maximum speech effort short of shouting. Examples analyzed according to the model are given.