Low Acoustic Power Research Articles

Measurements on a general thermoacoustic heat engine

Low‐power measurements have been made in a general test setup on a class of thermoacoustic devices operating at moderately low acoustic power levels (approximately 110 dB). The setup includes two loud speakers mounted on either end of a tube filled with a porous solid and air at one atmosphere. A heater and a cooling coil allow a temperature gradient to be established from one end of the tube to the other, while thermocouples and microphones allow the temperature and sound fields to be monitored by a computer (which also controls the speakers). Measured acoustic gains as a function of power level, frequency, and phasing will be reported. [Work supported by ONR.]

The Doppler signal: where does it come from and what does it mean?

Doppler sonographic measurement of blood velocity and associated physiologic parameters is a powerful diagnostic technique. State-of-the-art instrumentation incorporates velocity measurement with two-dimensional imaging capability; it uses intensity and color coding to display complex physiologic and anatomic data to the observer in an easily understood format. Although the concepts underlying Doppler sonography are not complex, mastery of the technique requires extra training and commitment. The principal features and clinical practicalities associated with Doppler sonography are summarized in the following paragraphs. Continuous-wave Doppler is very sensitive to small vessels and has no upper velocity limit. In addition, the instrumentation is not complex and produces relatively low acoustic power. A significant drawback to continuous-wave Doppler is that there is no depth sensitivity, and thus complex structures or multiple vessels can give conflicting information. Pulsed Doppler (including duplex and color-flow) instrumentation has the capability of depth resolution and a variable sample volume. Pulsed Doppler equipment is prone to aliasing (false velocity indications) under some circumstances and also produces higher peak power levels than does continuous-wave equipment. Duplex equipment is more complex and expensive than continuous-wave equipment because the two-dimensional and Doppler modes must be synchronized in operation and display. Color-flow equipment is extremely complex and expensive. Color flow provides information of a qualitative and limited quantitative value. Absolute measurement still requires range-gate measurements. Technical and anatomic factors will affect the measured velocity profiles. Thus, it is important to fully appreciate the anatomy of the vessel and the angle between the vessel and the ultrasound beam when making quantitative measurements. Measurements that evaluate the velocity waveform and make use of ratios, such as the pulsatility index, eliminate the need for angular corrections; however, artifacts due to unappreciated anatomic or wall characteristics may lead to incorrect information if all parameters are not fully understood. Doppler sonographic measurements may be used to determine the presence of flow, determine the direction of flow, identify time-varying velocity characteristics, and detect velocity disturbances. Because flow and velocity are related, it is possible to estimate flow from velocity measurements with careful calibration and proper precautions. Velocity is related to flow, which, in turn, is related to both pressure and vascular resistance.(ABSTRACT TRUNCATED AT 400 WORDS)

Vane Stagger Angle and Camber Effects in Fan Noise Generation

The problem of sound generated by the interaction of velocity disturbances with stator vanes in an annular duct is considered theoretically. The duct carries a uniform subsonic main flow and is assumed to be anechoic. In this problem it seems consistent to model the vanes by flat plates parallel to the duct axis. However, this modeling may yield an unrealistically low acoustic power. In the present paper, a nonplanar lifting surface approximation of the vanes shows that at frequencies prevailing in current turbofans even a small vane inclination significantly affects the sound generation process. The lower the circumferential periodicity \m of the cut-on sound field, the more pronounced are the acoustic effects of vane stagger angle and camber. Calculations for a typical example of a turbof an stator show at \m = 1 a rise of 23 dB in upstream acoustic power when the stagger angle is increased from 0 to 15 deg. Furthermore, even at \m - 16 there is still a rise of 5 dB. Besides the acoustic power, the modal distribution of the sound field is very sensitive to vane camber and stagger angle.

Acoustic power dissipation on radiation through duct terminations: Experiments

This paper describes the acoustic transmission characteristics of ducts, nozzles, orifices, and perforated plates, studied under an experimental program using an acoustic impulse technique. In this technique high intensity pulses, generated by discharging a capacitor across a spark gap, were used as the sound source. The test conditions include heated and unheated flows, with and without simulated flight. Results for a straight round duct, three convergent nozzles, a suppressor nozzle, 12 orifice plates, and 10 perforated plates are presented. A low frequency acoustic power loss phenomenon was observed for all configurations at all test conditions including the no flow condition. It was suspected that the power loss phenomenon at the no flow condition could be due to the conversion of acoustic energy into vortical energy due to non-linear propagation of high intensity pulses. However, a small amount of low frequency power loss was noticed even when tests were repeated with low intensity sound. Detailed flow visualization results were also obtained to complement the acoustic results.

Effect of 80-keV Ne+ ion implantation on acoustic surface wave attenuation in LiNbO3

Acoustic surface wave attenuation in LiNbO3 caused by ion implantation was studied as a function of fluence and surface wave frequency. 80-keV Ne+ ions were implanted at five fluences covering the range 1014–1016 ions/cm2. The yz Rayleigh surface waves were generated spanning the interval 100–1000 MHz by interdigital transducers operating at odd harmonics. Attenuation was measured directly by a standard laser probe using reflected light. At low acoustic power densities, the data can be fit by P(z)/P0=10−Af2z, where P is acoustic power, f is the surface wave frequency, and A is a constant for each fluence used. The values of A show a definite peak with fluence at around 1015 ions/cm2, where A=9 × 10−5 dB cm−1 MHz−2±10%. This is about twenty times the value of A for an unimplanted surface.

Measurement of Acoustic Powers in the Microwatt-Milliwatt Range

The recent increase in the use of ultrasonics in medical diagnosis has led to the need for methods to measure very low acoustic power outputs in the megahertz frequency range. A system similar to that described by Kossoff [J. Acoust. Soc. Amer. 38, 880–881 (1965)] has been developed which uses a Cahn electrobalance to measure acoustic radiation force on an absorbing target. At present the system can easily detect power outputs of 30 μW. Sources of error and how they can be eliminated in such measurements will be discussed. Recently questions have been raised about the validity of the classical theoretical expression for Langevin radiation pressure applicable to the case of a plane traveling wave impinging on an absorbing target in an open vessel. However, a critical review of the theory using a formulation based on Euler's momentum theorem leads one to the conclusion that the Langevin radiation pressure is just equal to the classical value of the energy density even when nonlinearities of the propagation medium and distortion of waveform are considered. Experimental tests of this theory are in progress. [This work was supported by the HAS Project at the University of Vermont, and by the National Institutes of Health.]

Ultrasonics and uranium: Myers, J. L.Ultrasonic News, 6, no. 2, p. 8 (1962)

The aim of this study was to investigate the inertial cavitation inside a phantom treated by pulsed HIFU (pHIFU). Basic bovine serum albumin (BSA) phantoms without any inherent ultrasound contrast agents (UCAs) or phase-shift nano-emulsions (PSNEs) were used. During the treatment, sonoluminescence (SL) recordings were performed to characterize the spatial distribution of inertial cavitation adjacent to the focal region. High-speed photographs and thermal coagulations, comparing with the SL results, were also recorded and presented. A series of pulse parameters (pulse duration (PD) was between 1 and 23 cycles and pulse repetition frequency (PRF) was between 0.5 kHz and 100 kHz) were performed to make a systematic investigation under certain acoustic power (APW). Continuous HIFU (cHIFU) investigation was also performed to serve as control group. It was found that, when APW was 19.5 W, pHIFU with short PD was much easier to form SL adjacent to the focal region inside the phantom, while it was difficult for cHIFU to generate cavitation bubbles. With appropriate PD and PRF, the residual bubbles of the previous pulses could be stimulated by the incident pulses to oscillate in a higher level and even violently collapse, resulting to enhanced physical thermogenesis. The experimental results showed that the most violent inertial cavitation occurs when PD was set to 6 cycles (5 μs) and PRF to 10 kHz, while the highest level of thermal coagulation was observed when PD was set to 10 cycles. The cavitational and thermal characteristics were in good correspondence, exhibiting significant potentiality regarding to inject-free cavitation bubble enhanced thermal ablation under lower APW, compared to the conventional thermotherapy.