Direct measurements of time-averaged spatially integrated output power radiated into reflectionless water loads can be made with high accuracy using techniques which exploit the radiation pressure exerted by sound on all objects in its path. With an absorptive target arranged to intercept the entirety of an ultrasound beam, total beam power can be determined as accurately as the radiation force induced on the target can be measured in isolation from confounding forces due to buoyancy, streaming, surface tension, and vibration. Pulse modulation of the incident ultrasound at a frequency well above those characteristics of confounding phenomena provides the desired isolation and other significant advantages in the operation of the radiation force balance (RFB) constructed in 1974. Equipped with purpose-built transducers and electronics, the RFB is adjusted to equate the radiation force and a counterforce generated by an actuator calibrated against reference masses using direct current as the transfer variable. Improvements made during its one overhaul in 1988 have nearly halved its overall measurement uncertainty and extended the capabilities of the RFB to include measuring the output of ultrasonic systems with arbitrary pulse waveforms.

No abstract

In response to increased interest in the use of calibrated sources of ultrasonic energy, we have developed a system comprising components grouped to facilitate the accurate transfer of calibration. Electronic circuitry supplied with and built into each ultrasonic transducer obviates both the use of not-readily-available radiofrequency equipment and the measurement of anything more exotic than dc voltage. Prototype transducers have shown good output at frequenices up to 78 MHz. Units now available to the public can be calibrated at output powers ranging from 5 mW to 500 mW at frequencies between 1 and 20 MHz.

We have developed a transducer which allows the benefits of Line-Focus Beam (LFB) acoustic minoscopy to be realized over large areas, using a conventional pulserreceiver. Experimental evidence is presented to show that the transducer is correctly modeled in detail by Green's function theory, and that all relevant wave speeds can also be predicted using a much simpler geometrical ray model. Data obtained by simply rotating the transducer a fixed distance above the specimen are presented using grey-scale plots which establish the ease with which anisotropy can be revealed.Finally, a grey scale plot of rotational-scan data recast in terms of velocity is shown to demonstrate the simultaneous detection of both surface and pseudo-surface waves in the same crystallographic orientation of a silicon specimen.

In a specially designed radiation force balance (RFB), low-frequency pulse modulation of the incident ultrasound allows high-accuracy measurement of time-averaged spatially-integrated ultrasound power radiated into a reflectionless water load. Errors characteristic of force sensors are precluded by operating the RFB as a force comparator, without directly measuring force. Equipped with purpose-built transducers and electronics, the RFB is adjusted to equate the radiation force and a counterforce generated by an actuator calibrated against reference masses using direct current as the transfer variable. Special techniques enable RFB measurements of the output of continuous-wave and pulsed diagnostic medical systems. Nearly twenty years ago, customer feedback indicated that the accuracy of power levels replicated using radiation conductance was seriously compromised by limitations of equipment typically available. Accordingly, an alternative scheme, based on a stable radio frequency (rf) voltage sensor treated as part of the transfer standard, was developed which allows ultrasound power-transfer measurements to be achieved with relative standard uncertainty components as low as 2 10-3 (1), using equipment already available to most customers. A detailed description is given of the dc-level method (DCLM), which since 1985 has been, by a wide margin, the method most frequently requested.