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In 1966, William Russell Abel, Ansel Cochran Anderson and John Charles Wheatley sent acoustic waves through bulk liquid helium-3 at $0.32\mathrm{atm}$ and frequencies $15.4$ and $45.5\mathrm{MHz}$. A quartz delay line and receiving acoustic transducer bounded a $0.138\mathrm{mm}$ liquid gap; shielding reduced electrical pickup. Powdered cerium magnesium nitrate provided magnetic thermometry during a slow warm-up. Pulse amplitudes measured acoustic attenuation; phase difference comparison measured velocity. Calibration pulses checked gain; early received cycles avoided multiple reflections. On cooling through a maximum, acoustic attenuation changed from $\alpha \propto {\omega }^2/T^{\ast 2}$ to $\alpha \propto T^{\ast 2}$ with a velocity shift supporting zero sound; $T^{\ast }$ is their magnetic temperature scale. Weak acoustic attenuation was harder to resolve.^[ref]

This 1966 test followed Lev Landau's 1962 Nobel Prize in Physics.^[ref]

The experiment concerned normal liquid helium-3, distinct from the later discovery of superfluid helium-3.^[ref] Landau's original papers are theoretical calculations, not reports of an apparatus.

An acoustic transducer converts between an acoustic disturbance and another form of signal or actuation. A microphone converts sound-induced motion into an electrical signal; a loudspeaker performs the opposite conversion. A piezoelectric transducer can use reciprocal coupling between deformation and an electric field to transmit and receive ultrasound.

Its frequency response, sensitivity and acoustic impedance affect what is emitted or measured. In a pulse-echo experiment, the same device can launch a short pulse and record the returning reflection. Travel time constrains distance only after the sound speed and the outgoing-plus-return path are accounted for.

Echocardiography uses ultrasonography to form moving images of the heart. Returning sound waves reveal the dimensions and motion of its chambers and heart valves, making structure and pumping visible during the same examination.

In the common transthoracic approach, an acoustic transducer transmits pulses through the chest and receives reflected signals. A computer reconstructs their spatial and temporal pattern into images. The resulting measurements contribute to clinical diagnosis alongside the clinical history and other observations; an image and its interpretation are distinct stages of the assessment.^[ref]

Endoscopic ultrasonography combines an endoscope with an acoustic transducer to obtain ultrasonography images from within the gastrointestinal tract. Positioning the source and receiver near a target shortens the intervening acoustic path and can reveal the wall and nearby structures. A probe integrated into the endoscope tip and a probe passed through its working channel are different apparatus arrangements.^[ref]

A microphone is an acoustic transducer that converts an acoustic wave into an electrical signal. In an acoustic time-of-flight speed measurement, its response affects which recorded feature is identified as arrival.

Time reversal acoustics uses recorded sound to send a field back toward the region that originally emitted it. An array of acoustic transducers records a pulse at multiple positions, reverses each recording in time, and emits the reversed signals from the corresponding positions. In a stationary reciprocal medium, the different paths can then contribute coherently near the original source.

A laboratory demonstration places a short-pulse source in a tank, records it at an array, removes or silences that source, and re-emits the reversed records. Mapping the returning pressure pulse tests the refocusing directly. Multiple propagation paths need not prevent focusing if they remain stable and are recorded; they can supply additional path diversity. Finite recording duration, limited bandwidth, incomplete spatial coverage and acoustic attenuation limit the reconstruction. Reversing recordings therefore implements a particular wave-control experiment, rather than undoing every dissipative process in the medium.