Third sound is a film thickness and temperature fluctuation on a thin superfluid helium film (Figure 1, below). Such fluctuations can be generated by thermal means or by electrostriction. The techniques which are used to measure third sound include time of flight of individual pulses with signal averaging, and resonance techniques in which the amplitudes, damping and phase of the resonance can be determined. A good reference for pulsed third sound is the early review by Atkins and Rudnick .
Current ongoing research on third sound in the labs here involves study of third sound itself and the use of third sound to investigate problems of wider interest in Condensed Matter Physics. Some of our recent and more current work involves: (1) a detailed study  of the evolution of third sound pulse shapes as a function of temperature, film thickness and thermal drive ampitude (e.g. Figure 2, below), (2) studies of third sound pulses in head-on collisions  (Figure 3, below) as a function of the just mentioned parameters, and (3) measurements of third sound in 3He - 4He mixture films. Discussion of some of this work can be found in references [4-7]. Work planned for the future includes (1) a study of third sound in mixture films to extremely low temperatures as part of a search for a new superfluid phase of 3He, (2) measurements of third sound on hydrogen plated substrates, and (3) further study of third sound in the presence of patterned roughness.
Figure 1: An example of a third sound pulse recorded by a bolometer. To create the pulse, a resistive strip deposited on a glass substrate is driven with a current pulse. The local heating creates the third sound pulse in the adsorbed 4He film which is a few atomic layers thick. The temperature swing (a few micro Kelvin in this temperature range) associated with the pulse is detected by a transition edge superconducting bolometer. The temperature of the transition edge of the bolometer can be tuned (lower) somewhat by the application of a magnetic field and/or a bias current, and by this means maximum sensitivity can be achieved at the temperature of interest.
Figure 2: An example of the evolution of the third sound pulse shape at T = 1.10 K for a 4He film thickness of 4.9 atomic layers on glass as the amplitude of a square wave current pulse to the driver is increased. The third sound is observed to saturate in amplitude and then broaden as the energy in the pulse increases.
Consider the bottom trace of the left panel. Here pulse A is launched first, and is detected at the bolometer first, followed by the detection of pulse B. Because of this timing, Pulse A arrives at the bolometer in virgin condition, passes over it, and then collides with pulse B which is on its way toward the bolometer. Thus, by the time pulse B arrives, it has suffered a prior collision with pulse A. As we move up the figure, this collision event becomes visible as the "delay" is changed. At the top of the figure, the "delay" is of a value such that pulse B arrives first in its virgin state, followed by the arrival of pulse A. In this case, pulse A has collided with B prior to pulse A's arrival at the bolometer.