Phase, lock-in, synchronous, etc. detection is used to measure a
muddied signal. One great use is in second sound where one finds the
speed by at least two methods.
Digression: One method to find the sound speed is time of flight of
the thermal pulses generated by reverse use of a bolometer. In the
method w/ which I'm familiar a second bolometer is used for detection
at the other end of a cylindrical cavity. If one tunes the PPR to
match the resonance freq. of the cavity , sensitivity is increased so
one may find the speed nearer to the T(crit.) This resonance method
is used when the "heat" signal is sinusoidal and, obviously, lock-in
detection will enable one to more closely approach T(crit.)
The question: May one use a lock-in amp. w/ narrow thermal pulses,
wherein there is less heating of the helium.