Mechanical coupling causes the skin to vibrate in synchrony
with motions of the heart. Such vibrations generate acoustic
pressure waves, referred to as heart sounds, that can be picked
by microphones or by trained human ears using stethoscopes.
Such sounds provide important information about cardiac
health. For example, consider the characteristic lub/dub rhythm
present within each heart beat, referred to as the first heart
sound (S1) and second heart sound (S2), respectively. These
sounds are produced by the turbulent flow of blood against
the closed atrioventricular and semilunar valves, respectively,
and are thus sensitive to the condition of these valves. The
resting value and short-term variability of the heart rate, which
are both of clinical importance [1], can also be measured
from such acoustical recordings, known as phono-cardiograms
(PCGs).