A type of acoustic energy that propagates in one direction while being confined in the other two directions, in this case by the borehole wall. Normal modes are propagated as reflections off the borehole wall, and exist only in hard rock. They are highly dispersive, starting with the formation shear velocity at a certain cutoff frequency and decreasing at high frequencies to the borehole fluid velocity. Below the cutoff frequency, they do not exist. Normal mode #0 is often considered to be the tube wave and starts at zero frequency. Normal mode #1 is called the pseudoRayleigh, and starts at around 5 kHz. The other normal modes start at increasingly higher frequencies.