The Hartman effect has also been observed with acoustic waves. Yang, et al. propagated ultrasound pulses through 3d phononic crystals made of tungsten carbide beads in water. For frequencies inside the stop band they found that the group delay saturated with sample thickness. By converting the delay to a velocity through v = L / T, they found a group velocity that increases with sample thickness. In another experiment, Robertson, et al. created a periodic acoustic waveguide structure with an acoustic bandgap for audio frequency pulses. They found that inside the stop band the acoustic group delay was relatively insensitive to the length of the structure, a verification of the Hartman effect. Furthermore, the group velocity increased with length and was greater than the speed of sound, a phenomenon they refer to as "breaking the sound barrier."