A mad doctor believes that baldness can be cured by warming the scalp with sound waves. His patients sit underneath the Bald-o-Matic loudspeakers, where their heads are bathed with 92dB of soothing 800 {\rm Hz} sound waves. Suppose we model a bald head as a 16-cm-diameter hemisphere.
If 0.23J of sound energy is considered an appropriate "dose," how many minutes should each therapy session last?