Sound amplification by stimulated emission of radiation implies that lasers should be as feasible with sound as they are with light. In the 21st century, it is easy to produce low frequency sound in the range that humans can hear (~20 kHz), in either a random or orderly form. However, at the terahertz frequencies in the regime of phonon laser applications, more difficulties arise. The problem stems from the fact that sound travels much slower than light. This means that the wavelength of sound is much shorter than light at a given frequency. Instead of resulting in orderly, coherent phonon laser structures that can produce terahertz sound,
Stimulated emission given by where the proportionality constant "B" is known as the "Einstein B coefficient" for that particular transition, and "ρ"("ν") is the radiation density of the incident field at frequency "ν". The rate of emission is thus proportional to the number of atoms in the excited state "N", and to the density of incident photons. At the same time, there will be a process of atomic absorption which "removes" energy from the field while raising electrons from the lower state to the upper state. Its rate is given by an essentially identical equation, The rate of absorption is thus proportional