The illustration shows the transient response of a draw note recorded with the TurboHarp in slow motion. These and many other records show that the reed vibration always builds up gradually.
The energy is supplied "bit by bit". The energy once transmitted to the reed remains stored in it for the most part, so that the total energy increases more and more.
Finally, one comes into a saturation range. The mechanism for the energy supply works there no longer. The maximum volume is reached at a given blowing or suction pressure.