Friday, May 23, 2008
When a pulse is applied to an NMR probe in a strong magnetic field, the oscillating rf current in the circuit induces mechanical (acoustic) oscillations in metal parts of the probe. These mechanical oscillations in turn generate rf signals detected by the coil. The so called "acoustic ringing" is seen in the FID and (depending on the specific frequency and probe) usually decays within several tens to hundreds of microseconds after the pulse. It is more of a problem at high fields and low frequencies and a particular problem when wide spectral widths (short dwell times) are employed. The acoustic ringing may impede the observation of very broad lines and cause baseline and phasing problems when observing sharp lines with large spectral widths. An example of the latter case is shown in the figure below.