An example of a narrow band frequency would be 154.8525 currently in use by the Millville, Ma. Police. Most current scanners allow direct entry of this frequency. There are some scanners which require that you change the frequency step from it's present default step, whatever it is, to a different step. The current spacing of frequencies has been changed to 7.5 kHz. from a previous 15 kHz. There are other steps, 6.25 being common also. There are even smaller steps, an example of 3.25 in the new 700 allocation.
The newer radios will enter a frequency like 154.0175 while older scanners allow 154.015 or 154.02.
Audio sounds a bit softer on some narrow band frequencies.