Bandwidth can also be defined as the
frequency difference between two
arbitrary attenuation values.
For
example: The 3
dB bandwidth of my
audio amplifier is 20 k
Hz. Translation: The difference between the highest and lowest frequency of
interest that is 3 dB (half
power) is 20 kHz. 3 dB is how far down from
maximum (or whatever you
choose) that you
allow for the
signal to be
acceptable to you. If the low frequency is 30 Hz then the upper 3 dB frequency is 20,030 Hz.
You can
specify whatever attenuation
factor you want for your
system but generally an unmarked bandwidth will
assume 3 dB.
This definition also holds true for
EM waves.