P108 Lab 5B&1
Often a system has natural frequencies of vibration.
In the case of the column of air in the
PVC tube we are studying, these natural frequencies are called the harmonic frequencies,
A vibrating source, such as the speaker, is said to
the column of air.
, of the driver (speaker) is slowly varied, the amplitude of the driven
system (air column) gets larger and larger until it reaches a peak at one of its own resonant
Therefore, as we sweep the speaker frequency past one of the
harmonic frequencies of the air column, we hear the increase in amplitude of the vibrating
air column as an increase in loudness of the radiated sound from the tube.
At resonance the
driving system passes energy very efficiently on to the driven system and the amplitude of
the driven system reaches a maximum called
(See Rossing, The Science of Sound,
A graph of the amplitude as a function of frequency is shown here.
Note that the resonant amplitude peaks at frequency
and that the peak has a width,
The linewidth is usually measured at an amplitude of 71% of
For a system which loses energy rapidly through damping or friction, the maximum
, is small and the linewidth large, and the resonance is said to be
Similarly, for a resonating system which loses energy very slowly, the maximum
amplitude is very large and the linewidth is very small.
The resonance is said to be