Resonance in Open Air Columns
Like a closed air column, if an air column is open at both ends resonance can only occur if a
standing wave is set up involving the air molecules in the column. Because both ends of the
column are open, they will both act as free ends forming anti-nodes.
The simplest pattern for a longitudinal standing wave in an open air column has an anti-node
at each end and a single node in the middle, combining to make an entire loop. The
fundamental frequency is therefore given by
f0 
v
2L
For each successive resonant length, one more node and anti-node
are added, adding half the wavelength of sound each time.
Like a closed air column, if the length of the open air column
remains constant and the pressure within the air column is
increased, the wavelength becomes successively shorter and the frequency correspondingly
higher. This means that an air column can resonate to a whole series of different
frequencies.
Example: An air column, open at both ends, is 1.00 m long. If the speed of sound is 340 m/s
and resonance occurs, what are the two lowest resonant frequencies?
Assigned questions: #9, 10
Assigned questions: #9, 10