Please explain to me the 4th point

Dear student,
A sound waves in air is an example of a longitudinal (compressional) wave – the direction of the air particles’ vibrational motion is in the same direction that the wave is propagating.
As frequency is inversly proprtional to length of flute so different frequencies is produced by varying its effective length and this done by opening and closing of holes present in the flute.
In flute a , air is blown inside the instrument(energy is given) , and air inside them vibrates i.e. air column vibrates and sweet sound is produced .
Hope this information will clear your doubts about topic 
Regards 
 

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