For a cylinder that has both ends open resonant frequency is given by the following formula:
[tex]f= \frac{nv}{2L} [/tex]
Where n is the resonance node, v is the speed of sound in air and L is the length of a cylinder.
The fundamental frequency is simply the lowest resonant frequency.
We find it by plugging in n=1:
[tex]f_0= \frac{v}{2L}=\frac{343}{2\cdot 2779}=0.062 Hz[/tex]
To find what harmonic has to be excited so that it resonates at f>20Hz we simply plug in f=20 Hz and find our n:
[tex]20= \frac{n343}{2\cdot 2779} =n\cdot f_0[/tex]
We can see that any resonant frequency is simply a multiple of a base frequency.
Let us find which harmonic resonates with the frequency 20 Hz:
[tex]20=n\cdot f_0\\ n=\frac{20}{0.062}=322.58[/tex]
Since n has to be an integer, final answer would be 323.
