Recorder is an musical instrument that is widely used. Its pitch depends on the volume of air that is vibrating. A larger volume vibrates more slowly. The sound produce has a longer wavelength and thus has a lower pitch. A smaller volume vibrates more quickly. It has a shorter wavelength and a higher pitch.
In the experiment, when I covered holes that are closer to the mouthpiece, the volume of air that is vibrating in the recorder is very small as the air went out from the holes quickly, and the particles are vibrating quickly. When I covered holes that are further from the mouthpiece, the volume of air needed to keep vibrating all the way through the covered holes until the vibration was sent out of the recorder through the hole that was really far away. As the air inside the recorder vibrates for a longer time, the vibration reduces and the particles are vibrating slowly. This is how the high and low pitch in the recorder is made.
The pitch of a recorder can be easily changed by the way how we covered the holes. Even a small gap we leave when we are covering a hole can make the pitch of the sound made totally different as the frequency of the vibration is changed.