You should ask students where they have seen or experienced similar phenomena. Blowing through the reeds causes them to vibrate. It also causes the column of air in the straw to vibrate, producing the pitch characteristic to that length. High pitched sounds characteristically have shorter wavelengths. Their frequency of vibration is higher.
Frequency is measured in cycles per second or hertz.
5000 cycles per second. Low pitched sounds have longer wavelengths and lower frequencies. Piano tuners will tune the middle A to 440, meaning 440 hertz. The range for normal human hearing is from 15 cycles per second on the low end of the scale to 15,000 cycles per second at the high end. The reeds of most straw oboes vibrate between 200 and 600 cycles second.