एक प्रगामी तरंग (progressive wave) का कुल ऊर्जा घनत्व किसके द्वारा दिया जाता है (जहाँ प्रतीकों के अपने सामान्य अर्थ हैं)?

When a transverse wave strikes normally against a rigid wall and gets reflected, then:

The fundamental frequency of a closed organ pipe is equal to the frequency of the first overtone of an open organ pipe of length 60 cm. The length (in cm) of the closed organ pipe is: