In the constituents section of a fertilizer bag, NPK along with some numerical values such as 10-15-20 etc is mentioned.
This NPK signifies the constituents of the fertilizer where N stands for Nitrogen, P stands for Phosphorus and and K stands for Potassium.
Also the number such as 10-15-20 signifies the percentage amount of NPK with in the fertilizer such as a fertilizer bag of mark 10-15-20 contains 10 % nitrogen, 15 % phosphorus and 20% potassium.
It works just like a human tympannic membrane. They hear sounds through this structure as sound waves vibrate against the membrane. The men rane acts like a drum skin.