Meaning/Usage/Translation: 耳膜 vs. 鼓膜?
The dictionary takes extra caution and says it's the membrane in the ear, because 鼓膜 literally means "drum membrane", which might be taken by someone as the membrane on a real drum. 

In real usage, however, 鼓膜 is just the eardrum. The membrane on a drum instrument, or the drum head, is called 鼓皮.