Enrico is having trouble telling the difference between the sound of a tuba and the sound of a piccolo. even though a piccolo produces much shorter, faster sound waves than does a tuba, he has trouble picking out the differences in the of these sounds.

pitch                  

Explanation:

The answer is "pitch".

A pitch of the sound corresponds to the frequency of the sound, meaning whether the sound wave is of higher frequency or of lower frequency.

Sounds are made of vibrating particles called sound waves. If the sound waves vibrate fast they are said to have higher pitch or if the sound waves vibrate slow, the sound waves are said to have lower pitch.

Thus Enrico is having trouble identifying the sound waves between a tuba and a piccola based on the pitch of these sounds.

Thus the answer is

"pitch".