A sound unit (7)
I believe the answer is:
decibel
This could be a double definition.
'a sound' is the first definition.
I know that 'sound' relates to this answer. I also know that both the answer and definition are singular nouns
'unit' is the second definition.
(decibel is a kind of unit)
Can you help me to learn more?
(Other definitions for decibel that I've seen before include "Unit for measuring intensity of sound" , "Sound level unit" , "Measure of sound" , "Noise unit" , "Unit of volume" .)