Amount of sound indicated by central part of soundboard? (7)

I believe the answer is:

decibel

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'amount of sound' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

(Other definitions for decibel that I've seen before include "Measure of sound" , "Sound level unit" , "Unit of volume" , "Unit of measurement for loudness" , "Measure of noise intensity" .)

I've seen this clue in The Times.

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