The Night Flying Restrictions detail which aircraft types are and aren't allowed to operate at Heathrow during the night and, for those that are, they specify the amount of Noise Quota each movement consumes (based on certificated noise values for the type in question).
That's distinct from actual noise per departure, which comes from readings from fixed monitors located at the 1000' point (6.5 km from the start of roll). That data is used to established whether or not a noise infringement has occurred.