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Laying second hand carpet

I have sourced good 2nd hand carpet and need a carpet layer to size and lay the carpet for 1 bedroom approx 4.5m x 5.0m and a small hallway approx 1.5 m x .7m. This is an upstairs apartment. I would be interested to know if the old carpet could remain as an underlay, it has been commercially cleaned. The neighbour below complains of noise and I figure this would help if it were possible.