Dont know about soundproofing underlay, but an underlay makes a world of diffrence to a carpet, and carpet does make wood floors quieter.
In a rented out I was in they had not put underlay under the tread of a step between the front and rear of the house, although they rest of the house had it, and it was very noticable.
Hi i am a carpet fitter of 30yrs
no you dont need underlay but you will feel a massive diffrance with and without.
also if there is a fault on your carpet forsome reason and your carpet needs underlay and did not put it down some manufactures will not replace your carpet for that reason.
if your carpet is hessian or action back i would say put underlay down then wooden grippers must be used to get tention on yous carpet.
hope this helps.
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