There are lots of different underlays but they generally fall into 3 categories of “PU Foam”, “Felt” and “Rubber” underlay.

The best underlay for the carpet depends on the type of carpet which is being fitted.

Most carpets will have either PU Foam (which is eco friendly) or Rubber underlay, both are good for sound proofing.

If it is a Wilton or similar the best would be a Felt underlay so the carpet can “bed in” over time.