In most situations where acoustical performance is needed there should be some type of cushion underlayment under flooring. The underlayment can serve two purposes. One is noise control, the other is a micro leveling layer that can hide imperfections in the subflooring.
There are several types of underlayment. The materials vary and so do the installation requirements.
Thin foam pads do little for STC noise control, but will give the floor a slight leveling effect. These typically are very thin and can not be glued down. The foam is typically only allowed for use under floating floors with locking edges. These will add some minimal IIC (Impact) noise control, and add little if any STC (Airborne) noise control.
Rubber underlayment comes in a variety of thicknesses. Typically the thinner ones are approved for use under the LVT,and Hardwood, while the thicker versions are only allowed under wood and sometimes ceramic or stone tile type floors, depending on the tile industry approvals. The rubber gives a good vibration absorbing layer and can be loose laid or glued down. Some Rubber needs to be be glued down with a special adhesive to keep the oils in the rubber from casting through the LVT type flooring. Please research the brands, adhesives, and installation recommendation of the manufacturers before installation. These will add good IIC (Impact) noise control, and add some but minimal STC (Airborne) noise control.
statements about increased noise control of 5 IIC points. IIC is an increasingly increasing level of difficulty. An IIC 5 point improvement starting at an IIC 30 is fairly easy. The same 5 IIC point improvement starting at 50 is 20 Times harder to accomplish.