Designing Kitchens and Bathrooms - Installing air-jet tub in tub enclosure

We are in the process of building a tub enclosure for an air-jet tub. I remember seeing something a while back regarding preparing the base so the floor of the tub feels stable, not like it is rocking or feeling as though the tub was moving (this is being used as a shower as well). Does anyone know where I could get more info on this? We are concerned that standing in the tub will be an unsettling experience. Thanks so much for your response!

Many plastic/fiberglass tubs require support underneath to prevent the tub floor from moving. Instrutions provided by the manufacturer should tell what is needed. But mortar/concrete, plaster, and foam are some of the things used.