Sound deadening and insulation, whether purpose-made or simply walls/trees/incidental stuff. Fifty people in a 200sq/ft space will almost always louder than twenty-five people in two 100sq/ft spaces connected by a doorway.

Unless the space has amazing purpose-built acoustic qualities, put physics obstructions between groups of people (walls, doors, bushes, trees, fences, whatever).

If a house-party is unbearably loud, there's just too many people for the space, or there's some anomaly that is concentrating too many people in one area.