The most impactful ways to decrease the sound resonance in a room would be to concentrate on a few significant points. The most impactful and easiest change is the source of the sound that is having resonance. A more suitable location not orthogonally facing a wall will decrease echo. Sound absorption is paramount to creating a space that does not meaningfully reflect sound waves in a way that causes resonance. Focus on corners and large flat walls to prevent resonance. The material and shape can distort the sound waves enough to prevent any audible artifacts from the resonance.