Level is a synonym of Flat. Flat is related to Stage. 
Boom is a type of Noise. Noise is related to Horn. Horn is a type of Device. Device is related to Lock. 
Level <--Synonym--> Flat <--RelatedTo--> Stage 
Boom --IsA--> Noise <--RelatedTo--> Horn --IsA--> Device <--RelatedTo--> Lock 