A macrocellular foam is described having improved cell size and Fire-test-response Characteristics, among other features, which is obtained by selecting a particle size less than 1 micron for the flame retardant adjuvant. The inventors found that the amount of fire retardant adjuvant can be increased for a given foam cell size or the foam cell size can be increased for a given amount of fire retardant adjuvant, allowing the production of foams having exceptionally large, well-formed, cells that have excellent Fire-test-response Characteristics. The benefits are especially noteworthy in relation to thermoplastic foams and inorganic flame retardation adjuvants, due to the unexpected reduction in the nucleation effect of the adjuvant. The foams are useful for improving the acoustic performance of products that are required to meet certain Fire-test-response Characteristics. It may be used in automotive and other transportation devices, building and construction, household and garden appliances, power tool and appliance and electrical supply housing, connectors, and aircraft as acoustic systems for sound absorption and insulation.