To inexpensively provide energy conversion fibers which are excellent in lightweightness and spacing saving characteristics and have high sound absorbability in a specific frequency region, more particularly a low-frequency region below 1,000 Hz, and near the frequencies of conversation sounds as interior materials of for example, automobile, etc., an acoustic material consisting of such energy conversion fibers, a sound insulation structure and an interior material for vehicles.

Inorganic powder and granular materials 4 consisting of a piezoelectric substance as represented by, for example, barium titanate (BaTiO3) and lead titanate zirconate (PZT) are incorporated into a portion or the whole of the fibers essentially consisting of a thermoplastic resin and gaps 6 are formed between the inorganic powder and granular materials 4 and the thermoplastic resin 3 around the same and the inorganic powder and granular materials are fixed in the state of coming into partial contact with the thermoplastic resin 3 around the same.