摘要
The classical normal-mode theory expresses the steady-state sound field in an enclosure produced by a sound source as a series of normal modes of vibration. Experimental facts are not often explained by this theory, and it was conjectured that the normal-mode expression is not the complete solution of the wave equation in the enclosure, but only the reverberant part of it, and there should be an additional term representing the direct spherical radiation to make the solution complete. The problem is examined by critically reviewing the derivation of the normal-mode expression, and by theoretical analysis of the steady-state sound field in the room and experimental measurements therein. The conjecture is thus confirmed, and it is definitely shown that the sound field should contain the direct wave as well as the standing waves (normal modes) formed by the confinement of the boundary surfaces. Relevant mathematical expressions are derived.
The classical normal-mode theory expresses the steady-state sound field in an enclosure produced by a sound source as a series of normal modes of vibration. Experimental facts are not often explained by this theory, and it was conjectured that the normal-mode expression is not the complete solution of the wave equation in the enclosure, but only the reverberant part of it, and there should be an additional term representing the direct spherical radiation to make the solution complete. The problem is examined by critically reviewing the derivation of the normal-mode expression, and by theoretical analysis of the steady-state sound field in the room and experimental measurements therein. The conjecture is thus confirmed, and it is definitely shown that the sound field should contain the direct wave as well as the standing waves (normal modes) formed by the confinement of the boundary surfaces. Relevant mathematical expressions are derived.
