混响室中单极子的声功率发射

POWER EMISSION OF A MONOPOLE IN REVERBERATION CHAMBERS

在本文中推出了混响室内一个简单声源的功率发射的准确公式,并用简单分析方法求得声功率发射的重要特性。其一就是声源在混响室内的平均功率并不等于其在自由声场中的功率,而是大于后者,这是前人所未料及的。还进一步推出一平滑的统计公式,后者一直可用于波长接近于室内平均自由程的低频率。由这公式可看到室中有很多功率发射峰值的点,在室内形成规则的立体网格,接近周围墙壁则为高发射区。统计公式在预测声功率发射沿与混响室某长、宽、高方向的相对变化时,基本是准确的,只有平滑时引起的误差,用于室内声场的一般估计时可能有较多误差。为说明问题,文中举了几个数字实例以比较简正波理论和统计理论的结果。混响室内声功率发射的变化影响声源的使用和测量,必须予以注意。

An exact sound power emission formula of a simple source in a reverberation chamber is derived from normal mode theory as a function of its position in the room, and important properties of the emission are found from simple analyses. For example, the average emission of the source in the room 'is not 'equal to the free-space emission but greater, on the contrary to the common notion. A smooth statistical formula is also derived for the variation of average sound power in an axial direction, which holds even at low frequency at which the wave length approaches th mean free path in the room. The sound power is found to have numerous peaks in the room forming a three-dimensional lattice structure with high power barriers around the boundary walls. The space factor in- an axial direction is essentially exact, but the general formula of power emission formed with the product of space factors in three axial directions might have some error. Numerical examples are presented to check this. The variability of sound power emission in reverberation chambers should be taken into account in the application and measurement of sound sources.