Thompson Stephen C, LoPresti Janice L
Applied Research Laboratory, The Pennsylvania State University, PO Box 30, State College, Pennsylvania 16804, USA.
J Acoust Soc Am. 2008 Mar;123(3):1364-70. doi: 10.1121/1.2832314.
Expressions are derived for the acoustical impedance of a rectangular enclosure and of a finite annular cylindrical enclosure. The derivation is valid throughout the frequency range in which all dimensions of the enclosure are much less than the wavelength. The results are valid throughout the range from adiabatic to isothermal conditions in the enclosure. The effect is equivalent to placing an additional, frequency-dependent complex impedance in parallel with the adiabatic compliance. As the thermal boundary layer grows to fill the cavity, the reactive part of the impedance varies smoothly from the adiabatic value to the isothermal value. In some microphones, this change in cavity stiffness is sufficient to modify the sensitivity. The resistive part of the additional cavity impedance varies as the inverse square root of frequency at high frequencies where the boundary layer has not grown to fill the enclosure. The thermal modification gives rise to a thermal noise whose spectral density varies asymptotically as l/f(3/2) above the isothermal transition frequency.
推导了矩形封闭空间和有限环形圆柱封闭空间的声阻抗表达式。该推导在封闭空间所有尺寸远小于波长的整个频率范围内均有效。结果在封闭空间从绝热到等温条件的整个范围内均有效。其效果相当于在绝热柔顺性上并联一个额外的、与频率有关的复阻抗。随着热边界层增长以填满腔体，阻抗的电抗部分从绝热值平滑地变化到等温值。在一些麦克风中，腔体刚度的这种变化足以改变灵敏度。在边界层尚未增长到填满封闭空间的高频情况下，额外腔体阻抗的电阻部分随频率的平方根倒数而变化。热修正会产生一种热噪声，其频谱密度在等温转变频率以上渐近地随(1/f^{(3/2)})变化。
