17.7
声音强度
声源的响度与声源的振动能量有关，振动能量会使传播介质的分子产生振动。 为了测量声源的响度，须关注的物理量是强度。 这被定义为垂直于声波传播方向的每单位面积每单位时间发射的能量。 由于如果声源振动时间较长且面积较大，则总能量较大，因此将发射的能量除以时间和面积变量即可得出声源的固有响度。
声源每秒通常…
声音的响度与人耳对其产生的感知有关。它取决于声源振动的剧烈程度。
响度由一个物理量——强度来描述。声源的强度定义为声源在单位面积上垂直于传播方向所发射的功率。
通常情况下，功率随时间波动。因此，强度被定义为单位垂直面积上的平均功率。
功率的国际单位制单位是瓦特，面积的国际单位制单位是平方米。因此，强度的国际单位制单位是瓦特每平方米。
如果声源向各个方向均匀地发射声波，则与传播方向垂直的面积即为包围声源的球体表面积。因此，声强随距离的平方成反比衰减。
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Q1: What is sound intensity and how is it measured?
Sound intensity is the average power emitted by a sound source per unit area perpendicular to the direction of propagation. It is measured in watts per square meter, the SI unit derived from power in watts and area in square meters. Intensity quantifies the loudness of a sound source based on how energetically the source vibrates.
Q2: Why does sound intensity decrease with distance from the source?
When a sound source emits waves uniformly in all directions, the sound spreads over an increasingly larger spherical area. Since intensity is power divided by area, and the sphere's surface area increases with the square of the distance, intensity falls off as the square of the distance from the source.
Q3: How is sound intensity related to pressure amplitude?
Sound intensity is proportional to the square of the pressure amplitude creating oscillations in the medium. Since oscillation amplitude is proportional to pressure amplitude, and intensity increases with oscillation amplitude, the relationship between intensity and pressure amplitude is quadratic. This connection is explored in intensity and pressure of sound waves.
Q4: Why is average power used instead of instantaneous power in the intensity definition?
Sound sources oscillate hundreds or thousands of times per second, but the human ear perceives only the time average of these rapid oscillations. Using average power accounts for what humans actually hear rather than the fluctuating instantaneous values, making the intensity definition physically meaningful for perception.
Q5: What factors determine a sound source's intrinsic loudness?
A sound source's intrinsic loudness depends on how energetically it vibrates and the pressure amplitude it generates in the propagation medium. The intensity definition divides emitted energy by time and area to isolate the source's intrinsic loudness from factors like duration of vibration or size of the radiating surface.
Q6: How does the medium's particle vibration affect sound intensity?
Sound propagates through vibration of the medium's particles, so intensity increases with the pressure amplitude driving these oscillations. The greater the pressure amplitude, the larger the oscillation amplitude of medium constituents, and consequently the higher the sound intensity produced by the source.
Q7: What is the relationship between oscillation amplitude and sound intensity?
Sound intensity is proportional to the square of the oscillation amplitude of the medium's constituent particles. This quadratic relationship means that doubling the oscillation amplitude increases intensity by a factor of four, making amplitude a critical factor in determining how loud a sound appears.