17.7
Sound Intensity
The loudness of a sound source is related to how energetically the source is vibrating, consequently making the molecules of the propagation medium vi…
The loudness of a sound is related to the perception it creates on the human ear. It depends upon how energetically the source of sound is vibrating.
The loudness is described by a physical quantity, the intensity. A source's intensity is defined as the power emitted by the source per unit area perpendicular to the direction of propagation.
Generally, the power fluctuates with time. Hence, the intensity is defined as the average power per unit perpendicular area.
The SI unit of power is watt and that of area is meter squared. Hence, the SI unit of intensity is watt per meter squared.
If the source emits the sound waves uniformly in all directions, the area perpendicular to the propagation is that of a sphere covering the source. Hence, the intensity falls off as the square of the distance.
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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.