How would you expect the directivity of the piston to vary as the temperature of air decreases? (Hint: Low temperature -> decrease of sound speed)
1.Directivity of the piston is independent of the temperature.
2.Directivity of the piston becomes smooth (low directivity) because the wavelength increases.
3.Directivity of the piston becomes sharp (high directivity) because the wavelength decreases.
To determine how the directivity of the piston would vary as the temperature of air decreases, we need to understand the relationship between temperature and sound speed.
When the temperature decreases, the speed of sound in air also decreases. This is because the speed of sound is directly proportional to the square root of temperature. Therefore, at lower temperatures, the speed of sound is lower.
Now, let's consider how the directivity of the piston is affected by the speed of sound. Directivity refers to the concentration of sound energy in a particular direction. It is influenced by the wavelength of the sound.
When the speed of sound decreases (as the temperature decreases), the wavelength of the sound increases. This is because the speed of sound is inversely proportional to the frequency of the sound, and wavelength is inversely proportional to frequency. Therefore, at lower temperatures, the wavelength of the sound is longer.
Based on this information, we can determine the expected variation in directivity:
Option 1: "Directivity of the piston is independent of the temperature" is incorrect. The temperature affects the speed of sound and, subsequently, the wavelength of the sound.
Option 2: "Directivity of the piston becomes smooth (low directivity) because the wavelength increases" is incorrect. When the wavelength increases, the directivity of the piston actually becomes sharper (higher directivity) because the sound waves tend to concentrate in a narrower beam.
Option 3: "Directivity of the piston becomes sharp (high directivity) because the wavelength decreases" is the correct answer. As the temperature decreases and the speed of sound decreases, the wavelength of the sound decreases. This results in a more focused and concentrated sound beam, leading to higher directivity.
In summary, as the temperature of air decreases, the directivity of the piston would become sharper (higher directivity) because the wavelength of the sound decreases.