Final answer:
The observed frequency of the siren sound at ground level will be 250 Hz.
Step-by-step explanation:
When a sound wave is emitted by a source, it spreads out in all directions. As it spreads, the wavefronts form a hemispherical shape, resulting in what is known as hemispherical divergence.
In the case of a siren operating at 500 Hz in air and at a small height above the ground, the frequency observed at ground level will be lower than the emitted frequency due to the Doppler effect. The Doppler effect occurs when the source of sound is moving relative to the observer, causing a shift in perceived frequency.
The observed frequency of a siren sound at ground level would still be the same as the frequency at which it operates, regardless of the hemispherical divergence or the height above the ground, provided there are no relative motion effects involved like the Doppler Effect.
The propagation of sound in this scenario does not affect the frequency; rather, it may affect the intensity or loudness of the sound. Therefore, if a siren operates at 500 Hz in the air and at a small height above the ground, the observed frequency at ground level would still be 500 Hz.
In this scenario, as the siren moves closer to the ground, the observed frequency will be lower than 500 Hz. Therefore, the correct answer is 250 Hz.