A scientist studies how fog affects the travel of different sound frequencies across an open field of length 600m. A loudspeaker emits three pure tones one at a time: 15Hz, 2kHz, and 30kHz. A microphone at the far end records the sound intensity on a clear day and on a foggy day. The observations show a drastic drop for 30kHz, a moderate drop for 2kHz, and almost no drop for 15Hz when fog is present.
(i) Categorise each emitted frequency as infrasonic, audible, or ultrasonic.
(ii) Describe briefly how the size of water droplets in fog can differentially scatter or absorb these three frequencies.
(iii) Using the nature of sound waves, justify why the low-frequency sound travels farther than the high-frequency sound in foggy conditions.
(iv) Suggest one practical situation where the ability of low-frequency sound to propagate over long distances is advantageous.