During a lunar construction project, an alarm siren is mounted on the open surface of the Moon, which is essentially a vacuum. A pressurised habitat dome nearby contains air for astronauts.
(i) Explain why an astronaut working inside the habitat cannot hear the siren when it sounds on the lunar surface outside.
(ii) Engineers install a sealed, straight helium-filled tube of length 17m that connects the siren to the interior of the habitat. The siren emits a steady frequency of 500Hz. Given that the speed of sound in helium is 1000ms−1 and in air is 340ms−1, calculate the wavelength of this sound in (a) helium, (b) air.
(iii) Which property of the sound wave—frequency, wavelength, or speed—remains unchanged as it passes from helium into air? State the reason.
(iv) Assuming the sound travels only through the helium-filled tube and then through the air inside the habitat, determine the minimum time after the siren is activated for the first sound to reach the astronaut’s ear.