The behavior of a sound wave is affected by the density of the medium, the temperature of the medium and the pressure of the medium if it is a gas. Motion of the medium independent of the sound wave may also affect the way sound propagates.
Yes, you may check-in for your Delta Air Lines (DL) flight with your mobile device starting 24 hours and ending 60 minutes before the scheduled departure time (depending on your departure city and destination) to receive a mobile boarding pass on your mobile device.
Airline Policy. Always arrive at the ticket counter by the airline's check-in deadline. Airlines differ slightly with their required check-in times for domestic flights. American Airlines requires that you check in 60 minutes before the flight if you are traveling without baggage and 90 minutes if you have luggage. Delta Air Lines and United Airlines require a 45- to 60-minute check-in, depending on which airport you are flying from, for travelers with bags.