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.
The UMNR must be brought to the airport by a parent/guardian. Delta recommends arriving at the airport at least 90 minutes prior to departure time for domestic flights and 2 hours prior to departure time for international flights. Please keep in mind, some airports have specific cut off times for check-in.