Question

When sound comes from a moving source, the sound waves in front of the source are
compressed, causing a shift toward higher frequency
compressed, causing a shift toward lower frequency
expanded, causing a shift toward higher frequency
expanded, causing a shift toward lower frequency

Answer 1

here you go. this will answer your question. this is called the Doppler Shift. http://www.physicsclassroom.com/class/waves/Lesson-3/The-Doppler-Effect