Experiments you can run yourself.
Mechanical waves: how wavelength, frequency and wave speed are linked (v = λf) and what happens at a constant speed. Interactive simulation.
The received frequency rises as the source approaches and falls as it recedes: f′=f·v_sound/(v_sound±v_z). That is why the pitch of a passing ambulance falls after it goes by.
On a string fixed at both ends, standing waves form with frequencies f_n=n·v/(2L), with nodes and antinodes at fixed positions. Shorter string → higher pitch.
Waves in phase reinforce each other; in antiphase they cancel.
The dB scale is logarithmic: 10× the intensity is +10 dB.