The lowest frequency is called the fundamental . The next frequency is called the second harmonic . Longitudinal wave means the vibration is in the direction of the wave motion, i.e. back and forth instead of perpendicular, such as, up and down. The velocity of the wave depends on the medium and not on the frequency. The higher the temperature the higher is the velocity.
Sound waves have energy and are measured in decibels.
Every 10-decibel change the energy levels by a factor of 10. Whenever the source or the observer has a velocity, the frequency changes. Remember the velocity depends on the medium. This change of wavelength depends on the velocity.