Vibration of an object is what produces sound waves. The vibrating object moves in one direction and compresses the air directly in front of it. As the vibrating object moves in the opposite direction, the pressure on the air is lessened so that an expansion, or rarefaction, of air molecules occurs. One compression and one rarefaction make up one longitudinal wave. The vibrating air molecules move back and forth parallel to the direction of motion of the wave receiving energy from adjacent molecules nearer the source and passing the energy to adjacent molecules farther from the source. Pitch is determined by the frequency of the tone that the ear receives. High notes are produced by an object that is vibrating a greater number of times per second than for a low note. The loudness of a sound depends upon the subjective effect of intensity of sound waves on the ear. In general, more intense sounds are louder, but the ear does not respond similarly at all frequencies. Two tones of the same intensity but with different pitches may then appear to have different loudness.