The frequency of a sound wave is called pitch. Hz are called ultrasonic waves. The amplitude or volume of a sound wave is the amount of pressure exerted by a sound source to air molecules. The higher the pressure, the harder the molecules will collide and the farther the wave will travel. Scientists measure the amplitude in atmospheres.
Humans can detect from less than a billionth of an atmosphere to values one million times higher.
However, it is hard to deal with this huge range of different values. Instead, the pressure is measured by the intensity of the sound. The sound produced by the source is always the same, however, John is receiving more oscillations per second because the source is moving toward him. The sound that reaches John's ear has a higher pitch than the sound that reaches Bob's ear. This effect is called Doppler effect.