Sound waves have different characteristics. Perceived pitch is determined by frequency, or by how fast an object vibrates back and forth. Frequency is the number of times that an object, or sound wave it produces, vibrates in one second. The higher the frequency of an object, the higher the pitch. The normal human ear can hear sounds with frequencies between 20 and 20,000 vibrations per second. Each vibration is considered to be one Hertz.
The amount of energy flowing in the sound waves is referred to as the intensity of sound. The loudness of the sound is based on the strength of the sensation received by the eardrum and sent to the brain. The same intensity of sound may produce different degrees of loudness for different people. Intensity and loudness of a sound depend on four factors: (1) how far the distance is from the source of the sound (especially in outdoor situations), (2) the amplitude of the vibration, (3) how dense the medium is through which the sound travels, and (4) the area of the vibrating object.
The intensity and loudness of a direct sound decrease as the distance increases between a person and the source of the sound. This happens because sound waves move out from their source in all directions. The energy flowing in the sound waves spreads over a greater area and decreases the farther away the sound travels.
The amplitude of vibration is the distance that a vibrating object moves as it vibrates. The larger the amplitude of vibration of a sounding body, the louder and more intense the sound. The amplitude of a sound wave is the degree of motion of air molecules within the wave, which correspond to the extent of rarefaction and compression that accompanies the wave.
In air, the forward movement of vibrating objects pushes molecules together. This is called compression. When the vibrating object moves back in the opposite direction, the air is separated, causing the molecules to move farther apart. This is called rarefaction. The amplitude of a sound wave can be expressed in terms of absolute units by measuring the actual distance of displacement of the air molecules, or the pressure differential in the compression and rarefaction, or energy involved.
One way of explaining this phenomenon is to use the tuning fork as an example. As we know the vibrating prongs of a tuning fork produce sound. As the sound of the tuning fork move forward in one direction, it compresses the air molecules in front of it. Then the tuning fork swings in the opposite direction, and the space that it just occupied is nearly empty of air molecules. The surrounding air molecules begin to crowd into the partly empty space, but the tuning fork, swinging forward again, compresses them once more. This process of compressing and rarefying the air around the tuning fork continues as long as the tuning fork vibrates.
The compressed air molecules are pushed against those that are a little farther away from the tuning fork. This push, or impulse, moves farther and farther outward, compressing air molecules as it travels. A space of rarefied air follows each compression. Thus, the vibrating tuning fork sends a continual series of alternating compressions and rarefactions through the air. Each pair of compressions and rarefactions makes up one sound wavelength.