Joseph Culp, age 14, of Dover, Del., for his question:

IS THERE A DIFFERENCE BETWEEN LOUDNESS AND INTENSITY OF SOUND?

When talking about sound, many people use the words "loudness" and "intensity" as if they mean the same thing. But the words definitely have different meanings. The "intensity" of sound refers to the amount of energy flowing in the sound waves. The "loudness" of sound is the apparent 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. As an example, one person may barely hear a sound that seems perfectly clear to another.

For any one person, both the intensity and loudness of the sound depend on four factors: the distance from the source of the sound; the amplitude of vibration; the density of the medium through which the sound travels; and the area of the vibrating object.

The intensity and loudness of a 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. It requires more energy for a violin string to have a large amplitude of vibration than a small amplitude. The larger the amplitude of vibration, the louder and more intense the sound.

The loudness and intensity of the sound decreases as the density of the medium decreases. A sound is much louder under water than in air because water is denser than air.

The area or size of a vibrating object also affects the loudness and intensity of a sound. You can prove this by hitting a table fork so that its prongs vibrate. Then quickly put the handle of the fork against the top of a table. The loudness of the sound made by the vibrating fork immediately increases because the fork makes the table top vibrate. This increases the area of vibration.

Pitch of a sound affects the loudness but not the intensity. The sensitivity of the ear varies with the pitch of a sound. The ear has low sensitivity to low pitches. But its sensitivity increases as the pitch increases, until the pitch reaches about 1,000 vibrations a second. After this point, its sensitivity to sound decreases again.

Each sound we hear is caused when something moves back and forth in quivering motions called vibrations. These vibrations travel through the air to our ears and we hear them as sounds.

The air can carry many vibrations at the same time and the vibrations can travel in many different directions. You may sit in a room and hear a radio playing, a bird singing, a friend talking and an airplane passing overhead, all at the same time.

Pitch is determined by how rapidly an object vibrates or by its frequency. Frequency is the number of times an object, or the sound wave it produces, vibrates in a second. The higher the frequency, the higher the pitch.