Vibration is movement and sound that comes from objects which vibrate. For example, guitar strings, when touched, vibrate and make a sound; and drum skins, when hit, vibrate and make a sound. Vibrations are described in terms of amplitude and frequency. In the  case of a guitar, the amplitude. or loudness, is the distance the string moves; and in the case of a drum, the skin moves at a certain speed and vibrates a certain number of times each second.

If the skin, or the string, vibrates 440 times per second, then we say it has a frequency of 440 Hertz (or 440 Hz for short). If it moves faster or slower, then it  has a higher or lower frequency. The human ear cannot detect all sounds. Sounds must have a certain amplitude, and a frequency between 40 Hz and 16,000 Hz.

Vibrations above or below these will not be detected by the human ear even if they are extremely loud. Many animals have better hearing  than us. Dogs, for example, can hear higher frequencies; and bats can hear sounds with incredibly high frequencies – up to 48,000 Hz. The vibrating object first causes the molecules in the air around it to vibrate at the same frequency and amplitude. These molecules then cause other molecules to vibrate and so it continues until molecules of  air inside our ears vibrate.

Finally our eardrums vibrate and cause minute, i.e. very small, electrical signals to be sent to the brain. All sounds come from vibrations. But not all sounds are the same. Some are pleasant to hear, such as music. Others are unpleasant and these we call noise. What’s the difference between the two? This is a  difficult question to answer.

But the sounds of musical instruments, which are usually good to hear, do have a special characteristic: musical instruments, such as the guitar and the drum, vibrate at more than one frequency. Thus, when a guitar string produces the note of A, the vibration of greatest amplitude has a frequency of 440 Hz.

But there are vibrations of other frequencies present, too. They have less amplitude, and so we do not consciously hear them. But thev add to the sound and form a pattern of frequencies which is pleasant to hear. This is called harmonics. It is harmonics which help us to identify the musical instrument we hear. 35 Of course, there are other characteristics of music, too. One of these is rhythm, the sequence of sounds. Rhythm is not exclusive to musical sound; but it is one of the factors which help make music pleasant to hear.

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