The frequency of a sound wave

Imagine once again a guitar string that has been plucked and is vibrating back and forth.The string will be oscillating at a certain frequency,that is, it will vibrate back and forth a certain number of times per second,producing a new wave each time.'Frequency' is defined here as the number of oscillations per second.A high frequency vibration produces high frequency sound waves.An object vibrating more slowly produces lower frequency sound waves.Frequency is measured in 'Hertz'.One Hertz is simply one oscillation per second.A guitar string vibrating back and forth 150 times each second is said to be vibrating at 150 Hertz (abbreviated '150Hz').The letter 'f'is commonly used to denote frequency.The frequency of a sound wave determines the pitch (the particular note) that we hear. A high frequency sound wave is heard by the human ear as a high pitch,or a high note, while a low frequency wave is heard as a low pitched sound.This relationship between frequency and pitch is actually readily observable by the human eye.If you look inside a piano or look carefully at a guitar while it is played, you will be able to see that the larger,fatter strings vibrate more slowly.These low frequency oscillations produce the low notes.The small,thin strings vibrate much more rapidly and produce the high notes.These waves travel through the air from the instrument to the listener's ear.Note that there is not any mass of air that travels from the source to the listener's ear;it is simply the disturbance of the air that travels.These waves enter our ears, causing the ear drum to vibrate, producing the sensation of hearing. Just as the sound waves are produced at a certain frequency,they enter the ear at a certain frequency.If you are listening to a guitar string that has been plucked,the frequency at which the string oscillates will be the same as the frequency of the sound wave that enters your ear,and you will hear the corresponding note.A vibration of about 256 Hz produces the note Middle C.A vibration at twice this frequency will produce a note exactly one octave higher.A vibration of about 440 Hz will produce the note A. A vibration of 880 Hz will produce an A one octavehigher.In general,doubling the frequency of the vibration increases the pitch by one octave.The human hear can detect sounds within a certain requency range.The lowest sound a human can hear is around 20 Hz.The highest sound the ear can detect is usually around 20,000 Hz.The upper limit,however,usually decreases as a person ages.While young people can hear sounds as high as 20,000 Hz,older people may only be able to hear sounds as high as 15,000 or even 10,000 Hz.Some animals can hear sounds that are out of the range of human hearing.Dogs and bats,for example,can hear ultrasonic sounds, sounds to high for the human ear to detect.Some birds are believed to be able to hear infrasonic sounds,which are below the range of human hearing.