Physics - Sound Part I



  • The to and fro or back and forth motion of an object is known as vibration. Therefore, when a tightly stretched band is plucked, it vibrates and when it vibrates, it produces sound.

  • In some cases, vibration can be easily observed, but in most of the cases, their amplitude is so small that it is very difficult to see them with naked eye; however, their vibration can be easily felt in the form of sound. E.g. Tabla, Harmonium, Flute, Sitar, etc.

  • In human beings, the sound is produced by the larynx (also known as voice box).

  • One can feel the vibration by keeping fingers on the throat; this is the part that is known as the voice box.

Sound Produced By Human Beings

  • Two vocal cords (as shown in the given image), are stretched across the voice box (or larynx) in such a way that it leaves a narrow slit between them for the passage of air; this is how sound is produced.

  • The vocal cords in men are about 20 mm long.

  • The vocal cords in women are about 15 mm long and children’s vocal cords are even more shorter; this is the reason that men, women, and children have different voice.

Voice Box

Human Ears

  • The part through which we hear is known as ear.

  • The shape of the outer part of the ear is similar to a funnel; therefore, when sound enters in it, it keeps going down through a canal to the end. At the end, there is a thin membrane stretched tightly; it is known as the eardrum.

  • The eardrum is very similar to a stretched rubber sheet and sound vibrations make the eardrum vibrate.

  • The eardrum sends vibrations to the inner ear and from there, the signal goes to the brain; this is how we hear the sound clearly.

Frequency of A Vibration

  • The vibration motion is known as oscillatory motion.

  • The number of oscillations per second is known the frequency of oscillation and the frequency is expressed in hertz (Hz).

  • Amplitude and frequency are the two significant features of any sound.

  • The loudness of sound depends on its amplitude; if amplitude is higher, then the sound is louder and if the amplitude is lesser, then the sound is feeble.

  • The loudness of sound is expressed in a unit and it is expressed in decibel (dB).

  • The following table illustrates the loudness of sound generated from various sources −

Source of Sound Loudness of Sound
Normal breathing 10 dB
Soft whisper (at 5m) 30 dB
Normal conversation 60 dB
Busy traffic 70 dB
Average factory 80 dB
  • The frequency determines the pitch or shrillness of the sound; therefore, if the frequency of vibration is higher, then the sound has a higher pitch and shrillness is higher and vice versa.

  • The frequencies of sound less than about 20 vibrations per second (i.e. 20 Hz) cannot be perceived by the human ear.

  • The frequencies of sound higher than about 20,000 vibrations per second (i.e. 20 kHz) cannot be perceived by the human ear.

  • For a human ear, the range of audible frequencies roughly range between 20 and 20,000 Hz.

  • Some of the animals can hear the sounds of frequencies higher than 20,000 Hz, e.g. dogs.

Noise and Pollution

  • Unpleasant sounds are called noise.

  • Presence of excessive or annoying sounds in the environment is called noise pollution.

  • Noise pollution may cause many health related problems. Such as Lack of sleep, hypertension (high blood pressure), anxiety, impairment of hearing, etc.

  • Plantation on the roadside and other places especially in the city region and industrial area can effectively reduce the noise pollution.