NCERT Class 9 • Science • Chapter "Sound" | NCERT Class 8 • Science • Chapter "Sound"

Sound and Its Properties

Sound is produced by a vibrating body and travels as a wave through a material medium. It cannot travel through vacuum, which is why no sound reaches us from outer space, and this single fact separates sound from light straight away.

Nature and Characteristics

• Sound travels as a longitudinal wave, in which particles vibrate along the direction of the wave through compressions and rarefactions.
• Frequency is the number of vibrations per second and is measured in hertz; it decides the pitch of the sound.
• A high frequency gives a shrill high pitched sound and a low frequency a deep low pitched one.
• Amplitude is the maximum displacement of a particle and decides the loudness, which is measured in decibel.
• Quality or timbre is what lets us tell two instruments apart even when they play the same note at the same loudness.
• The speed of sound is greatest in solids, less in liquids and least in gases, because particles are closer in solids.
• Speed of sound in air increases with temperature and humidity.

Echo, Reverberation and Ultrasound

• An echo is the repetition of sound caused by reflection from a distant surface.
• To hear a distinct echo the reflecting surface must be far enough that the reflected sound arrives after about 0.1 second.
• Reverberation is the persistence of sound in a hall due to repeated reflection, and it is reduced with curtains, carpets and rough surfaces.
• Audible range for a normal human ear is about 20 hertz to 20000 hertz.
• Infrasound is below that range and ultrasound above it.
• Ultrasound is used to clean parts, detect cracks in metal, and in medical imaging such as sonography and echocardiography.
• Sonar uses ultrasound to find the depth of the sea and to locate underwater objects.

Echo — the repetition of a sound caused by reflection from a distant surface, heard after about one tenth of a second.