CBSE Class 9 Physics, Sound

•   Sound is a mechanical energy which produces sensation of hearing. Sound is produced due to vibration of different objects.

•   Sound wave propagates as compressions & rarefactions in the medium. Sound waves are longitudinal waves.

      Characteristics of a Sound Wave

      Sound waves are produced due to variations in pressure & density of the medium.

•   Compression is the portion of the medium where a temporary increase in volume & a decrease in density takes place when a sound wave passes through the medium.

•   Rarefaction is the portion of the medium where a temporary increase in volume & consequently a decease in density takes place when sound wave passes through the medium.

•   Crest is the portion of the medium where the density (or pressure) has a value larger than its average value.

•   Trough is the portion of the medium where the density (or pressure) has a value smaller than the average value.

•   Amplitude is the magnitude of the maximum disturbance in the medium on either side of the mean position.

•   Oscillation is the change in density (or pressure) from maximum value to the minimum value and again to the maximum value.

•   Frequency enables us to know as to how many times a particular event occur in a given time.

•   Time Period is the time taken for one complete oscillation in density (or pressure) of the medium.

•   Wavelength is the distance between two consecutive compressions or two consecutive rarefactions.

      Characteristics of Sound

•   Loudness or intensity.

•   Pitch or frequency.

•   Quality or timbre.

      Reflection of Sound

•   An Echo is the phenomenon of repetition of sound of a source by reflection from an obstacle.

•   Multiple echoes are heard when sound is repeatedly reflected from a number of obstacles at suitable distance.

•   Megaphone, stethoscope ear trumpet, hearing aid etc are based on phenomenon of multiple reflection of sound.

•   Reverberation is the phenomenon of persistence of audible sound after the source has stopped emitting sound.

      Reverberation is reduced by carpeting the floor, upholstering furniture & covering the walls with some absorbing material like curtains etc.

      Range of Frequencies

•   Audible range of hearing of average human being is in the frequency range of 20Hz to 20KHz.

•   Infrasound has a frequency below 20Hz. Ultrasound has a frequency above 20 kHz.

•   Ultrasound finds application in industry, medical service & communications.

      Human Ear

•   Outer ear collects sound waves.

•   Middle ear amplifies the sound waves.

•   Inner ear converts the amplified sound energy into electrical energy & conveys to the brain as nerve impulse for interpretation.