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Class 9, Science, Chapter-11, Lecture-5, Ultrasound (Notes)

SONIC BOOM:

The loud sound produced due to shock waves generated by objects moving with supersonic speed is termed as sonic boom. 

Light Travels Faster than Sound.

  • Man hears the sound of thunder only after he sees the flash of lightning. This is because light travels at a very great speed as compared to the speed of sound, we see the flash before the sound.
  • The flash of a gunshot reaches us before the sound of the gunshot. This is because light travels at a very great speed as compared to the speed of sound, we see the flash before the sound.

REFLECTION OF SOUND: 

The bouncing back of sound when it strikes the surface of a solid or liquid is called the reflection of sound.
Hard surfaces are good reflectors of sound while soft surfaces are bad reflectors of sound. 

Laws of reflection of sound:

  1. The incident sound wave, the reflected sound wave, and the normal at the point of incidence all lie in the same plane.
  2. The angle of incidence of a sound wave is equal to the angle of reflection of the sound wave.

USE OF REFLECTION OF SOUND:

  1. Loudspeaker, Megaphone, bulb horn:
    Megaphones, horns, and musical instruments like trumpets, shehnais etc. are designed to send sound by multiple reflections in a particular direction without spreading in all directions
  2. Stethoscope:
    Doctors listen to sounds from the human body through a stethoscope. The sound of a heartbeat reaches the doctor's ears through multiple reflections.
  3. Soundboard:
    Generally, the ceilings of cinema halls and auditoriums are curved so that sound after multiple reflections reaches all parts of the hall.

ECHO:

The sound returning towards the source after getting reflected from a distant hard surface is termed as an echo.
An echo is heard clearly if the time interval between the original sound and the reflected one(echo) is at least 0.1 seconds.
An echo is heard clearly if the distance between the source and the reflector is at least 17.2 m.

REVERBERATION: 

Persistence of sound wave for a long time because of repeated (multiple) reflections of sound are called reverberation.
Sound becomes too blurred and distorted to be heard in big concert halls because of reverberation
Reverberation can be prevented by stopping the reflection of sound by using soft sound absorbent materials, such as curtains, plant fibre, compressed fireboard, carpets. These materials absorb undesired reflected sound and reduce reverberation

TYPES OF SOUND BASED ON FREQUENCY:

  • Audible Range of sound 
    The sound between frequencies of 20 Hz to 20,000 Hz is the audible sound.
  • Infrasound or Infrasonic sound 
    Sound, below the frequency of 20Hz is called infrasonic or infrasound. Infrasound is produced because of very slow vibration. 
    Example: Simple pendulum, Earthquakes. 
    Human beings cannot hear infrasound. 
    Many animals such as whales, elephants, rhinoceros, etc. can produce and hear infrasound.
  • Ultrasound or Ultrasonic Sound 
    Sound, above the frequency of 20000 Hz is called ultrasound. 
    Example: Volcanic eruptions also produce ultrasound 
    Humans cannot hear the ultrasonic sound. 
    Animals such as dogs, cats, bats, monkeys, and deer can hear ultrasound. 
    Human children can also hear ultrasound.

USES OF ULTRASOUND:

  • Detection of any deformities (flaws, cracks) in metal blocks or sheets.
  • Ultrasound is generally used to clean parts located in hard-to-reach places like spiral tubes.
  • Ultrasound is used by bats to find their prey. Bats produce high-pitched ultrasonic squeaks. These high-pitched squeaks are reflected by objects such as prey and returned to the bat’s ear. This allows a bat to know the distance of his prey.
  • Dolphins use ultrasound to find fish and to detect sharks that may attack them.
  • Ultrasound may be employed to break stones formed in the kidneys into fine grains.
  • An ultrasound scanner is used to detect problems in the internal organs.
  • Ultrasonography is the process of getting pictures of parts of the body by using multiple reflections.
  • Echocardiography- is the use of ultrasound to detect the activities of the heart.
  • Ultrasonography is used to study the development of a foetus during pregnancy to detect any abnormalities.