What Is Multiple Reflection Of Sound

Reflection Of Sound

  • Sound waves like light waves also get reflected from plane and spherical surfaces. During reflection, sound waves obey the laws of reflections. The reflection of sound from a hard surface can be observed by performing a simple experiment on the equipment as shown in fig.
    Reflection-of-sound-from-hard-surface
  • Sound waves have much longer wavelength than the light waves. Therefore unlike light waves, sound waves do not need smooth surfaces for suffering reflection. That is why, a brick wall, a wooden board, a row of trees, a hill etc. serve as the reflectors of sound waves.
  • To have an appreciable reflection of sound waves from any surface, it should have dimensions equal or larger than the wavelength of the sound waves falling on it. That is, a smaller object will not reflect the sound waves of larger wavelength.

REVERBERATION
The repeated reflection that results in the persistence of sound in a large hall is called reverberation.
reverberation
Excessive reverberation in any auditorium/hall is not desirable because the sound becomes blurred and distorted. The reverberation can be minimised/reduced by covering the ceiling and walls with sound absorbing materials, such as, fiber-board, rough plaster, draperies, perforted cardboard sheets etc.

Practical Applications Of Multiple Reflection Of Sound

Some simple devices based on multiple reflection of sound are,

  1. Stethoscope
  2. Megaphone, Loudhailer, Horns
  3. Trumpet, Shehanais
  4. Curved ceiling of concert hall/conference hall/cinema hall
  5. Soundboards

Stethoscope: Stethoscope is a medical diagnostic instrument based on multiple reflection of sound waves. This is used by doctors for listening to the sounds produced inside the body, particularly in the heart or lungs.
Stethoscope

Megaphone: Megaphone is a horn-shaped tube. Megaphones are used for addressing a small group of people.
Speaking tube is a hollow tube– one end is the speaker’s end, whereas the other one is the listener’s end.
In these devices, the sound waves suffer reflection repeatedly and the energy of the waves remains confined to the tube. The sound waves are prevented from spreading out.
Megaphone
It is for this very reason that loudspeakers also have horn-shaped opening.

Ear Trumpet (or Hearing Aid): Ear trumpet or hearing aid is used by the persons who are hard of hearing. The sound waves received by the wide end of the trumpet are reflected into a much narrower area, leading it to the ear. This increases amplitude of the vibrating air inside the ear and helps in improving hearing.

Sound Boards and Curved Ceiling and Walls in Large Halls: The arched ceiling and walls of large halls or auditorium often reflect the sound waves. These reflected sound waves interfere with the words of the speaker. This problem is solved by hanging curtains, putting up screens or by using sound boards. A sound board is often a concave rigid surface. The speaker is located at the focus of the sound board placed behind the speaker. The sound board the reflected sound waves parallel. This enable the sound to reach large distances.
Sound-Boards-and-Curved-Ceiling

The sound board prevents the spreading out of the sound waves in different directions.

Analysing Reflection of Waves

Analysing Reflection of Waves

  1. Figure shows the reflection of sea waves by a retaining wall built near the beach.
    Analysing Reflection of Waves 1
  2. Reflection of waves is a phenomenon that occurs when all or a part of the waves return after they encounter an obstacle. The obstacle is known as a reflector.
  3. A ripple tank kit is a very useful apparatus for studying waves in the laboratory. Figure shows a ripple tank in use.
    Analysing Reflection of Waves 2(a) When a steady train of waves move across the water surface, each wave crest acts like a convex lens and concentrates the light on the screen as shown in Figure. Flence, a bright pattern on the screen corresponds to a crest of the waves.
    Analysing Reflection of Waves 3(b) A stroboscope is normally used to observe wave patterns on the screen. By rotating the stroboscope at the same frequency as the motor of the ripple tank kit, the motion of the waves can be frozen for more detailed observation.

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Characteristics of Reflected Waves

Figure shows incident waves being reflected by a reflector. The angle of incidence, i is the same as that of the angle of reflection, r.

  • Direction: The direction of the propagation of the waves changed after reflection.
  • Angle of reflection: The angle of reflection, r is equal to reflection the angle of incidence, i.
  • Wavelength: The wavelength of the reflected waves is the same as that of the incident waves.
  • Frequency: The frequency of the reflected waves is the same as that of the incident waves.
  • Speed: The speed of the reflected waves is the same as that of the incident waves.

Analysing Reflection of Waves 4

Reflection of Plane Waves Experiment

Aim: To study the characteristics of reflection of plane waves in a ripple tank, light and sound waves.
Material: White paper as screen
Apparatus: Ripple tank and its accessories, stroboscope, protractor
Method:

Analysing Reflection of Waves 5

  1. The ripple tank is arranged as shown in Figure.
  2. The legs of the ripple tank are adjusted so that the base of the tank is horizontal.
  3. A horizontal beam vibrator is lowered until it touches the surface of the water to create plane (straight) waves.
  4. The speed of the motor is adjusted to produce a train of waves that can be clearly observed on the screen with the help of a stroboscope.
  5. A straight reflecting barrier (reflector) is placed on the tank so that the angle of incidence of the waves, i = 30°.
    Analysing Reflection of Waves 6
  6. The angle of the reflected waves, r is measured and recorded.
  7. The wavelengths of the incident and reflected waves are observed and compared.
  8. Steps 5 to 7 are repeated for i = 45° and 60°.

Results:

Analysing Reflection of Waves 7

Observation:
The wavelength of the reflected waves is the same as the incident waves.
Discussion:

  1. From the results, the angle of reflection of the waves is equal to the angle of incidence of the waves.
  2. The wavelength of the waves does not change after reflection.
  3. The frequency of the incident and reflected waves are the same as that of the source of the waves.
  4. From 2 and 3, and by using the formula v = fλ, the speed of the reflected waves is the same as that of the incident waves.

Reflection of Light Experiment

Aim: To study the characteristics of reflection of light.
Materials: A piece of white paper, plasticine, pencil Apparatus: Plane mirror, ray box, power supply, protractor
Method:

Analysing Reflection of Waves 8

  1. The apparatus is set up as shown in Figure.
  2. The activity is carried out to determine the angle of reflection, r corresponding to the angle of incidence, i = 30°, 45° and 60°.

Results:Analysing Reflection of Waves 9Discussion:
From the results, the angle of reflection of the light, r is equal to the angle of incidence of the light, i.

Reflection of Sound Waves Experiment

Aim: To study the characteristics of reflection of sound waves.
Materials: Round cardboard tubes, a piece of soft board, a piece of plywood, a piece of white paper
Apparatus: Protractor, mechanical stopwatch as source of sound
Method:

Analysing Reflection of Waves 10

  1. The apparatus is set up as shown in Figure.
  2. A stopwatch is placed in a cardboard tube with one end open and the other end closed.
  3. The tube is arranged at an incident angle, i = 30°.
  4. Another cardboard tube with both ends open is adjusted so that one ear is placed close to one end of the tube as to hear the loudest possible ticking sound of the stopwatch.
  5. The angle of reflection, r of the sound wave is measured and recorded.
  6. Steps 3 to 5 are repeated for i = 45° and 60°.

Results:
Analysing Reflection of Waves 11Discussion:
From the results, the angle of reflection, r is equal to the angle of incidence, i of the sound waves.
Conclusions:

  1. The direction of propagation of the reflected waves is different from that of the incident waves.
  2. The angle of reflection, r is equal to the angle of incidence, i of the waves.