Experiment of Reflection of Sound

Introduction

Everyday there are many instances in our lives where the sound source is not directly before us, but still we hear its sound. Many times we need to call somebody from another room under the same roof, though that person is not directly before us. How does our sound reach there?

This takes place by a phenomenon called the reflection of sound and its essential laws. Let us perform a simple experiment to see what it is, and how it works.

Table of Contents

• Aim

• Theory

• Procedure

• Observations

• Result

• Precautions

Aim

To study the reflection of sound.

Apparatus Required

• A Stopwatch.

• Two 70 cm hollow cylindrical cardboard tubes.

• One 100 cm × 30 cm hard cardboard sheet.

• One plain A4 white sheet.

• 5-6 Paper pins.

Theory

According to the laws of reflection of sound,

1. The angle of incidence is equal to the angle of reflection.

2. The incident ray, the normal at the point of incidence and the reflected ray, all lie in the same plane.

Law of Reflection of Sound

In this experiment, we are verifying the laws of reflection of sound by changing the angle of incidence and noting the corresponding angle of reflection.

Procedure

Experimental Setup for the Demonstration of Reflection of Sound Waves

1. Using a drawing board and a pencil, first make a rough sketch of the apparatus, comprising of a wall in the form of a straight line (PQ), a line perpendicular to this line at a mid-point on one side (the normal, OR), and two lines each making an angle of 45° with this normal at the point of intersection of normal and wall-lines, i.e., OA and OB (consider the above figure as reference).

2. Place this sheet on the floor beside a smooth wall, such that the line depicting the wall in the sketch is parallel to the actual wall.

3. Hold the cardboard sheet above the line depicting the normal line in the sketch accordingly, so that the cardboard sheet and the wall are perpendicular to each other. One side of the cardboard sheet should be placed on the floor.

4. Now for the line of incident wave, place one cylindrical tube joining the clock and the intersection point of cardboard and wall.

5. On the side of the reflected wave in the sketch, place another cylindrical tube joining the intersection of cardboard and normal, facing you.

6. Turn on the alarm clock, so that incident sound waves emerge.

7. Put your ear towards the open end of the cylindrical tube on the other side of the clock and listen carefully.

8. Change the positions of both the tubes accordingly and note the angles using a protractor between each tube and the cardboard for the direction for which the intensity of sound appears maximum in your ear. The first angle on the clock's side is called the angle of incidence (∠i) while the one on your side is called the angle of reflection (∠r).

9. Verify if both the angles are equal.

10. Verify if both the tubes share the same plane in which they are kept.

Observations

Make a table of three columns - Sr-No, Angle Of Incidence (∠i) and Angle Of reflection (∠r).

For each case, vary ∠i by 10o and note down your observations for ∠r according to the maximum observed intensity of sound.

Observation Table

Result 

Within the experimental limit, for all values of ∠i,

1. The angle of reflection ∠r was observed to be equal to the angle of incidence ∠i. This proves the first law of reflection. 

2. Both the tubes and the sound waves share the same plane of operation. This proves the second law of reflection.

Precautions

1. Place the cardboard properly so that the direct sound coming from the watch is minimum.

2. Carefully examine the most optimum position so as to receive the maximum intensity of sound using this apparatus.

3. The sound from the source should not be too low or too high. It can cause errors in observation.

Lab Manual Questions

1. What are the requirements of a wood surface to act as a reflector of the sound waves and light waves?

Ans: A wooden surface can be polished or rough for the reflection of the sound waves, while it should be flat and well polished for the reflection of light waves.

2. Why do we prefer to use the pipes of small diameter and larger lengths in this experiment?

Ans: Here, we consider the sound wave source to be directional, and to avoid the sound waves present in the surroundings we use the pipes of smaller diameter and larger lengths.

3. In this experiment, what is the requirement of using a low amplitude sound source?

Ans: The low amplitude source travels through the hearing pipe and therefore, we can hear it through the pipe. If we use a source of larger amplitude, then we can hear the sound of waves outside the tube as well.

4. If the whole experimental setup is submerged in a viscous medium what changes will you observe?

Ans: The speed of the sound waves increase in the denser medium as compared to the rarer medium, for example, in air the speed of sound is 343 m/s, while it is 1482 m/s in water, therefore for a submerged setup in viscous medium we will hear the sound waves faster.

Viva Questions

1. How is a sound wave produced?

Ans: A sound wave is produced by the rapid vibrations of a moving body. The vibrations set the nearby particles of the material medium into to-and-fro motion, creating a disturbance wave of the medium's particles due to transfer of energy. This wave reaches the observer as a sound wave.

2. Are sound waves longitudinal or transverse?

Ans: sound waves are longitudinal in nature as the particles oscillate along the direction of propagation of the wave.

3. In which medium does the sound travel fastest?

Ans: sound travels fastest in a solid due to least inter-particle separation. This makes the transfer of energy quickest from one particle to another.

4. Can you observe the reflection of sound in a vacuum?

Ans: No, sound waves cannot travel in a vacuum.

5. Can humans perceive all frequencies of sound waves?

Ans: No, human beings can perceive sound only in the range 20 Hz - 20 kHz.

6. State the laws of reflection of sound.

Ans: According to the laws of reflection of sound, the angle of incidence is equal to the angle of reflection, and the incident ray, the normal at the point of incidence and the reflected ray, all lie in the same plane.

7. Give the characteristics of a sound wave.

Ans: sound waves have a frequency (f), a wavelength (L), a velocity (v), a time period (T) and an amplitude (A).

8. How does temperature affect the velocity of sound in a medium?

Ans: As the temperature increases, the kinetic energy of the particles of the medium increases and they vibrate with a greater frequency. Hence the energy transfer is quicker and the sound travels at a greater velocity.

9. Give one example of longitudinal waves other than sound.

Ans: Seismic waves caused by earthquakes are also longitudinal in nature.

10. How do sound absorbing materials work?

Ans: When the sound wave strikes such materials, they either get deformed or decay the vibrations of particles using their pores. In both cases, they absorb sound energy.

Practical Based Questions

1. The SI unit of frequency of sound waves is:

1. Hertz (Hz)

2. Meter (m)

3. Second (s)

4. Meter per second (m/s)

Ans: (A) The SI unit of frequency of sound waves is Hertz (Hz).

2. The human audible range of sound frequencies include:

1. 0 - 20 Hz

5. 20 - 20,000 Hz

6. Above 20,000 Hz

7. None of the above

Ans: (B) The human audible range of sound frequencies include 20 - 20,000 Hz.

3. Velocity of sound in air is about:

1. 230 m/s

2. 300 m/s

3. 320 m/s

4. 343 m/s

Ans: (D) The velocity of sound in air is about 343 m/s.

4. Which surface is best suitable for studying the reflection of sound?

1. Steel

2. Plastic

3. Plain mirror

4. Wood

Ans: (C) Plain mirror is best suitable for studying the reflection of sound due to its           even and regular surface.

5. Time taken by sound waves to reach the observer is maximum in:

1. Low density ammonia gas

2. Air

3. Oil

4. Solid

Ans: (A) It will be maximum in low density ammonia gas due to less number of  medium's particles.

6. What kind of wave is sound?

1. Regular

2. Disturbance

3. Plain

4. Spherical

Ans: (B) A sound wave is a disturbance wave.

7. The study of sound is called:

1. Optics

2. Mechanics

3. Acoustics

4. Seismology

Ans: (C) The study of sound is called Acoustics.

8. Propagation of sound is carried out by:

1. Water waves

2. Particle waves

3. Light waves

4. None of the above

Ans: (B) propagation of sound is carried out by the particle waves of the medium.

9. In which media does the sound travel fastest?

1. Air

2. Water

3. Steel

4. Benzene

Ans: (C) sound travels fastest in steel (solid).

10. Choose the option where sounds do not find applications.

1. SONAR

2. Music player

3. Medical science

4. X-rays

Ans: (D) Sound does not find application in X-rays.

Conclusion

From this article, we conclude the central notion of sound. We have covered the basics of the mechanism of sound - its properties, propagation and laws of reflection. We have also demonstrated a simple home experiment for verifying the laws of reflection of sound. These concepts will help in building a strong foundation for the reader regarding the same.