An Overview of Experiment For Class 9 Physics 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,
The angle of incidence is equal to the angle of reflection.
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
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).
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.
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.
Now for the line of incident wave, place one cylindrical tube joining the clock and the intersection point of cardboard and wall.
On the side of the reflected wave in the sketch, place another cylindrical tube joining the intersection of cardboard and normal, facing you.
Turn on the alarm clock, so that incident sound waves emerge.
Put your ear towards the open end of the cylindrical tube on the other side of the clock and listen carefully.
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).
Verify if both the angles are equal.
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,
The angle of reflection ∠r was observed to be equal to the angle of incidence ∠i. This proves the first law of reflection.
Both the tubes and the sound waves share the same plane of operation. This proves the second law of reflection.
Precautions
Place the cardboard properly so that the direct sound coming from the watch is minimum.
Carefully examine the most optimum position so as to receive the maximum intensity of sound using this apparatus.
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:
Hertz (Hz)
Meter (m)
Second (s)
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:
0 - 20 Hz
20 - 20,000 Hz
Above 20,000 Hz
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:
230 m/s
300 m/s
320 m/s
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?
Steel
Plastic
Plain mirror
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:
Low density ammonia gas
Air
Oil
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?
Regular
Disturbance
Plain
Spherical
Ans: (B) A sound wave is a disturbance wave.
7. The study of sound is called:
Optics
Mechanics
Acoustics
Seismology
Ans: (C) The study of sound is called Acoustics.
8. Propagation of sound is carried out by:
Water waves
Particle waves
Light waves
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?
Air
Water
Steel
Benzene
Ans: (C) sound travels fastest in steel (solid).
10. Choose the option where sounds do not find applications.
SONAR
Music player
Medical science
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.
FAQs on Experiment For Class 9 Physics Reflection Of Sound
1. What are the most important types of questions to prepare from the Class 9 Physics chapter on Sound for the 2025-26 exams?
For the Class 9 Sound chapter, you should focus on several types of questions to score well. Based on the CBSE pattern, important questions include:
- Numerical Problems: Questions based on calculating echo distance, frequency, and wavelength.
- Difference-based Questions: Such as the difference between echo and reverberation, or longitudinal and transverse waves.
- Application-based Questions: Especially on the uses of ultrasound and SONAR.
- Diagram-based Questions: The structure and working of the human ear is a frequently asked 5-mark question.
- Definition-based Questions: Key terms like amplitude, frequency, and time period are often asked for 1 mark.
2. State the laws of reflection of sound. How can these laws be verified experimentally?
The two laws of reflection of sound are:
1. The angle of incidence of the sound wave is equal to the angle of reflection of the sound wave.
2. The incident sound wave, the reflected sound wave, and the normal to the reflecting surface at the point of incidence all lie in the same plane.
These laws can be verified using two identical long tubes. One tube is used to direct sound waves from a source (like a clock) onto a hard surface (like a drawing board). The other tube is positioned to capture the reflected sound. By adjusting the angles and measuring them with a protractor, you can show that the angle of incidence equals the angle of reflection, and all components are on the same plane.
3. What is the fundamental difference between an echo and reverberation? Why is it important to control reverberation in large halls?
An echo is a single, distinct reflection of a sound wave that is heard after the original sound has ceased. It requires a minimum distance between the source and the reflector. In contrast, reverberation is the persistence of sound in an enclosed space due to multiple, continuous reflections. The reflections overlap and blend together.
Controlling reverberation in large spaces like concert halls or auditoriums is crucial because excessive reverberation makes sound blurry and unclear. It causes different sounds to merge, reducing the clarity of speech and music. Sound-absorbent materials like carpets, curtains, and special ceiling tiles are used to minimise these multiple reflections.
4. A bat emits an ultrasonic sound which is reflected by a prey and is received by the bat 0.5 seconds later. If the speed of sound in air is 340 m/s, how far is the prey from the bat?
This is a typical numerical problem based on the reflection of sound. Here's how to solve it:
Given:
Time taken to hear the reflection (t) = 0.5 s
Speed of sound in air (v) = 340 m/s
Let the distance between the bat and the prey be 'd'. The sound travels a total distance of 2d (to the prey and back).
We know that, Distance = Speed × Time
2d = v × t
2d = 340 m/s × 0.5 s
2d = 170 m
d = 170 / 2
d = 85 m
Therefore, the prey is 85 metres away from the bat.
5. List three important applications of ultrasound and explain the principle behind its use in medical diagnostics.
Three important applications of ultrasound are:
- Medical Diagnostics: Used in sonography or ultrasonography to create images of internal organs like the heart (echocardiography), foetus, and kidneys.
- Industrial Cleaning: High-frequency ultrasound waves are used to dislodge dirt and grease from intricate parts that are hard to reach.
- Detecting Flaws: Used to detect cracks and flaws in metal blocks or structures without damaging them.
6. Why are sound waves unable to travel through a vacuum, while light waves can? How is this an important conceptual question for exams?
This is a Higher Order Thinking Skills (HOTS) question. Sound waves are mechanical waves, which means they need a material medium (like air, water, or solids) to propagate. They travel by causing the particles of the medium to vibrate and pass the energy along. In a vacuum, there are no particles to vibrate, so sound cannot travel.
On the other hand, light waves are electromagnetic waves. They are composed of oscillating electric and magnetic fields that do not require a medium for propagation. They can travel through the vacuum of space, which is why we can see the sun and stars. This distinction is crucial for exams as it tests the fundamental understanding of wave types.
7. What is SONAR and what is its primary application? Explain why ultrasound is used in this technology instead of audible sound.
SONAR stands for SOund NAvigation and Ranging. It is a technology that uses sound propagation to navigate, communicate with or detect objects on or under the surface of the water, such as submarines, shipwrecks, and schools of fish. Its primary application is to measure the depth of the sea (bathymetry).
Ultrasound is used in SONAR for several important reasons:
- High Energy and Directionality: Ultrasonic waves have high frequency and energy, allowing them to travel long distances in water without significant loss of strength. They can also be confined to a narrow beam, improving accuracy.
- Inaudibility: Since the frequency is above the human hearing range, it does not cause noise pollution for the crew on the ship.
- Better Resolution: The short wavelength of ultrasound provides better resolution for detecting smaller objects.
8. For a 5-mark question, how should you describe the working of the human ear?
To answer a 5-mark question on the human ear for the CBSE Class 9 exam, you should include a labelled diagram and explain the process step-by-step:
1. Outer Ear (Pinna): It collects sound waves from the surroundings and directs them into the auditory canal.
2. Middle Ear: At the end of the canal, the sound waves strike the eardrum (tympanic membrane), causing it to vibrate. These vibrations are amplified by three tiny bones: the hammer, anvil, and stirrup.
3. Inner Ear: The amplified vibrations are transmitted to the cochlea. The fluid inside the cochlea moves, stimulating tiny hair cells.
4. Signal to Brain: These hair cells convert the sound vibrations into electrical signals.
5. Interpretation: The auditory nerve carries these electrical signals to the brain, which interprets them as sound.
A simple, well-labelled diagram is essential to score full marks.
9. Why is the minimum distance required to hear a distinct echo approximately 17.2 metres? Explain the concept behind it.
This is a frequent conceptual question. The ability to hear a distinct echo depends on a phenomenon called the persistence of hearing. The sensation of any sound persists in our brain for about 0.1 seconds. Therefore, for a reflected sound to be perceived as a separate echo, it must reach our ear at least 0.1 seconds after the original sound was produced.
We can calculate the minimum distance using this time interval:
- Time (t) = 0.1 s
- Approximate speed of sound in air (v) = 344 m/s
2d = Speed × Time = 344 m/s × 0.1 s = 34.4 m
d = 34.4 / 2 = 17.2 m
Therefore, the minimum distance to the reflecting surface must be 17.2 metres for a distinct echo to be heard.
