Reflection of Sound

Sound is defined as oscillations or sensations evoked by oscillations in particles. Sound is the energy that travels in waves through the air, water, or another medium. Sound can be heard, and it propagates through air as a longitudinal wave. Sound can neither be created nor be destroyed but can be changed from one form to another. For instance, Sound produced by an electric bell here, electrical energy is converted into sound energy. Sound is produced due to the vibration of an object.

Reflection of Sound waves

What is Sound?

A sound is a form of energy like light energy and heat energy. Sound is a wave made of vibrations in the air. These vibrations create sound waves which move through mediums which can be solid, liquid, or gas, before reaching our ears. Sound does not travel in a vacuum. Sound, therefore, is not audible in space. Just like light, the sound gets reflected at the surface of a solid or liquid. This bouncing back of sound when it strikes any surface is the reflection of sound. 

Here are some important terms defined below to understand the reflection of sound in a better way as:

• Echo – Repetition of sound caused by the reflection of the sound wave is called an echo.
• Reflection of Sound Waves – The bouncing back of the sound wave on striking a surface such as a wall, metal sheet, plywood, etc. is called the reflection of the sound wave.
• Reverberation – If the distance is less than 17 m, then the original sound mixes with the reflected sound. Due to repeated reflections at the reflecting surface, the sound gets prolonged. This effect is known as reverberation.

Types of Waves

Elastic or mechanical waves:

The sound waves necessarily require a medium for their propagation. These waves travel in a medium through the vibrations of the medium particles about their mean positions. Hence, they are called elastic or mechanical waves

Longitudinal waves:

The vibrations of medium particles are along the direction of propagation. Compressions and rarefactions are formed in the medium

Transverse waves:

The vibrations of medium particles are perpendicular to the direction of propagation. Crests and troughs are formed in the medium

Conditions for formation of an echo:

An echo is heard only if the distance between the person producing the sound and the rigid obstacle (or reflector) is long enough to allow the reflected sound to reach the person at least 0.1 seconds after the original sound is heard

Conditions for hearing the echo distinctly:

The minimum distance in air between the source of sound and the reflector must be 17 m. The size of the reflector must be large enough as compared to the wavelength of the sound wave. The intensity of sound should be sufficient so that the reflected sound reaching the ear is audible.  

Use of echoes by bats, dolphins, and fisherman

Use of echoes by sonar (sound navigation and ranging)

Characteristics of sound

Loudness – It is the property by virtue of which a loud sound can be distinguished from a faint one, both having the same pitch and frequency. Loudness is expressed in a unit called decibel (dB). It is directly proportional to the amplitude of vibration.

Intensity – The amount of sound energy passing each second through a unit area is called the intensity of sound.

Pitch – Pitch is that characteristic of sound by which an acute (or shrill) note can be distinguished from a grave or flat note. It is not the same as frequency. It refers to the sensation as perceived by the listener.

Quality / Timbre – The quality or timbre of the sound is that characteristic that enables us to distinguish one sound from another having the same pitch and loudness, but emitted by two different instruments.

Factors affecting the loudness of sound

Loudness ∝ (Amplitude)²

Loudness ∝ 1/(Distance)²

Loudness depends on the surface area of the vibrating body. The larger the vibrating area the louder the sound heard. Loudness depends on the density of the medium. More the density of medium more is the loudness. Loudness depends on the presence of resonant bodies. The presence of resonant bodies near the vibrating bodies increase the loudness

Reflections of sound

Bouncing back of sound waves from the surface is called reflection of sound or we can say that when sound travels in a given medium it strikes the surface of another medium so that it returns in some other direction, this phenomenon is called reflection of sound.

The reflection of sound is similar to the reflection of light as it follows the laws of reflections. As a sound wave travels across a room and touches a wall, a reflective wave is produced that will reintroduce a portion of that wave back into the room.  

A polished or rough and big obstacle is necessary for the reflection of sound. The reflection of sound waves is also affected by the shape of the surface.

Incident sound waves: The sound waves that travel towards the reflecting surface are called the incident sound waves.

Reflected sound waves: Sound waves that return from the reflecting surface are called reflected sound waves.

Sound Transmission

Sound Transmission occurs with the portion of energy that can transmit through the wall. The sound wave is transferred as energy from molecule to molecule through either medium. Sound Reflection is the energy that is returned to the original medium.

Laws of Reflection of Sound

• The angle of reflection is always equal to the angle of incidence(Angle of Reflection = Angle of Incidence)
• The incident wave, the reflected wave, and the normal at the point of incidence lie on the same plane.

Applications of reflection of sound

Reflection of sound is used in many devices:

1. Echo: Echo is the repetition of a sound caused by the reflection of sound waves. Echo arrives at the listener with a delay after the direct sound. If this gap between the object is greater than 1 second, the reflection is called an echo.  Bats and Dolphins use echo to detect obstacles. Echo is also used in the SONAR (Sound Navigation And Ranging) technique used for the detection and location of objects inside the water, such as submerged submarines, icebergs.
2. Stethoscope: A stethoscope is used to hear the sounds of the internal organs of a patient.
3. Megaphone: Megaphones prevent the spreading out of sound waves by successive reflections, thus confining them to the air in the tube.
4. Soundboard: Soundboards are curved surfaces that are placed in such a way that the sound source is at the focus. The sound waves are made to reflect equally throughout the hall or an auditorium thus enhancing their quality.
5. Hearing aid: A hearing aid is a device used by people with difficulty in hearing.

Sample Problems

Problem 1: Does sound follow the same laws of reflection as light does? Explain.

Solution:

Yes, sound and light follow the same laws of reflection given below :

(a) Angle of incidence at the point of incidence = Angle of reflection.

(b) At the point of incidence, the incident sound wave, the normal and the reflected sound wave lie in the same plane.

Problem 2: How does the sound produced by a vibrating object in a medium reach your ear?

Solution:

Air is the commonest material through which sound propagates. When vibrating objects, like prongs of a tuning fork move forward, they push the molecules of the air in front of them. This in turn compresses the air, creating a region of high pressure and high density called compression. When the prongs of the tuning fork move backward, they create a region of low pressure in the air called rarefaction. This region has low pressure, low density and more volume. As the tuning fork continues to vibrate, the regions of compression in the air alternate with the regions of rarefaction. The energy of vibrating tuning fork travels outward. This energy which reaches the ears, makes the eardrums to vibrate, and thus we hear sound.

Problem 3: How does the sound produced by a musical instrument, reach your ears? Astronauts need radio transmitters to talk to each other on the moon. Why? 

Solution:

The sound produced by the musical instrument makes the molecules of air vibrate. These vibrations are carried forward by the other molecules till they reach our ear and then vibrate our eardrum to produce sound. Since sound requires a medium to propagate, therefore, sound cannot travel between astronauts on the moon, hence they use radio transmitters.

Problem 4: How the experiment of reflection of sound is different from the experiment on laws of reflection of light?

Solution:

For reflection of light we need to take a polished and smooth surface but not transparent whereas for the reflection of sound the surface may not be polished and may be transparent. For sound reflection we need to choose such materials which do not absorb sound.

Problem 5: Sound made in front of a tall building 18 m away is repeated. Name the phenomenon and briefly explain it.

Solution:

The phenomenon is known as Echo. We get echoes because of the reflection of sound. The reflected sound reaches the observer after a time 2d/v, where d is the distance of the reflecting wall from the source and v is its velocity. For the human ear, this time must be at least 0.1 second to ensure that the reflected sound does not get mixed up with the direct sound. Since the velocity of sound in air is nearly 340 ms-1, we must have 2d/340 > 0.1 or d > 17 m. Since the building is at a distance of 18 m, we can hear the reflected sound in the form of an echo clearly.

Problem 6: Explain how sound is produced by your school bell.

Solution:

When the school bell vibrates, it forces the adjacent particles in air to vibrate. This disturbance gives rise to a wave and when the bell moves forward, it pushes the air in front of it. This creates a region of high pressures known as compression. When the bell moves backwards, it creates a region of low pressure know as rarefaction. As the bell continues to move forward and backward, it produces a series of compressions and rarefactions. This makes the sound of a bell propagate through air.

Problem 7: The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?

Solution:

Frequency is defined as the number of oscillations per second. It is given by the relation.

Number of oscillations = Frequency × Total time

Given, Frequency of sound = 100 Hz

Total time = 1 min = 60 s

Number of oscillations/Vibrations = 100 × 60 = 6000

Hence, the source vibrates 6000 times in a minute, producing a frequency of 100 Hz.

Problem 8: When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sounds on a hotter day?

Solution:

An echo is heard when the time interval between the original sound and the reflected sound is at least 0.1 s. The speed of sound in a medium increases with an increase in temperature. Hence, on a hotter day, the time interval between the original sound and the reflected sound will decrease. Therefore, an echo can be heard only if the time interval between the original sound and the reflected sound is greater than 0.1 s.