Reflection of sound occurs when sound waves strike a hard surface and bounce back into the same medium, following the laws of reflection. This phenomenon is commonly experienced as an echo, heard when sound reflects off walls in an empty hall. Sound requires a medium to propagate and can travel through solids, liquids, and gases, but unlike light, it cannot travel through a vacuum.
An obstacle is necessary for the reflection of sound. The shape of the surface also affects the reflection of sound waves. Various terminologies are used in the reflection of sound.
1. Incident Sound Waves: The sound waves that travel toward the reflecting surface are called incident sound waves. The angle formed by the incident sound wave with the normal to the surface is called the angle of incidence.
2. Reflected Sound Waves: Sound waves that return from the reflecting surface are called reflected sound waves. The angle formed by the reflected sound wave with the normal to the surface is called the angle of reflection.
Echo
An 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 the gap between the original sound wave and the reflected sound wave is greater than 1/10th of a second, the reflection is called an Echo.
Bats and Dolphins use echoes 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 and icebergs.
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 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.
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 is prolonged. This effect is known as reverberation.
Characteristics of Sound
1. 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 the vibration.
2. Intensity
The amount of sound energy passing each second through a unit area is called the intensity of sound.
3. 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.
4. 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 Loudness of Sound
Factors affecting the loudness of the sound are,
1. Loudness ∝ (Amplitude)2
2. Loudness ∝ 1/(Distance)2
Some other factors influencing the loudness of the sound are,
- Loudness depends on the surface area of the vibrating body.
- The larger the vibrating area, the louder the sound is heard. Loudness depends on the density of the medium.
- Greater density gives better transmission. Loudness depends on the presence of resonant bodies.
- The presence of resonant bodies near the vibrating bodies increases the loudness
Learn more about Characteristics of Sound Waves
Laws of Reflection of Sound
- The angle of reflection of the sound wave is always equal to the angle of incidence of the sound wave. (Angle of Reflection = Angle of Incidence)
- For sound waves, the incident wave, the reflected wave, and the normal at the point of incidence lie on the same plane.
Applications of Reflection of Sound
1. Stethoscope
A stethoscope is a medical device that is used to hear the sounds of the internal organs of a patient. The sound coming from the heart is of low intensity, and when it passes through the pipe of the stethoscope, it undergoes multiple reflections and hence amplifies the sound to make it audible.
2. Megaphone
A megaphone is a device that is used to increase the intensity of the sound. It prevents the spreading out of sound waves by successive reflections, thus confining them to the air in the tube.
3. Soundboard
Soundboards are curved surfaces that are placed in such a way that the sound waves from the sound source are at the focus of the board, increasing the intensity of the sound at a particular point. The cinema hall and auditorium are made keeping the above concept in mind.
4. Hearing Aid
A hearing aid is a device that is used by people who are unable to hear normal sounds. A hearing aid has a microphone that allows the sound from the environment to enter, then the sound goes to the amplifier, which amplifies it with multiple reflections, and the speaker allows the sound to exit the device and enter the person's ear.