Reflection of Sound: Shaping Auditory Perception

The phenomenon of sound reflection plays a crucial role in our auditory experience. When sound waves encounter a surface and bounce back, it’s known as reflection. This occurrence is fundamental in understanding acoustics, shaping the way we perceive sound in diverse environments, from concert halls to everyday surroundings.

Reflection of Sound Fundamentals: Following Laws, Shaping Acoustic Experience

• Follows Law of Reflection:  Sound bounces off a solid or a liquid like a rubber ball bounces off a wall. 
  • Like light, sound gets reflected at the surface of a solid or liquid and follows the laws of reflection. 
  • The directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three are in the same plane. 
  • An obstacle of large size which may be polished or rough is needed for the reflection of sound waves

Echo: Reflection of Sound at a Distance

• Meaning: An echo is a phenomenon that occurs when sound waves bounce off a surface and return to the listener after travelling a certain distance. 
  • It is essentially the reflection of sound. 
• Minimum Distance: It is required for a distinct echo to be heard is approximately 17 metres. 
  • This is because sound travels at about 343 metres per second in air, and it takes some time for the sound wave to travel to the surface and back. 
  • To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s. 
• Echoes may be heard more than once due to successive or multiple reflections. 
  • Examples: The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and the land. 
  • Sonar (Sound Navigation and Ranging) technology is based on the principle of echo.  

Reverberation: Prolonged Sound in Enclosed Spaces

• Meaning: Reverberation is the persistence of sound in an enclosed space after the original sound source has stopped producing sound. 
  • It occurs due to multiple reflections of sound waves from the walls, floor, and ceiling of a room.
  • A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. 
  • The repeated reflection that results in this persistence of sound is called reverberation. 
• To reduce unwanted reverberation, the roof and walls of the auditorium or halls are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies. 
• The seat materials are also selected on the basis of their sound absorbing properties. 

Uses of Multiple Reflection of Sound: Directing Sound Through Reflection

• Megaphones or loudhailers, horns, musical instruments such as trumpets and shehnais, are all designed to send sound in a particular direction without spreading it in all directions. 
• Stethoscope is a medical instrument used for listening to sounds produced within the body, mainly in the heart or lungs.
• The sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflections of sound.
• The ceilings of concert halls, conference halls and cinema halls are mostly curved so that sound after reflection reaches all corners of the hall. 

Hearing Spectrum: Audible and Inaudible Sound Frequencies

• Sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. 
  • Such sounds are called inaudible. 
  • On the higher side, sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are also not audible to the human ear.
  • Thus, for the human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz.
• Children under the age of five and some animals, such as dogs can hear up to 25 kHz (1 kHz = 1000 Hz). 
  • As people grow older their ears become less sensitive to higher frequencies. 
• Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound. Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. 
  • Whales and elephants produce sound in the infrasound range.
• Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.
  • Ultrasound is produced by animals such as dolphins, bats and porpoises.