12. Sound | connecting-the-dots

1. Introduction to Sound and Wave Propagation

Sound Production: Sound is produced due to the vibration of different objects.
Medium: The matter or substance through which sound is transmitted is called a medium, which can be solid, liquid, or gas (e.g., air, water, metals).
Wave Motion: A wave is a disturbance that moves through a medium when the particles of the medium set neighboring particles into motion. The particles of the medium oscillate back and forth about their position of rest but do not move forward; the disturbance (energy) is carried forward.
Sound in Vacuum: Sound cannot travel in a vacuum as it is a mechanical wave and requires a material medium to propagate.

Feature	Longitudinal Wave	Transverse Wave
Medium Requirement	Needs a medium for propagation.	May or may not need a medium for propagation.
Particle Movement	Particles of the medium move in a direction parallel to the direction of propagation of the disturbance.	Particles of the medium move in a direction perpendicular to the direction of propagation of the disturbance.
Example	Sound wave.	Light wave, seismic wave.

Sound travels as successive regions of high pressure and density called Compressions (C), and regions of low pressure and density called Rarefactions (R).
- Crest: The peak of a wave, representing the region of maximum compression (high density/pressure).
- Trough: The valley of a wave, representing the region of maximum rarefaction (low density/pressure).

A sound wave can be described by its frequency, amplitude, and speed.

Characteristic	Definition	Determined by	Unit
Wavelength ( $\lambda$ )	The distance between two consecutive compressions (C) or two consecutive rarefactions (R).	–	Metre (m)
Frequency ( $\nu$ )	The number of complete oscillations (changes in density from maximum to minimum and back to maximum) per unit time.	Pitch	Hertz (Hz)
Time Period (T)	The time taken for one complete oscillation in the density of the medium (or the time taken by two consecutive C or R to cross a fixed point).	–	Second (s)
Amplitude (A)	The magnitude of the maximum disturbance in the medium on either side of the mean value.	Loudness or Softness of sound.	Metre (m) (or unit of density/pressure)
Speed (v)	The distance travelled by a point on a wave (C or R) per unit time.	Nature and temperature of the medium.	Metre per second (m/s)

Relationship between Speed, Wavelength, and Frequency: $\text{Speed } (v) = \text{Wavelength } (\lambda) \times \text{Frequency } (\nu) \quad \text{since } \nu = 1/T$
Pitch: How the brain interprets the frequency of an emitted sound. The faster the vibration (higher the frequency), the higher the pitch. A bird makes a high-pitched sound, while a lion makes a low-pitched roar.
- In short, a bird's small vocal cords can vibrate many times per second (high frequency $\rightarrow$ high pitch), while a lion's larger, heavier vocal cords vibrate fewer times per second (low frequency $\rightarrow$ low pitch)
Loudness: Determined by the amplitude of the wave. A sound wave with a larger amplitude is louder. Loudness is expressed in decibels (dB).
Quality/Timbre: The characteristic that enables us to distinguish one sound from another having the same pitch and loudness.

Law of Reflection of Sound: The directions of incident and reflected sound make equal angles with the normal to the reflecting surface at the point of incidence, and the three lie in the same plane.
Echo: The distinct sound heard after reflection of sound from a large surface. For a distinct echo, the distance of the reflecting surface from the source should be more than 17.2 m (based on a minimum time interval of 0.1 s).
Reverberation: The persistence of sound in an auditorium due to the repeated reflections of sound.
Applications of Reflection of Sound:
- SONAR (Sound Navigation and Ranging): Ships use this technique to locate the depth, distance, direction, and speed of underwater objects.
- Ceilings of concert halls are curved so that sound reaches all corners after reflection.

The ear is a sensitive device that converts pressure variations into electrical signals that travel to the brain via the auditory nerve.

Outer Ear:
- Pinna: Collects sound from the surroundings.
- Auditory Canal: Sound waves pass through this canal.
- Tympanic Membrane (Eardrum): Receives the vibrations of sound. It is pushed inward by compressions (high pressure) and moves outward by rarefactions (low pressure).
Middle Ear:
- Consists of three small bones: hammer, anvil, and stirrup.
- These bones receive the eardrum's vibrations and amplify them before passing them to the inner ear.
Inner Ear:
- Consists of the cochlea and auditory nerve.
- The cochlea turns the amplified pressure variations into electrical signals.
- The auditory nerve sends these electrical signals to the brain, where they are interpreted as sound.

Audible Range: The range of frequencies a human ear can hear is roughly from 20 Hz to 20,000 Hz (20 kHz).
Inaudible Sounds: Sounds with frequencies less than 20 Hz (infrasound) and greater than 20 kHz (ultrasound).
Ultrasound Applications:
- Used to detect cracks and flaws in metal blocks.
- Used in Echocardiography (reflects waves from the heart to form an image).
- Used in Ultrasonography (to detect organ images or abnormalities).
- Used by bats and porpoises for navigation and locating prey.

Tone: A sound of a single frequency.
Note: Sound produced due to a mixture of several frequencies that is pleasant to listen to.
Noise: Unpleasant sound, often a mixture of several frequencies.
Noise Pollution: Unwanted, unpleasant sound in the surroundings that may cause harmful hearing impairment, high blood pressure, and heart attack.

Sonic Boom: Caused when an object attains a supersonic speed (speed greater than the speed of sound), creating shock waves that result in a large change in air pressure.
Speed of Sound and Temperature: The speed of sound increases as the temperature increases (e.g., sound travels faster in summer than in winter).