# AP Board 9th Class Physical Science Notes Chapter 11 Sound

Students can go through AP State Board 9th Class Physical Science Notes Chapter 11 Sound to understand and remember the concept easily.

## AP State Board Syllabus 9th Class Physical Science Notes Chapter 11 Sound

→ Sound is produced by a vibrating body.

→ Sound is transmitted through a medium and received by our ears.

→ A sound is a form of energy.

→ Sound travels through disturbances in the form of waves.

→ The matter or the substance through which sound is transmitted is called the medium. If the particles of the medium vibrate along the direction of the wave, then the wave is called a longitudinal wave. Sound waves are longitudinal.

→ Longitudinal waves consist of compressions and rarefactions.

→ The region of the high density of particles in the medium during propagation of sound is called compression and low-density regions are called rarefaction.

→ The distance between two consecutive compressions or two successive rarefactions is called the wavelength of a sound wave.

→ If the particles of the medium vibrate perpendicular to the direction of the wave, then the wave is called a transverse wave.

→ The maximum variation in density or pressure from the mean value is called amplitude. The number of oscillations of the density of the medium at a place per unit time is called the frequency of the sound wave.

→ The time taken to complete one oscillation of the density of the medium is called the time period of the sound wave.

→ Frequency and time period are related as T = 1/u or o = 1/T

→ The distance by which a point on the wave, such as a compression or rarefaction travel in unit time is called the speed of the sound wave.

→ The relation between the speed of a sound wave, wavelength, and frequency is v = uA.

→ The sounds which produce pleasing effects on the ear are called musical sounds, while the sounds which produce unpleasant effects are called noises.

→ The characteristics by which one can distinguish a musical note from another are pitch, loudness, and quality.

→ Pitch is a characteristic of sound which distinguishes between a shrill sound and a deep, low sound.

→ The loudness of sound is defined as the degree of sensation produced in the ear, measured in decibels.

→ The quality of sound is the characteristic that enables us to distinguish between musical notes emitted by different musical instruments.

→ Sound reflects as light reflects.

→ A reflection of sound, arriving at the listener in more than 0.1 s after the direct sound is called an echo.

→ A reflection of sound, arriving at the listener in less than 0.1 s after the direct sound is called reverberation.

→ The human ears are able to hear sound in a frequency range of about 20 Hz to 20 kHz. Sounds of frequency less than 20 Hz are known as infrasonic sounds.

→ Sounds of frequency higher than 20 kHz are known as ultrasonic sounds.

→ Ultrasonic waves are used in drilling holes or making cuts of different shapes, cleaning, detection of defects in metals, in the medical field to imaging of organs, surgeries, etc.

→ SONAR stands for Sonographic Navigation and Ranging.

→ SONAR is equipment or a system for finding objects underwater using sound waves.
→ Mechanical energy: Energy produced by mechanical means like pushing, pulling, beating, vibrating, etc. is known as mechanical energy.

→ Tuning fork: A tuning fork is an acoustic resonator. It is a steel bar, bent in U-shape, with a handle at the bend. It resonates at a specific constant pitch when set into vibration by a string with a rubber hammer.

→ Longitudinal wave: If the particles of the medium vibrate along the direction of the wave, then the wave is called a longitudinal wave.

→ Transverse wave: If the particles of the medium vibrate perpendicular to the direction of the wave, then the wave ¡s called a transverse wave.

→ Compression: Compression is the region where density, as well as the pressure of particles in the medium, is high.

→ Rarefaction: Rarefaction is the region where density, as well as the pressure of particles in the medium, is low.

→ Crest: Crest is a point where the amplitude of a wave is maximum in a transverse wave.

→ Trough: A trough is a point where the amplitude of a wave is minimum in a transverse wave.

→ Pressure: Pressure is force per unit area applied in a direction .perpendicular to the surface of an object. The SI unit of pressure is the newton per square meter, which is called the pascal (Pa) after the seventeenth-century philosopher and scientist Blaise Pascal.

→ Wavelength: The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength of a sound wave.

→ Amplitude: The maximum variation in density or pressure from the mean value is called amplitude.
(or)
The maximum disturbance of the particles of a medium from their mean position is called amplitude.

→ Frequency: The number of oscillations of the density of the medium at a place per unit time is called frequency.

→ Pitch: Pitch is a characteristic of sound which distinguishes between a shrill sound and a deep, low sound.

→ Loudness: Loudness of sound is defined as the degree of sensation produced in the ear.

→ Quality of sound: The quality of sound is the characteristic that enables us to distinguish between musical notes emitted by different musical instruments.

→ Echo: A reflection of sound, arriving at the listener in more than 0.1 s after the direct sound is called an echo.

→ Reverberation: A reflection of sound, arriving at the listener in less than 0.1 s, after the direct sound is called reverberation.

→ Infrasonic: Sound of frequency less than 20Hz is known as infrasonic sound.

→ Sonic: Of or relating to audible sound. e.g: a sonic wave.

→ Ultrasonic: Sound of frequency greater than 20 kHz is known as ultrasonic sound.

→ SONAR: SONAR stands for Sonographic Navigation And Ranging. It is equipment or a system for finding objects underwater using sound waves.

→ Heinrich Rudolf Hertz:
Heinrich Rudolf Hertz was born on 22 February 1857 in Hamburg, Germany, and educated at the University of Berlin. He was the first to conclusively prove the existence of electromagnetic waves. He laid the foundation for the future development of radio, telephone, telegraph, and even television. He also discovered the photoelectric effect which was later explained by Albert Einstein. The SI unit of frequency was named Hertz in his honor.