Class 9 Sound

Cheatsheet Content

### Introduction to Sound - **Sound:** A form of energy that produces the sensation of hearing. - **Production:** Produced by vibrating objects. - *Examples:* Vocal cords, guitar strings, tuning fork. - **Medium:** Requires a material medium (solid, liquid, gas) for propagation. Cannot travel through a vacuum. ### Wave Nature of Sound - **Sound Wave:** A mechanical wave, meaning it requires a medium to travel. - **Longitudinal Wave:** Particles of the medium vibrate parallel to the direction of wave propagation. - **Compressions (C):** Regions of high pressure and density where particles are crowded. - **Rarefactions (R):** Regions of low pressure and density where particles are spread out. - **Transverse Wave:** (For comparison, light is a transverse wave) Particles vibrate perpendicular to the direction of wave propagation. Sound is NOT a transverse wave. ### Characteristics of Sound Waves - **Wavelength ($\lambda$):** The distance between two consecutive compressions or rarefactions. Unit: meter (m). - **Frequency ($\nu$ or $f$):** The number of oscillations (compressions + rarefactions) per unit time. Unit: Hertz (Hz). - $1\text{ Hz} = 1\text{ oscillation/second}$. - **Time Period ($T$):** The time taken for one complete oscillation. Unit: second (s). - Relationship: $T = 1/\nu$. - **Amplitude ($A$):** The maximum displacement of the particles of the medium from their mean position. Unit: meter (m). - Determines the **loudness** or **softness** of the sound. Larger amplitude = louder sound. - **Speed of Sound ($v$):** The distance a point on the wave (e.g., a compression) travels per unit time. Unit: meter/second (m/s). - Relationship: $v = \lambda \times \nu$. - Speed of sound is generally highest in solids, then liquids, then gases. - *Example:* Air ($\approx 343 \text{ m/s}$ at $20^\circ\text{C}$), Water ($\approx 1500 \text{ m/s}$), Steel ($\approx 5100 \text{ m/s}$). ### Pitch, Loudness, and Quality - **Pitch:** Determined by the **frequency** of the sound wave. - High frequency = high pitch (e.g., a child's voice, whistle). - Low frequency = low pitch (e.g., a man's voice, drum beat). - **Loudness:** Determined by the **amplitude** of the sound wave. Measured in decibels (dB). - Larger amplitude = louder sound. - Also depends on the sensitivity of the ear. - **Quality (Timbre):** That characteristic of sound which enables us to distinguish between sounds produced by different sources, even if they have the same pitch and loudness. - Due to the mixture of different frequencies (overtones) present in the sound. ### Reflection of Sound - **Echo:** The phenomenon of hearing the same sound again due to reflection from a distant obstacle. - Minimum distance for a distinct echo: $\approx 17.2 \text{ m}$ (at $20^\circ\text{C}$). - **Calculation:** $2d = v \times t$ (where $d$ is distance to obstacle, $v$ is speed of sound, $t$ is time taken for echo). - **Reverberation:** The persistence of sound in a large hall due to repeated reflections from walls, ceiling, and floor. - Too much reverberation can make sound unclear. - **Uses of Reflection:** - **Megaphone/Horn:** Directs sound in one direction. - **Stethoscope:** Sound of heart/lungs reaches doctor's ears through multiple reflections. - **Soundboards:** Curved surfaces behind the stage in auditoriums to spread sound evenly. ### Range of Hearing - **Audible Range:** The range of frequencies that human ears can detect. - Approximately $20\text{ Hz}$ to $20,000\text{ Hz}$ ($20\text{ kHz}$). - **Infrasound:** Sound waves with frequencies below $20\text{ Hz}$. - Produced by elephants, whales, earthquakes, volcanoes. - **Ultrasound:** Sound waves with frequencies above $20,000\text{ Hz}$. - Produced by bats, dolphins, dogs. ### Applications of Ultrasound - **Medical:** - **Echocardiography:** Imaging of the heart. - **Ultrasonography:** Imaging of internal organs (e.g., fetus during pregnancy). - Breaking kidney stones into fine grains. - **Industrial:** - Cleaning hard-to-reach parts of machinery, electronic components. - Detecting cracks and flaws in metal blocks. - **SONAR (Sound Navigation And Ranging):** - Uses ultrasonic waves to measure the distance, direction, and speed of underwater objects. - **Principle:** A transmitter sends ultrasonic waves, which reflect off objects and are detected by a receiver. - **Calculation:** $2d = v \times t$. ### Structure of the Human Ear - **Outer Ear (Pinna):** Collects sound waves from the surroundings. - **Middle Ear:** - **Ear Canal:** Directs sound waves to the eardrum. - **Eardrum (Tympanic Membrane):** Vibrates when sound waves strike it. - **Ossicles (Malleus, Incus, Stapes):** Three tiny bones that amplify the vibrations from the eardrum and transmit them to the inner ear. - **Inner Ear:** - **Cochlea:** Converts pressure variations into electrical signals. - **Auditory Nerve:** Transmits these electrical signals to the brain, where they are interpreted as sound.