Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define sound as energy from vibrations.
• Explain how sound travels through different mediums.
• Describe the structure and function of the human ear.
• Differentiate between frequency (pitch) and amplitude (loudness).
• Identify the dangers of loud sounds and how to protect hearing.

Lesson summary

Sound is all around us, from the chirping of crickets in the Karoo to the vibrant music of kwaito booming from taxis in Johannesburg. Understanding sound allows us to appreciate the world in a richer way and also helps us to solve practical problems related to noise pollution and hearing protection, important issues facing many communities in South Africa, particularly those living near busy roads, airports, or industrial areas. This week, we will explore what sound is, how it travels, and how our ears enable us to hear. We'll also learn about the properties of sound, such as frequency and amplitude, and how these relate to pitch and loudness.

Lesson notes

What is Sound? Sound is a form of energy that is produced when objects vibrate. These vibrations create disturbances that travel through a medium (like air, water, or solids) to our ears. When these disturbances reach our ears, our brains interpret them as sound. Imagine hitting a drum – the drum skin vibrates, pushing and pulling on the air around it. These compressions and rarefactions (regions of high and low pressure) spread outwards as a sound wave.

How Sound Travels: Sound travels as a longitudinal wave. This means that the particles of the medium vibrate parallel to the direction the wave is traveling. Think of a slinky – if you push and pull one end, the compressions and rarefactions travel along the slinky.

Mediums: Sound needs a medium to travel. It cannot travel through a vacuum (like space) because there are no particles to vibrate.

Speed of Sound: The speed of sound depends on the medium. Sound travels faster in solids than in liquids, and faster in liquids than in gases. This is because the particles are closer together in solids and liquids, allowing vibrations to be transmitted more quickly. Temperature also affects the speed of sound; sound travels faster in warmer mediums.

Example: Sound travels at approximately 343 meters per second (m/s) in air at 20°C. In water, it travels at about 1480 m/s, and in steel, it travels at around 5000 m/s. Imagine a train approaching. You'll hear the rumble through the steel tracks first (if you put your ear to the track – don't do this unsafely!), then you'll hear it through the air.

Properties of Sound Waves: Frequency (Pitch): Frequency is the number of vibrations (cycles) per second, measured in Hertz (Hz). A high frequency means a high pitch (like a whistle), and a low frequency means a low pitch (like a bass drum).

Example: A mosquito buzzing has a high frequency (high pitch), while the rumble of a distant truck has a low frequency (low pitch).

Amplitude (Loudness): Amplitude is the size of the vibration. A large amplitude means a loud sound, and a small amplitude means a soft sound. Loudness is measured in decibels (dB).

Example: A whisper has a small amplitude (soft sound), while a loud rock concert has a large amplitude (loud sound).

The Human Ear: The ear is a complex organ that allows us to hear.

It consists of three main parts: Outer Ear: Collects sound waves and funnels them into the ear canal. The pinna (the visible part of the ear) helps to direct sound.

Middle Ear: Amplifies sound waves. The sound waves cause the eardrum (tympanic membrane) to vibrate. These vibrations are then passed on to three tiny bones called the ossicles (malleus, incus, and stapes), which amplify the vibrations and transmit them to the inner ear.

Inner Ear: Converts sound waves into electrical signals that the brain can interpret. The cochlea is a spiral-shaped, fluid-filled structure that contains tiny hair cells. When the vibrations reach the cochlea, they cause the fluid to move, which in turn stimulates the hair cells. These hair cells send electrical signals to the auditory nerve, which carries them to the brain.

Hearing Damage: Exposure to loud sounds can damage the hair cells in the cochlea, leading to hearing loss. This damage is often permanent. It's important to protect your hearing by avoiding loud noises and using earplugs or earmuffs when necessary. Noise pollution in many South African communities is a growing concern, particularly in areas with high traffic density, industrial activity or informal settlements close to such areas.

Worked example

Example 1: Calculating Frequency: A tuning fork vibrates 440 times in one second. What is its frequency?

Solution: Frequency is the number of vibrations per second.

Therefore, the frequency of the tuning fork is 440 Hz.

Commentary: This is a direct application of the definition of frequency.

Example 2: Comparing Speeds of Sound: A sound travels 1000 meters in air in 3 seconds and 1000 meters in water in 0.68 seconds. Calculate the speed of sound in each medium.

Solution:

Speed in air = Distance / Time = 1000 m / 3 s = 333.33 m/s (approximately)

Speed in water = Distance / Time = 1000 m / 0.68 s = 1470.59 m/s (approximately)

Commentary: This demonstrates how to calculate the speed of sound using the formula: Speed = Distance / Time. This also reinforces the concept that sound travels faster in water than in air.

Guided Practice (With Solutions)

Question 1: Explain why sound cannot travel through a vacuum.

Solution: Sound needs a medium (solid, liquid, or gas) to travel. A vacuum is a space that is empty of matter, meaning there are no particles to vibrate and transmit the sound waves.

Commentary: This question tests understanding of the fundamental requirement of a medium for sound transmission.

Question 2: A sound wave has a high frequency. How will this sound be perceived?

Solution: A high frequency sound wave will be perceived as a high-pitched sound.

Commentary: This tests the relationship between frequency and pitch.

Question 3: Describe the role of the eardrum in the process of hearing.

Solution: The eardrum (tympanic membrane) vibrates when sound waves reach it. These vibrations are then passed on to the ossicles in the middle ear, initiating the process of sound amplification and transmission to the inner ear.

Commentary: This assesses knowledge of the ear's structure and function.

Question 4: A sound is measured at 85 dB. Is this sound potentially harmful to your hearing? Explain.

Solution: Yes, prolonged exposure to sounds at 85 dB or higher can cause hearing damage. This is because loud sounds can damage the hair cells in the cochlea.

Commentary: This relates sound intensity to potential health risks.

Independent Practice (Questions Only)

List three different mediums through which sound can travel. Rank them in order of the speed of sound through each (fastest to slowest).

Explain the difference between amplitude and frequency in the context of sound waves. Give an example of a sound with high amplitude and a sound with low frequency.

Draw a simple diagram of the human ear and label the following parts: pinna, eardrum, ossicles, cochlea, auditory nerve.

Describe how the cochlea converts sound waves into electrical signals.

Why is it important to wear ear protection when attending a loud concert or working in a noisy environment? Explain in terms of sound wave properties.

A sound wave has a frequency of 20 Hz. Would you consider this a high-pitched or low-pitched sound? Would you hear it? Human hearing range is 20Hz to 20,000 Hz.

Explain how the temperature of air affects the speed of sound.

Research and compare the speed of sound in steel vs. wood. What differences do you find? Explain why.