Lesson Notes By Weeks and Term v4 - SHS 2
WAVES
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WAVES
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Subject: Physics
Class: SHS 2
Term: 1st Term
Week: 18
Grade code: 2.2.2.LI.2
Strand code: 2
Sub-strand code: 2
Content standard code: 2.2.2.CS.2
Indicator code: 2.2.2.LI.2
Theme: ENERGY
Subtheme: WAVES
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Sound is a fundamental part of our daily lives in Ghana. From the vibrant sounds of the market at Makola, the rhythm of Adowa or Kpanlogo drumming, to the horns of tro-tros and the preaching from a nearby church, we are surrounded by sound. However, the world of sound is much wider than what our ears can perceive. Just like there are colours of light we cannot see, there are sounds we cannot hear. Today, we will explore this invisible world of sound by learning how to classify sound waves based on their frequency. Understanding this helps us appreciate technologies like medical scans and how animals like elephants and bats communicate.
A. What is Sound and Frequency? Sound is a form of energy that travels as a mechanical wave. This means it needs a medium (like air, water, or a solid) to travel through. It is caused by vibrations. When an object vibrates, it causes the particles of the medium around it to vibrate and pass the vibration along. Frequency is the most important property for classifying sound. It is defined as the number of complete vibrations or cycles a wave makes in one second. Think of it like tapping a drum. If you tap it slowly, the frequency is low. If you tap it very fast, the frequency is high. The S.I. unit for frequency is the Hertz (Hz). 1 Hertz (Hz) = 1 vibration per second. A higher frequency corresponds to a higher-pitched sound (like a whistle), while a lower frequency corresponds to a lower-pitched sound (like a big bass drum). B. The Human Hearing Range: The Audible Spectrum
The human ear is an amazing organ, but it has limits. It cannot hear all possible sound frequencies. The range of frequencies that a healthy young person can typically hear is from 20 Hz to 20,000 Hz. This range is known as the Audible Range or the Audio-sonic Range. Frequencies within this range are called audible sounds or sonic sounds. As people get older, their ability to hear high-frequency sounds often decreases.
We can now classify all sound waves into three categories based on this human hearing range. C. The Three Classifications of Sound Infrasonic Sound (Infrasound) Definition: These are sound waves with frequencies below 20 Hz. The prefix "infra-" means "below". Characteristics: Humans cannot hear them. They have very long wavelengths. They can travel over very long distances and can pass through obstacles more easily than higher-frequency sounds. We sometimes *feel* them as rumbles or vibrations rather than hear them. Examples and Uses: Natural Sources: Earthquakes, volcanic eruptions, avalanches, and large ocean waves produce infrasound. Animal Communication: Elephants (like those in Mole National Park) and whales use infrasound to communicate over many kilometres. Technology: Scientists use infrasound detectors to monitor for earthquakes and nuclear explosions. Audible (Audio-sonic) Sound Definition: These are sound waves with frequencies between 20 Hz and 20,000 Hz (or 20 kHz, where 'k' means kilo or 1000). Characteristics: This is the range of sound that humans can hear. It covers everything from the lowest rumble of a bass guitar to the highest-pitched whistle. Examples and Uses: Human Speech: Typically falls between 85 Hz and 255 Hz. Music: All the sounds from instruments in Highlife, Afrobeats, or Gospel music fall within this range. Everyday Sounds: A dog barking, a car engine, rain falling on a roof. Ultrasonic Sound (Ultrasound) Definition: These are sound waves with frequencies above 20,000 Hz (20 kHz). The prefix "ultra-" means "beyond". Characteristics: Humans cannot hear them. They have very short wavelengths. They are very directional (travel in a straight beam) and can be used to create detailed images. Examples and Uses: Animal Communication & Navigation (Echolocation): Bats and dolphins produce ultrasound and listen for the echoes to navigate and find prey. This is how a bat can catch a mosquito in the dark. Medical Imaging: Ultrasound machines are used in hospitals (like Korle Bu or Komfo Anokye) to create images of internal body organs, such as checking on an unborn baby (sonogram), the heart (echocardiogram), or kidneys. This is safe because it does not use harmful radiation. Industrial Cleaning: High-frequency ultrasound is used to clean delicate items like jewellery and electronic components. Dog Whistles: These whistles produce a sound above 20,000 Hz, so dogs can hear it but humans cannot. Summary Table
| Category | Frequency Range | Can Humans Hear It? | Example(s) | | :--- | :--- | :--- | :--- | | Infrasonic | Below 20 Hz | No | Earthquake waves, elephant communication | | Audible | 20 Hz – 20,000 Hz | Yes | Human speech, music, a car horn | | Ultrasonic | Above 20,000 Hz | No | Bat echolocation, medical ultrasound scan |