Ch 7 · 3.4 Sound · O Level Physics 5054
Topic 7 of 7 in Waves — O Level Physics 5054 — 80 lessons.
Introduction
Every sound you hear, from a whisper to thunder, starts with something vibrating: your vocal cords, a guitar string, a loudspeaker cone. The vibration pushes on the air, and a wave of pressure travels to your ear. Bats and ships use sound too high for us to hear to find their way. (Syllabus section 3.4.)
Key ideas — How sound is made and travels
Sound is produced by vibrating objects. A vibrating loudspeaker cone moves forwards and backwards, pushing the air particles in front of it.
🧑🏫 Teacher's explanation
Imagine a long, tightly stretched Slinky spring. If you push one end forward and pull it back quickly, you see a pulse of coils being pushed together and then pulled apart. This pulse travels down the spring, but notice that the individual coils aren't traveling to the other end; they are just moving back and forth. This is exactly how sound works.
📖 Definitions
🔍 Worked example — Easy
A student uses an echo to find the distance to a large wall. The sound takes 0.080 s to travel to the wall and back. Calculate the speed of sound in air, assuming the speed is constant. (Use v = 2d/t)
🔍 Worked example — Standard
A ship uses sonar to detect the seabed. A sound pulse is sent from the ship and the echo is received 1.2 s later. If the speed of sound in water is 1500 m/s, calculate the depth of the sea.
🔍 Worked example — Exam-style
An ultrasound machine is used for medical scanning. If the frequency of the ultrasound is 40 kHz, how many oscillations does the wave complete in 0.001 s?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the speed of sound in air. [2] (b) State the type of wave sound is. [1] [3 marks]
Key ideas — Sound needs a medium
Sound is passed on by particles colliding with their neighbours, so it needs a medium (a solid, liquid or gas). It cannot travel through a vacuum, where there are no particles. This is why space is silent.
🧑🏫 Teacher's explanation
Think of sound as a 'domino effect' of collisions. Imagine a long line of people standing close together. If the person at the front pushes the person behind them, that push travels down the line. The people don't move from their spots, but the 'push' (the energy) moves through them. This is exactly how sound works in a medium.
📖 Definitions
🔍 Worked example — Easy
A sound wave travels through air at 340 m/s. If the frequency of the sound is 500 Hz, calculate the wavelength λ.
🔍 Worked example — Standard
A ship uses sonar to detect the seabed. A sound pulse is sent from the ship, reflects off the seabed, and returns to the ship after 4.0 s. If the speed of sound in water is 1500 m/s, calculate the depth of the seabed.
🔍 Worked example — Exam-style
A student performs an experiment to find the speed of sound. They stand 170 m from a large wall and clap their hands. They record the time between the clap and the echo as 0.98 s. Calculate the speed of sound in air.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Hearing range
Humans can hear frequencies from about 20 Hz to 20 000 Hz (20 kHz). The upper limit falls with age.
🧑🏫 Teacher's explanation
Think of sound as a series of invisible 'shoves' traveling through a crowd. When an object vibrates, it pushes the air molecules next to it, creating a region where molecules are packed tightly together (a compression) and a region where they are spread out (a rarefaction). This pattern of pushing and pulling travels through the medium as a wave.
📖 Definitions
🔍 Worked example — Easy
A tuning fork vibrates at a frequency of 440 Hz. Is this sound within the human audible range?
🔍 Worked example — Standard
A medical ultrasound machine uses a transducer with a frequency of 5.0 MHz. Calculate the frequency in Hz.
🔍 Worked example — Exam-style
A sonar system sends an ultrasonic pulse from a ship. The pulse hits the seabed and returns to the ship after 0.80 seconds. If the speed of sound in water is 1500 m/s, calculate the depth of the sea.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Loudness, pitch and quality
An oscilloscope connected to a microphone turns sound into a trace on a screen.
🧑🏫 Teacher's explanation
Think of sound as a series of physical pushes and pulls. When you pluck a guitar string, it vibrates back and forth. This movement pushes the air molecules next to it, creating a 'crowd' of molecules (a compression) followed by a 'gap' (a rarefaction). This pulse travels through the air to your ear.
📖 Definitions
🔍 Worked example — Easy
A sound wave is displayed on an oscilloscope. The peak-to-peak displacement of the wave is 4.0 mm. If the amplitude is doubled, what is the new amplitude?
🔍 Worked example — Standard
A tuning fork vibrates at a frequency of 440 Hz. If the frequency is increased to 880 Hz, describe the change in pitch and the change in the number of waves seen on an oscilloscope screen (assuming the time scale remains constant).
🔍 Worked example — Exam-style
An oscilloscope shows two sound waves. Wave A has an amplitude of 2.0 mm and a frequency of 500 Hz. Wave B has an amplitude of 4.0 mm and a frequency of 500 Hz. Compare their loudness and pitch.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe how the student could use the oscilloscope to determine if the tuning fork is producing a higher pitch or a lower pitch. [2] (b) The student notices that a violin and a flute produce the same note at the same volume. Explain why the oscilloscope traces for the violin and the flute look different. [2] [4 marks]
Key ideas — Echoes and reflection of sound
An echo is a reflection of sound from a hard surface such as a wall or a cliff. Hard, flat surfaces reflect sound well; soft surfaces (curtains, carpets, foam) absorb it.
🧑🏫 Teacher's explanation
Imagine you are shouting into a deep canyon. You hear your own voice coming back to you a split second later. That 'repetition' is an echo. It happens because sound waves are like bouncy balls; when they hit a hard, flat surface like a cliff face, they bounce back toward you instead of passing through or being soaked up.
📖 Definitions
🔍 Worked example — Easy
A person stands 170 m away from a large, hard cliff face. If the speed of sound in air is 340 m/s, calculate the time taken for the echo to be heard.
🔍 Worked example — Standard
A ship uses sonar to detect the depth of the ocean floor. A pulse of ultrasound is sent downwards and returns to the ship after 0.80 s. If the speed of sound in seawater is 1500 m/s, calculate the depth of the ocean at that point.
🔍 Worked example — Exam-style
An echo is heard 0.065 s after a clap is made. The speed of sound is 340 m/s. Calculate the distance to the reflecting surface to 2 significant figures.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — The speed of sound
🧑🏫 Teacher's explanation
Imagine a long, coiled spring (a Slinky) stretched out on a table. If you push one end forward and pull it back quickly, you see a pulse of tightly packed coils moving down the spring, followed by a section where the coils are spread apart. This is exactly how sound moves. Sound is a longitudinal wave, meaning the particles of the medium move back and forth in the same direction that the wave travels.
📖 Definitions
🔍 Worked example — Easy
A ship uses sonar to detect the depth of the ocean floor. A sound pulse is sent from the ship, hits the seabed, and returns to the ship. If the total time taken for the echo to return is 0.80 s and the speed of sound in water is 1500 m/s, calculate the depth of the ocean.
🔍 Worked example — Standard
A student measures the speed of sound in air using an echo method. They stand 170 m from a large wall. The time between emitting a clap and hearing the echo is 1.0 s. Calculate the speed of sound.
🔍 Worked example — Exam-style
An ultrasound machine is used for medical scanning. If the ultrasound wave has a frequency of 4.0× 10^6 Hz and a wavelength of 0.375 mm, calculate the speed of the wave in human tissue.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Ultrasound
Ultrasound is sound with a frequency higher than 20 kHz, too high for humans to hear.
🧑🏫 Teacher's explanation
Imagine you are standing in a large, empty hall and you clap your hands. A moment later, you hear the sound again. That 'ghost' of the sound is an echo. Ultrasound works on this exact same principle, but instead of using your ears, we use electronic sensors to detect waves that are too 'fast' (high frequency) for humans to perceive.
📖 Definitions
🔍 Worked example — Easy
A ship sends an ultrasound pulse to the seabed. The echo returns after 0.80 s. Calculate the depth of the sea if the speed of sound in water is 1500 m/s.
🔍 Worked example — Standard
An ultrasound machine is used for medical scanning. A pulse is sent into the body and an echo is received 0.0004 s later. If the speed of sound in human tissue is 1540 m/s, calculate the distance to the organ being scanned.
🔍 Worked example — Exam-style
A sonar system detects a shoal of fish. The pulse travels at 1530 m/s and the echo is detected after 0.12 s. Calculate the distance to the fish.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Practical notes
This shows sound needs a medium; the light from the bell still reaches you, because light can cross a vacuum.
Worked example
Example 1. A girl stands 170 m from a cliff and claps. She hears the echo 1.0 s later. Find the speed of sound.
🔍 Worked example — Easy
A girl stands 170 m from a cliff and claps. She hears the echo 1.0 s later. Find the speed of sound.
🔍 Worked example — Standard
A ship's sonar pulse returns from the sea bed after 0.40 s. Sound travels at 1500 m/s in sea water. Find the depth of the sea.
🔍 Worked example — Exam-style
A musical note has a frequency of 512 Hz. If the speed of sound in air is 340 m/s, calculate the wavelength of the sound wave.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Space is a vacuum; sound needs particles (a medium) to pass the vibrations on. (2) Compressions are regions where the particles are squashed close together (higher pressure); rarefactions are regions where they are spread apart (lower pressure). (3) (a) taller waves (larger amplitude); (b) waves closer together (higher frequency). (4) About 20 Hz to 20 000 Hz. (5) 1500 × 1.2/2 = 900 m. (6) Any two: cleaning delicate objects; prenatal scanning; other medical scanning; sonar.