Sound

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Notes

Sound Waves

Sound waves are **longitudinal** waves produced by vibrating sources.
They require a **medium** (solid, liquid, gas) to travel; cannot travel through a vacuum.
Longitudinal waves consist of **compressions** (regions of higher density) and **rarefactions** (regions of lower density).
Sound is a **pressure wave**; compressions and rarefactions cause pressure variations.
When sound hits a solid, the pressure variations cause the solid to vibrate in sync with the wave.

Measuring the Speed of Sound

Speed of sound in air: **330–350 m/s** (increases with temperature).
Sound travels fastest in **solids** (~5000 m/s), slower in **liquids** (~1500 m/s), slowest in **gases**.
**Experiment 1 (two-point method)**: Measure distance (e.g., 100 m) and time (stopwatch) between a sound source and observer. Speed = distance / time.
**Experiment 2 (oscilloscope method)**: Two microphones connected to an oscilloscope, distance ~2 m. Clap near first mic; oscilloscope measures time difference. More accurate than stopwatch.
**Accuracy**: Experiment 2 is more accurate because timing is automatic; Experiment 1 suffers from human reaction time (~0.2 s error).
Use **repeat readings** and **larger distances** to reduce random errors.

Effects of Sound Waves

**Pitch** depends on **frequency**: high frequency → high pitch; low frequency → low pitch.
**Loudness** depends on **amplitude**: large amplitude → loud sound; small amplitude → quiet sound.
**Echoes** are reflections of sound waves off hard, flat surfaces.
Distance to reflecting surface: **distance = ½ × speed × time** (time is total for sound to travel to surface and back).

Ultrasound

Human hearing range: **20 Hz – 20,000 Hz** (20 kHz).
**Ultrasound** is sound with frequency **above 20 kHz**.
Ultrasound is partially reflected at boundaries between different media.
**Uses**: medical scanning (foetus, soft tissues), non-destructive testing (detecting cracks in metal), sonar (depth measurement).
Depth calculation: **depth = ½ × speed × time** (same as echo formula).
Ultrasound is **non-invasive** and considered harmless.

Longitudinal sound wave showing compressions and rarefactions

Pitch and amplitude of sound waves

Echo reflection off a wall

Ultrasound depth measurement

Practice questions

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1.Which of the following frequency ranges is completely audible by a human with average hearing?
Easy
- A200 Hz to 200 kHz
- B5 Hz to 50 kHz
- C50 Hz to 5 kHz
- D50 kHz to 50 MHz
2.A tuba player plays a loud note on her tuba. She then plays a quiet note of the same pitch. Which property of the sound wave she produced has changed?
Easy
- AFrequency
- BWavelength
- CAmplitude
- DSpeed
3.A high frequency note is played on a violin. The violinist then reduces the frequency of the note. What change in the note do the audience hear?
Easy
- APitch increases
- BPitch decreases
- CVolume increases
- DVolume decreases
4.Sound wave W has amplitude A and frequency f. Sound wave X is louder and lower in pitch than sound wave W. What can be said about the amplitude and frequency of sound wave X?
Easy
- AAmplitude higher than A, frequency higher than f
- BAmplitude higher than A, frequency lower than f
- CAmplitude lower than A, frequency higher than f
- DAmplitude lower than A, frequency lower than f
5.Which of the following is not a use of ultrasound?
Easy
- AScanning a fetus during pregnancy
- BNon-destructive testing of materials such as to find leaks in copper pipes
- CMapping with sonar
- DIdentifying broken bones
6.When objects vibrate, they produce sound waves. An object in a room vibrates. However, a person in that same room does not hear anything. Why not?
Medium
- AThe room contains no air.
- BThe sound waves are travelling too fast.
- CThe amplitude of the sound waves is too great.
- DThe frequency of the sound waves is too great.
7.After a lockdown drill, the management notes the siren is too quiet and its pitch is too low. They ask the company to make the alarm louder and give it a higher pitch. What effect does this have on the resulting sound wave?
Medium
- AIt has a larger amplitude and a lower frequency.
- BIt has a larger frequency and a lower amplitude.
- CIt has a smaller frequency and a larger amplitude.
- DIt has a larger frequency and a larger amplitude.
8.A teacher uses a signal generator connected to a speaker to make sounds of different frequencies. Which frequency can be heard by a student?
Medium
- A10 Hz
- B30 kHz
- C440 Hz
- D120 kHz

Slides

Sound Waves

Sound waves are produced by vibrating sources.
They are longitudinal waves, meaning the particles vibrate parallel to the direction of energy transfer.
A medium is needed to transmit sound; sound cannot travel through a vacuum.

What type of wave is a sound wave?

Transverse
Longitudinal
Electromagnetic
Standing

Compressions and Rarefactions

Longitudinal waves consist of compressions and rarefactions.
A compression is a region of higher density where particles are bunched together.
A rarefaction is a region of lower density where particles are spread apart.
Sound is a pressure wave; compressions and rarefactions cause pressure variations.

In a longitudinal wave, a region where particles are bunched together is called a ____. A region where particles are spread apart is called a ____.

Pitch and Loudness

The frequency of a sound wave determines its pitch: high frequency = high pitch, low frequency = low pitch.
The amplitude of a sound wave determines its loudness: large amplitude = loud, small amplitude = quiet.
Frequency is measured in hertz (Hz); amplitude is the maximum displacement from the equilibrium position.

A sound wave has a large amplitude and a low frequency. Which describes the sound?

Loud and high-pitched
Loud and low-pitched
Quiet and high-pitched
Quiet and low-pitched

Echoes

Sound waves reflect off hard, flat surfaces. The reflection of a sound wave is called an echo.
The distance to the reflecting surface can be calculated using: distance = (1/2) × speed × time.
The time is the total time for the sound to travel to the surface and back.

The formula to calculate the distance to a reflecting surface using an echo is: distance = ____ × speed × time.

Measuring the Speed of Sound

The speed of sound in air is approximately 330–350 m/s, and it increases with temperature.
Sound travels fastest in solids (≈5000 m/s), slower in liquids (≈1500 m/s), and slowest in gases.
Two common methods: using two points with a stopwatch, or using an oscilloscope with two microphones.

A student claps 100 m from a wall and hears the echo after 0.59 s. What is the speed of sound?

340 m/s
170 m/s
59 m/s
100 m/s

Ultrasound

The human hearing range is approximately 20 Hz to 20,000 Hz.
Ultrasound is sound with a frequency higher than 20 kHz (20,000 Hz).
Ultrasound is used in medical scanning, sonar, and non-destructive testing of materials.

Which of these are uses of ultrasound?

Scanning a fetus during pregnancy
Checking for cracks in metal objects
Measuring the speed of light
Mapping the ocean floor with sonar

Ultrasound Calculations

The depth of a boundary can be found using: depth = (1/2) × speed × time.
This is the same formula as for echoes; the time is the total time from emission to detection.
Ultrasound transducers emit and receive pulses; the time difference is measured on an oscilloscope.

A sonar pulse is sent from a boat to the seabed and returns after 1.3 s. The speed of sound in water is 1500 m/s. How deep is the water?

975 m
1950 m
750 m
1500 m

Non-Destructive Testing with Ultrasound

Ultrasound can detect cracks inside metal objects without damaging them.
Waves reflect from boundaries; a crack causes an early reflection, shown as an extra pulse on an oscilloscope.
The distance to the crack can be calculated from the time delay and the speed of sound in the material.

In non-destructive testing, an oscilloscope shows a pulse from the start of the object, a pulse from the ____, and a pulse from the end of the object.

Key Equations Summary

Speed = distance / time (for calculating speed of sound).
Distance to reflector = (1/2) × speed × time (for echoes and ultrasound depth).
Remember: in echo calculations, the time is for the round trip.

A student stands 50 m from a wall and claps. She hears the echo after 0.29 s. What is the speed of sound?

345 m/s
172 m/s
50 m/s
100 m/s

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