Sound
邊玩邊學
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課程筆記
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.
A diagram illustrating compressions and rarefactions in a sound wave generated by a vibrating solid.
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.
Experiment setup for measuring the speed of sound using two microphones and an oscilloscope.
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).
A person clapping to create an echo off a wall, while another person times the event.
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.
Medical scanning using ultrasound
投影片
練習題
免費預覽——48 題中的 8 題。註冊即可查看全部。
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?
Easy- 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?
Easy- 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?
Easy- A10 Hz
- B30 kHz
- C440 Hz
- D120 kHz