Waves, Light & Sound
Sound: Speed, Pitch, Loudness, Echoes & Ultrasound
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓By the end of this lesson students will be able to define sound as a vibration and explain how it travels through a medium.
- ✓By the end of this lesson students will be able to describe how the speed of sound varies in different media and with temperature.
- ✓By the end of this lesson students will be able to distinguish between pitch and loudness of a sound and relate them to the frequency and amplitude of sound waves.
- ✓By the end of this lesson students will be able to explain the formation of an echo and calculate distances using the speed of sound and time taken for an echo.
- ✓By the end of this lesson students will be able to identify and describe practical applications of ultrasound.
Key concepts
Sound is produced by vibrations. These vibrations travel through a medium (like air, water, or solids) as a longitudinal wave. In a longitudinal wave, the particles of the medium vibrate parallel to the direction the wave is travelling. Sound cannot travel in a vacuum as it requires particles to transmit the vibrations.
The speed at which sound travels depends on the medium it is passing through and its temperature. Sound travels fastest in solids, slower in liquids, and slowest in gases. For example, in air at 0°C, the speed of sound is approximately 331 m/s, increasing by about 0.6 m/s for every 1°C rise in temperature. In water, it's about 1500 m/s, and in steel, about 5000 m/s.
Pitch refers to how high or low a sound is. It is determined by the frequency of the sound wave. A high-pitched sound has a high frequency (more vibrations per second), while a low-pitched sound has a low frequency. Frequency is measured in Hertz (Hz).
Loudness refers to how intense or strong a sound is. It is determined by the amplitude of the sound wave. A large amplitude wave corresponds to a loud sound, while a small amplitude wave corresponds to a quiet sound. Loudness is measured in decibels (dB). Prolonged exposure to very loud sounds can damage hearing.
An echo is a reflected sound. When a sound wave hits a hard surface, it bounces back. For an echo to be heard distinctly, the reflecting surface must be far enough away so that the reflected sound arrives at least 0.1 seconds after the original sound. This is because the human ear can distinguish between two sounds if they are separated by at least 0.1 seconds.
Ultrasound refers to sound waves with frequencies higher than the upper limit of human hearing, which is typically around 20,000 Hz (20 kHz). Humans cannot hear ultrasound. It has several important applications: * **Medical Imaging**: Used to create images of internal body structures (e.g., foetal scans, organ examination) because it is non-invasive and does not use ionising radiation. * **Sonar (Sound Navigation and Ranging)**: Used by ships to detect objects underwater, measure ocean depth, and locate fish shoals. * **Industrial Uses**: Cleaning delicate objects, detecting flaws in materials.
Key facts to remember
- 1Sound is produced by vibrations and travels as a longitudinal wave.
- 2Sound requires a medium to travel and cannot travel in a vacuum.
- 3The speed of sound depends on the medium and its temperature (fastest in solids, slowest in gases).
- 4Pitch is determined by the frequency of the sound wave (higher frequency = higher pitch).
- 5Loudness is determined by the amplitude of the sound wave (larger amplitude = louder sound).
- 6An echo is a reflected sound wave.
- 7Ultrasound refers to sound waves with frequencies above the human hearing range (> 20,000 Hz).
- 8Ultrasound has important applications in medical imaging, sonar, and industrial cleaning.
Worked examples
Example 1
A student claps their hands in front of a large wall and hears an echo 0.8 seconds later. If the speed of sound in air is 340 m/s, calculate the distance between the student and the wall.
Answer
136 m
Remember to divide the total time by 2 when calculating the distance to a reflecting surface for an echo.
Example 2
A tuning fork vibrates at 440 Hz, producing a note. Another tuning fork produces a note with a frequency of 256 Hz. Which tuning fork produces a higher-pitched sound? Explain your answer.
Answer
The tuning fork vibrating at 440 Hz produces a higher-pitched sound. This is because pitch is directly related to frequency; a higher frequency corresponds to a higher pitch.
Example 3
Explain why a person can hear a train approaching sooner by listening for vibrations in the railway track than by listening for the sound travelling through the air.
Answer
Sound travels faster through solids (like the railway track) than through gases (like air). Therefore, the vibrations from the approaching train will reach the listener's ear through the track sooner than the sound travelling through the air.
Common mistakes
- ✗Confusing pitch with loudness (i.e., thinking pitch is related to amplitude or loudness to frequency).
- ✗Forgetting to divide the total time by two when calculating the distance to a reflecting surface for an echo.
- ✗Believing that sound can travel through a vacuum.
- ✗Not understanding that the speed of sound varies significantly depending on the medium it travels through.
- ✗Incorrectly stating that all sound waves are transverse waves (they are longitudinal).
Exam tips
- ★Always state the formula you are using before substituting values in calculations and include correct units in your final answers.
- ★When explaining echoes, clearly state that the sound travels to the surface and then *back* to the listener.
- ★Be able to give specific examples of ultrasound applications and explain why ultrasound is used in those contexts.
- ★Practise distinguishing between the terms 'pitch' and 'loudness' and their corresponding wave properties (frequency and amplitude, respectively).
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