Sound Waves and Ultrasound
Sound Waves and Ultrasound
Sound is a longitudinal wave caused by vibrating objects. It requires a medium to travel through — sound cannot travel through a vacuum. Understanding how sound behaves and how ultrasound is used is important for both the exam and practical applications.
How Sound is Produced and Detected
Sound waves are produced when objects vibrate. The vibrating object pushes the surrounding air particles back and forth, creating regions of:
- Compression — where particles are pushed close together (high pressure)
- Rarefaction — where particles are spread apart (low pressure)
These compressions and rarefactions travel outward from the source as a longitudinal wave. The particles themselves do not travel — they oscillate about their equilibrium positions, passing energy from one to the next.
When sound waves reach the ear:
1. They cause the eardrum to vibrate
2. These vibrations are transmitted through small bones to the inner ear
3. The inner ear converts vibrations into electrical signals sent to the brain
Properties of Sound
Speed of sound depends on the medium:
| Medium | Approximate speed |
|---|---|
| Air (20 degC) | 340 m/s |
| Water | 1,500 m/s |
| Steel | 6,000 m/s |
Sound travels fastest in solids (particles are close together, so vibrations transfer quickly) and slowest in gases.
Pitch depends on frequency:
- High frequency = high pitch
- Low frequency = low pitch
- The human ear can hear frequencies between approximately 20 Hz and 20,000 Hz (20 kHz)
Loudness depends on amplitude:
- Large amplitude = loud sound
- Small amplitude = quiet sound
Sound on an Oscilloscope
A microphone converts sound into an electrical signal that can be displayed on an oscilloscope:
- A louder sound has a taller wave (greater amplitude)
- A higher-pitched sound has more waves in the same space (higher frequency, shorter period)
- A quieter sound has a shorter wave (smaller amplitude)
- A lower-pitched sound has fewer waves in the same space (lower frequency, longer period)
Echoes and Reflection of Sound
An echo is a reflected sound wave. Sound reflects off hard, flat surfaces. Soft, rough surfaces absorb sound (this is why concert halls use padded walls to reduce echoes).
Echoes can be used to measure distance:
distance = speed x time / 2
The division by 2 is because the sound travels to the reflecting surface and back.
Example: A ship sends a sonar pulse to the seabed. The echo returns after 0.4 seconds. The speed of sound in water is 1,500 m/s.
distance = 1,500 x 0.4 / 2 = 300 m
Ultrasound
Ultrasound is sound with a frequency above 20,000 Hz (above the range of human hearing). It has the same properties as normal sound but at higher frequencies.
Uses of Ultrasound
Medical imaging (prenatal scans):
1. An ultrasound transmitter sends pulses of ultrasound into the body
2. At each boundary between different tissues (e.g. skin to muscle, muscle to bone), some ultrasound is reflected and some is transmitted
3. The reflected pulses are detected and the time delay is measured
4. A computer calculates the distance to each boundary and builds up an image
Advantages over X-rays:
- Ultrasound is non-ionising — it does not damage DNA or cause cancer
- It is safe to use on pregnant women and unborn babies
- It can distinguish between soft tissues (X-rays are better for bones)
Industrial testing:
- Ultrasound pulses are sent through metal components (e.g. aircraft engine parts)
- If there is a crack or flaw inside the metal, some ultrasound reflects from the defect
- The reflected pulse arrives sooner than expected, revealing the location and size of the deflaw
- This is non-destructive testing — the part does not need to be cut open
Cleaning:
- Ultrasound vibrations in a liquid dislodge dirt and contaminants from delicate objects (jewellery, surgical instruments, electronic components)
Distance measurement:
- Ultrasound can measure the distance to objects (e.g. parking sensors in cars send ultrasound pulses and detect reflections from nearby objects)
Infrasound
Infrasound is sound with a frequency below 20 Hz. Humans cannot hear it, but it is produced by earthquakes, volcanic eruptions, elephants, and some weather patterns. Specialised equipment can detect infrasound and use it to monitor seismic activity.
Exam Tips
- Always state that sound is a LONGITUDINAL wave and requires a MEDIUM — it cannot travel through a vacuum
- For echo calculations, remember to HALVE the total distance (sound goes there AND back)
- When comparing ultrasound to X-rays for medical imaging, emphasise that ultrasound is non-ionising and safe for unborn babies
- On oscilloscope questions, carefully describe how both the amplitude AND the frequency (or period) appear on the trace