Ultrasound and the Physics of Hearing (Grade 10 NSC Matric Physical Sciences): Revision Notes
Ultrasound and the Physics of Hearing
Sound intensity and the decibel scale
Sound intensity measures how much energy is transmitted through a unit area each second. This gives us a way to compare how loud different sounds are compared to each other.
The unit used to measure sound intensity is the decibel (dB). The decibel scale helps us understand the enormous range of sound intensities that humans can detect, from the quietest whisper to the loudest rocket launch.
| Source | Intensity (dB) | Times Greater Than Hearing Threshold |
|---|---|---|
| Rocket Launch | 180 | 10¹⁸ |
| Jet Plane | 140 | 10¹⁴ |
| Threshold of Pain | 120 | 10¹² |
| Rock Band | 110 | 10¹¹ |
| Factory | 80 | 10⁸ |
| City Traffic | 70 | 10⁷ |
| Normal Conversation | 60 | 10⁶ |
| Library | 40 | 10⁴ |
| Whisper | 20 | 10² |
| Threshold of Hearing | 0 | 0 |
The table above shows how different everyday sounds compare to our hearing threshold (the quietest sound we can hear). Notice how the intensity increases dramatically - each 10 dB increase represents a sound that is 10 times more intense than the previous level.
Key intensity levels to remember:
- 0 dB: Threshold of hearing (quietest sound we can detect)
- 60 dB: Normal conversation
- 80 dB: Factory noise (where hearing damage can begin)
- 120 dB: Threshold of pain
- 140 dB: Jet plane (can cause immediate hearing damage)
The intensity differences are massive. For example, a rocket launch at 180 dB is times more intense than our hearing threshold - that's 1,000,000,000,000,000,000 times stronger!
Sound intensity varies depending on your distance from the source. Instruments like vuvuzelas produce different sound levels at your ear compared to what you'd measure further away. The sound intensity decreases as you move away from the source.
Ultrasound
What is ultrasound?
Ultrasound is sound that has a frequency higher than 20 kHz (20,000 Hz). This is above the upper limit of human hearing, which means we cannot hear ultrasonic waves.
Many animals can hear ultrasound frequencies. Dogs, dolphins, and bats have hearing ranges that extend well beyond 20 kHz, allowing them to detect and use ultrasonic waves for communication and navigation.
Applications of ultrasound
Ultrasound has many practical applications in medicine, industry, and everyday life. Different applications require different frequency ranges:
| Application | Lowest Frequency (kHz) | Highest Frequency (kHz) |
|---|---|---|
| Cleaning (e.g. jewellery) | 20 | 40 |
| Material testing for flaws | 50 | 500 |
| Welding of plastics | 15 | 40 |
| Tumour ablation | 250 | 2000 |
Medical uses
The most common medical use of ultrasound is imaging. Ultrasound imaging works by using the reflection and transmission of waves at boundaries between different materials.
When ultrasound waves travel through the human body, they encounter boundaries between different tissues (like bone and muscle, or muscle and fat). At each boundary:
- Part of the wave is reflected back
- Part of the wave is transmitted through to the next layer
The reflected waves are detected and used to create an image of the internal structure. This makes ultrasound incredibly useful for:
- Monitoring pregnancy and fetal development
- Examining internal organs
- Scanning soft tissues
Ultrasound is a safe, non-invasive method for looking inside the human body, which is why it's preferred for pregnancy scans.
Industrial and therapeutic uses
Beyond imaging, ultrasound has several other important applications:
Therapeutic uses:
- Generating localised heating in biological tissue for physical therapy
- Cancer treatment through focused ultrasound
- Breaking up kidney stones
Industrial Cleaning Process:
Ultrasonic cleaners work at frequencies of 20-40 kHz. They clean objects by creating microscopic bubbles in a cleaning fluid. When these bubbles collapse, they release energy that removes dirt and debris from jewellery, lenses, dental instruments, and surgical tools.
The physics of hearing
Structure of the ear
The human ear has three main sections: the outer ear, middle ear, and inner ear. Each section plays a crucial role in converting sound waves into signals that our brain can interpret.
The outer ear consists of the pinna (the visible part of the ear that collects and focuses sound waves) and the ear canal that channels sound waves towards the eardrum.
The middle ear contains the eardrum which vibrates when sound waves hit it, and three small bones - the malleus (hammer), incus (anvil), and stapes (stirrup) - that amplify the vibrations.
The inner ear houses the cochlea which contains fluid and converts vibrations into electrical signals, and the auditory nerve that carries these signals to the brain.
How we hear
The Hearing Process:
- Sound collection: The pinna collects sound waves and channels them into the ear canal
- Vibration: Sound waves cause the eardrum to vibrate
- Amplification: The three middle ear bones transmit and amplify these vibrations
- Signal conversion: Vibrations pass through the elliptical window into the fluid-filled inner ear
- Nerve transmission: The cochlea converts vibrations into electrical signals sent to the brain via the auditory nerve
The fluid in the inner ear also helps us maintain balance and detect movement.
Hearing damage and protection
Protecting our hearing is essential because damage can be permanent and significantly impact quality of life.
Dangerous sound levels:
- Sounds above 80 dB can cause hearing damage over time
- Sounds above 120 dB cause immediate pain and potential damage
- Prolonged exposure to loud sounds is particularly harmful
Workers in loud environments (like those using jack hammers or working near aircraft) must use appropriate hearing protection to prevent permanent hearing loss.
Protection strategies:
- Wear ear plugs or ear muffs in noisy environments
- Limit exposure time to loud sounds
- Increase distance from sound sources when possible
- Use proper safety equipment in industrial settings
Key Points to Remember:
- Sound intensity is measured in decibels (dB), with 0 dB being the threshold of human hearing
- Ultrasound is sound with frequencies above 20 kHz that humans cannot hear
- Ultrasound is widely used in medical imaging because it safely shows internal body structures
- The ear has three main sections that work together to convert sound waves into brain signals
- Hearing damage can occur from sounds above 80 dB, so protection is essential in loud environments
- The decibel scale is logarithmic - every 10 dB increase represents 10 times more intensity