SPI Physics · Formula sheet
Sound formulas
1 formula, 3 reference tables and 12 common traps for SPI Physics Sound, grouped by subtopic.
Sound Waves and Their Speed
Learn this subtopic in the notesSound as a mechanical wave: v = fλ
Wave speed
- speed of the wave, in m/s
- frequency, in hertz
- wavelength, in metres
- time period, in seconds
How fast sound travels, and what changes it
| Medium | Speed of sound |
|---|---|
| Vacuum | Sound cannot travel |
| Air at 0 °C | about 331 m/s |
| Air at 20 °C | about 343 m/s |
| Water | about 1500 m/s, over four times the speed in air |
| Steel | about 5000 to 6000 m/s Fastest of the media the questions use. |
Warmer air carries sound faster; colder air carries it slower.
Common traps
Thinking light needs a medium too
Light is an electromagnetic wave and crosses empty space from the Sun. Only mechanical waves, like sound, need a material medium.
Calling sound transverse
Sound in air is longitudinal: the air moves back and forth along the wave's path, making compressions and rarefactions.
Leaving kilohertz unconverted
Change kHz to Hz before using . A 2 kHz note at 340 m/s has m, not 170 m.
Sound is fastest in air
Air is the slowest of the three. Sound is fastest in solids such as steel, then liquids such as water, then gases.
The source sets the speed
Who or what makes the sound does not matter. In one medium at one temperature, every sound travels at the same speed.
Heat slows sound down
In air it is the reverse: the speed rises as the temperature rises.
Sound is slower in water than in air
Sound travels over four times as fast in water (about 1500 m/s) as in air (about 340 m/s), because a liquid's particles are closer together.
Hearing, Speech and Uses of Sound
Learn this subtopic in the notesHow we speak and how we hear
| Part | Its job |
|---|---|
| Vocal cords (in the larynx) | Vibrate to make the voice |
| Pinna (outer ear) | Collects sound and sends it down the ear canal |
| Eardrum | A thin membrane that vibrates when sound reaches it |
| Hammer, anvil and stirrup (middle ear) | Amplify the vibrations and pass them to the inner ear The stirrup is the smallest bone in the body. |
| Cochlea (inner ear) | Turns vibrations into electrical signals |
| Auditory nerve | Carries the signals to the brain |
Outer ear collects, middle ear amplifies, inner ear converts.
Uses of reflected sound and ultrasound
| Use | How sound is used |
|---|---|
| Curved soundboard, curved ceiling | Reflect sound evenly to every part of a hall |
| Megaphone, horn | Reflections send the sound forward in one direction |
| Stethoscope | The heartbeat's sound is reflected along the tube to the ear |
| Echocardiography | Ultrasound echoes give an image of the heart Ultrasound, not X-rays. |
| Ultrasonography | Ultrasound images of organs and of a baby in the womb |
| SONAR | Ultrasound echoes find the depth of the sea and locate submarines |
| Bats | Find their way and their prey by ultrasound echoes |
Reflected sound is used in halls; ultrasound echoes are used in hospitals and at sea.
Common traps
The middle ear bones raise the pitch
The three bones only make the vibrations stronger. The frequency, and so the pitch, stays the same.
The eardrum makes the voice
The eardrum vibrates when we HEAR. When we speak, it is the vocal cords in the larynx that vibrate.
A soundboard absorbs sound
A soundboard reflects sound, and its curve spreads it across the whole width of the hall, not to one spot.
Heart scans use X-rays
Echocardiography uses ultrasound echoes. Infrasound cannot form an image, and no X-rays are used.
Expecting an echo from a near wall
A wall closer than about 17 m sends the sound back within 0.1 s, so it merges with the original sound and no separate echo is heard.