Sound is the form of energy that produces the sensation of hearing, and it travels as a wave through a material medium. The sections below move from the nature of sound and its speed in different media, through its frequency ranges and everyday applications, to intensity, echo, Mach number and the SI units that examiners draw on for this topic.
Sound begins with vibration, and it needs something to travel through; understanding what kind of wave it is forms the foundation for everything else.
Sound waves are classified as which type of wave?
Why can sound not travel through a vacuum?
In gases, plasma and liquids, sound travels as a wave, with particles vibrating along the direction of travel.
Sound cannot travel through a because there are no particles to carry the vibration.
The speed of sound is not fixed โ it depends mainly on the medium and its temperature, and this gives several favourite exam points.
| Medium | Approx. speed |
|---|---|
| Air (at 0ยฐC) | โ 332 m/s |
| Air (room temperature, โ 20ยฐC) | โ 343 m/s |
| Water | โ 1500 m/s |
| Steel (solid) | โ 5000 m/s |
In which medium does sound travel fastest?
By approximately how much does the speed of sound in air rise per 1ยฐC rise in temperature?
The speed of sound in air at room temperature (about 20ยฐC) is approximately m/s.
Sound travels faster in humid (moist) air than in air.
On the basis of frequency, sound waves are divided into three classes, and only the middle band can be heard by the human ear.
| Type | Frequency range |
|---|---|
| Infrasonic waves | < 20 Hz |
| Audible sound waves | 20 Hz โ 20,000 Hz |
| Ultrasonic waves | > 20,000 Hz |
What is the audible frequency range for the human ear?
Waves with a frequency below 20 Hz are called:
Waves with a frequency above 20,000 Hz are called waves.
Some animals behave abnormally before an earthquake because it produces low-frequency ahead of the main shock waves.
Because ultrasound is high-frequency and can be focused and reflected, it is widely used in industry and medicine.
Which medical test images the heart using reflected ultrasonic waves?
A dog whistle (Galton whistle) emits which kind of sound?
Ultrasonic waves can break into small pieces that pass out through urine.
Ultrasound is used to clean zig-zag (hard-to-reach) and delicate parts.
SONAR uses reflected ultrasonic waves to "see" underwater, where ordinary light and radio do not reach well.
SONAR stands for:
SONAR determines the distance to an underwater object using:
SONAR finds the distance, direction and of underwater objects.
In SONAR, a signal called a is sent out and reflects back from the target object.
Loudness in the physical sense is measured by intensity, and its everyday unit is one you have already heard of.
| Unit | Value |
|---|---|
| 1 bel (B) | a very large unit |
| 1 decibel (dB) | = 1/10 bel |
The SI unit of sound intensity is:
One decibel (dB) is equal to what fraction of a bel?
Sound intensity is the power carried by sound waves per unit measured perpendicular to it.
The practical unit of sound level used in daily life is the .
An echo is simply sound heard again after it bounces back, but the ear can separate it from the original only under certain conditions.
For how long does the sensation of a sound persist in the ear?
What is the minimum distance of a reflector to hear a clear echo?
An echo is the sound heard when sound waves off a surface back to the listener.
The original and reflected sounds must be at least seconds apart to be heard separately.
For very fast objects, speed is often compared with the speed of sound itself, and that ratio is the Mach number.
| Mach value | Meaning |
|---|---|
| Mach 1 | equal to the speed of sound |
| Mach 2 | 2 ร the speed of sound |
| Mach 3 | 3 ร the speed of sound (and so on) |
| Category | Description | Mach range |
|---|---|---|
| Subsonic | speed less than the speed of sound | Mach < 1 |
| Supersonic | objects such as missiles | 1 < Mach โค 5 |
| Hypersonic | very fast objects, e.g. BrahMos-II missile | Mach > 5 |
Mach number is defined as the ratio of the speed of a body to the:
An object travelling at Mach 3 moves at what speed?
An object moving at a speed greater than Mach 5 is described as .
An object with a speed less than the speed of sound (Mach < 1) is called .
Sound questions often sit alongside a general SI-units table, so the standard physical quantities and their units are listed together here.
| Physical quantity | SI unit | Symbol |
|---|---|---|
| Distance | Metre | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Current | Ampere | A |
| Temperature | Kelvin | K |
| Luminous intensity | Candela | cd |
| Angle | Radian | rad |
| Solid angle | Steradian | sr |
| Force | Newton | N |
| Energy | Joule | J |
| Speed | Metre/second | m sโปยน |
| Angular velocity | Radian/second | rad sโปยน |
| Frequency | Hertz | Hz |
| Moment of inertia | Kilogram metre square | kg mยฒ |
| Momentum | Kilogram metre/second | kg m sโปยน |
| Angular momentum | Kilogram metre square/second | kg mยฒ sโปยน |
| Pressure | Pascal | Pa |
| Power | Watt | W |
| Surface tension | Newton per metre | N mโปยน |
| Viscosity | Newton second per metre square | N s mโปยฒ |
| Thermal conductivity | Watt per metre per kelvin | W mโปยน Kโปยน |
| Specific heat | Joule per kilogram per kelvin | J kgโปยน Kโปยน |
| Electric charge | Coulomb | C |
| Potential | Volt | V |
| Capacitance | Farad | F |
| Resistance | Ohm | ฮฉ |
| Inductance | Henry | H |
| Magnetic flux | Weber | Wb |
| Luminous flux | Lumen | lm |
| Illuminance | Lux | lx |
| Optical wavelength | Angstrom | ร |
| Impulse | Newton second | N s |
What is the SI unit of luminous intensity?
The SI unit of pressure is:
The SI unit of frequency is the (Hz).
Magnetic flux is measured in the SI unit (Wb).
A last look before the exam โ the lines below pull together the points most often asked.
Take 5 questions at a time โ tap an option to check. After each round, revise the notes above and take the retest for 5 fresh questions, until you've mastered the whole chapter.