Chapter 15 Sound

Question 1

Define sound.

Answer 1

Sound is a form of energy that is transferred from one point to another as a longitudinal wave.

Question 2

How is sound produced?

Answer 2

Sound is produced by vibrating sources placed in a medium.

Question 3

State the differences between a displacement-distance graphs, displacement-time graphs and pressure-distance graphs.

Answer 3

Displacement-distance graph:

• Describes the displacements of ALL particles at a particular point in time over a distance.
• Shows the position of all the particles at a particular instant.
• Obtain amplitude and wavelength

Displacement-time graph:

• Describes how a single particle moves over a time interval.
• Obtain amplitude and period

Pressure-distance graph:

• Shows a sound wave at a certain instant
• Obtain wavelength, maximum and minimum amount of air pressure, rarefactions and compressions

Question 4

What are the properties of the speed of sound?

Answer 4

• The speed of sound varies in different mediums; propagate faster in denser medium.
• Ratio of speed in gas: liquid: solid = 1 : 5 : 15

Question 5

Why does sound travel fastest through solids?

Answer 5

The particles in solids are closely packed.

Question 6

Define echo.

Answer 6

An echo is the repetition of a sound due to the reflection of sound.

Question 7

How are echoes formed?

Answer 7

An echo is formed when a sound is reflected off a hard, flat surface such as a large wall. The laws of reflection of light also apply to sound waves.

Question 8

State and give an example for the uses of echoes.

Answer 8

• Measure large distances; for example, the depth of the sea or the position of shoals of fish can be found by sending out a signal (a pulse of sound) and noting the time interval before the reflected signal (the echo) arrives
• Detect the location of objects (i.e. echolocation); for example, Sound Navigation And Ranging (SONAR) is a type of technology that works based on echoes. It is used by ships for navigation at sea and to detect the position of other vessels.

Question 9

Define the range of audibility for humans.

Answer 9

It is the range of frequencies in which a person can hear. For humans, this range is from 20 Hz to 20 000 Hz.

Question 10

Define ultrasound.

Answer 10

Ultrasound is a sound with frequency above 20 kHz.

Question 11

State and explain the uses of ultrasound.

Answer 11

SONAR (Sound Navigation And Ranging):
- Used to measure the depth of seabed. Sends pulses of sound wave (usually ultrasound) towards the seabed and measures the echo time.

Quality control:

• Check for cracks or cavities in concrete slabs
  1. Pulse of ultrasound sent through a metal plate
  2. Pulse is reflected from the other end of metal plate
  3. Sensor detects the the reflected pulse
  4. If there is a flaw, two pulses will be detected; one pulse from the other end of the plate and another from the location of the flaw

Medical applications (prenatal scanning):

• Detect abnormal growth or checking the development of the foetus
• Preferred over X-rays as it is less hazardous due to its lower energy.

Question 12

Define loudness.

Answer 12

• It is related to the amplitude of a sound.

- The larger the amplitude, the louder the sound, the more energy it possesses.

Question 13

Define pitch.

Answer 13

Pitch is related to the frequency of a sound wave. Faster vibration → higher frequency → higher pitch.

Question 14

How can frequency be increased?

Answer 14

• Increasing the tension
• Decreasing the length
• Using thinner string of smaller mass per unit length