1.3 Sound Production

Question 1

What is a mechanical wave?

Answer 1

A mechanical wave is a disturbance that is created by a vibrating object and subsequently travels through a medium from one location to another, transporting energy as it moves.

Question 2

What is the principle of superposition?

Answer 2

When two or more waves meet their total displacement is the sum of their individual displacements, taking their directions into account. Having met, the waves continue on their way as if they never met at all.

Question 3

What must waves be to have a consistent pattern?

Answer 3

The waves need to have the same frequency and speed, and ideally the same or very similar amplitudes.

Question 4

Give the example of standing waves in music.

Answer 4

The vibration of a device, such as a wire, drum, or reed, creates musical sounds.
The instrument causes the surrounding air to vibrate at the same frequency as the device.
Musical instruments usually fall into the categories of percussion, string or wind.

Question 5

What us the sound of an acoustic guitar a result of?

Answer 5

The shape and body of the guitar, the strings themselves do no produce a very loud sound because of their small amplitudes and the fact they cannot move a large volume of air.

Question 6

How does the string of a guitar create a larger amplitude of sound?

Answer 6

It effectively transfers its vibration of the body of the guitar which vibrates as a whole and makes much more air move.

Question 7

Why is the top plate and air within a guitar important?

Answer 7

The top plate is only a few millimetres thick and can vibrate easily. The air inside the body is important since it can vibrate just like the air in a bottle when a person blows across the top.

Question 8

What three things does the pitch of a vibrating string depend on?

Answer 8

The mass of the string: more massive strings vibrate more slowly.
The tension in the string: the tighter the string the higher the pitch.
The length of the string: shortening the strings gives a higher pitch.

Question 9

How are standing waves on strings formed?

Answer 9

They’re formed from the superposition of two identical waves travelling in opposite directions.

Question 10

Where do travelling waves move from in string instruments?

Answer 10

In string instruments travelling waves move from the point where string is plucked or bowed. They then travel to the ends of the string where they reflect and undergo a phase change of 180 degrees where a crest becomes a trough.

Question 11

In travelling waves, there are points when the total displacement always remands zero. What are these points called?

Answer 11

Nodes.

Question 12

In travelling waves, there are points when the displacement varies between a maximum in one direction and a maximum in the other direction, what are these points called?

Answer 12

Antinodes.

Question 13

Explain the harmonics of a string.

Answer 13

A string has a number of frequency at which it will naturally vibrate. These natural frequencies are known as the harmonics of string.

Question 14

What is the natural frequency at which a string vibrates at dependent on?

Answer 14

The tension of the string, the mass per unit length and the length of the string.

Question 15

What does doubling the frequency of a vibration do?

Answer 15

It halves the wavelength and means that two loops are formed.

Question 16

What does three times the fundamental frequency of a standing wave give?

Answer 16

The third harmonic.

Question 17

Where does the rich sound come from in a musical instrument?

Answer 17

Several harmonics, occurring at the same time, and it is the balance of their amplitudes that determine the timbre.

Question 18

Where does the vibration usually begin at in wind instruments?

Answer 18

The mouth piece- this is the device which classifies the wind instrument.

Question 19

How do reed instruments work?

Answer 19

Air is blown through pieces of reed plant which causes the reed to vibrate quickly. This opens and closes the pipe. The pulses of air passing through the reed cause the air in in the pipe to vibrate in sympathy and sets up a standing wave.

Question 20

How is sound produced in flutes and recorders?

Answer 20

They have a sharp ‘edge mouth piece’. The sound is produced by blowing a stream of air at this sharp edge. The stream of air vibrates above and blow the edge setting up a standing wave in the pipe.

Question 21

How is sound produced in brass instruments?

Answer 21

The players lips vibrate against each other and against each other and against the rim of the mouthpiece.

Question 22

How does the displacement of air particles work in a pipe?

Answer 22

There is a displacement antinode at the open end and there will be a displacement node at the closed end. Displacement rather than pressure is used to represent the standing waves in a pipe.

Question 23

What are beats?

Answer 23

If two musical notes are sounded together, when the waves meet they will superpose and form an overall wave in a similar way to how standing wave is set up- the frequencies are similar but not identical a regular rising and falling of amplitudes of the resultant wave will be heard.

Question 24

When the strings are not in tune what happens to the beat frequency?

Answer 24

It is equal to the difference between the frequencies of individual strings.

Question 25

A stationary wave is set up in a pipe that is closed at one end and open at the other. Explain why a pressure node is set up at the open end (displacement antinode) and a displacement node at the closed end.

Answer 25

The open end is open to the atmosphere and must be at atmospheric pressure.
As this is constant the pressure at the open end cannot change.
At the closed end the air molecules cannot move past the barrier and hence there must be a displacement node.

Question 26

Explain why the length of a pipe is slightly less than a quarter of a wavelength when the pipe is resonating in its fundamental mode?

Answer 26

When at the fundamental frequency there is a displacement node at the
closed end and a displacement antinode at the open end.
The distance from a node to an antinode is equal to λ/4.
In practice displacement antinode occurs slightly beyond the open end of the pipe.

Question 27

A pipe open at both ends and a pipe closed at one end have the same length. They are resonating in their fundamental mode. State and explain the relationship between the frequencies of the sound waves produced by the two pipes.

Answer 27

The frequency of open pipe is twice the frequency of the closed pipe.
The length of the open pipe is equivalent to a quarter wavelength whereas the wavelength of the open pipe is equivalent to a half wavelength.
Thus as pipes are the same length, the wavelength of the sound wave from the open pipe is half the wavelength of the closed pipe.

Question 28

State and explain why a pipe open at one end is capable of producing more harmonics than a similar pipe closed at one end?

Answer 28

With a pipe closed at one end even harmonics are not possible as there is
a node at one end and antinode at the closed end.
With a pipe open at both ends, odd and even harmonics are possible as both ends have antinodes.

Question 29

How can the sound of a musical instrument by synthesised electronically?

Answer 29

Create alternating voltages and add them together.

Question 30

Why is digital signal used in Mp3?

Answer 30

• Reduced file size when compressed compared to an uncompressed audio file
• Easier sharing and portability in binary form
• Reduced storage requirements
• Mp3 compression simplifies audio content so a smaller file with minimal audio degradation
• Mp3 can convert digital to analogue upon playback

Question 31

What is lossless compression?

Answer 31

With lossless compression, every single bit of data that was originally in the file remains after the file is uncompressed. All of the information is completely restored.

Question 32

What is lossy compression?

Answer 32

Lossy compression reduces a file by permanently eliminating certain information, especially redundant information. When the file is uncompressed, only a part of the original information is still there (although the user may not notice it).