Unit 11

Question 1

Sound

Answer 1

1. Consists of small rapid fluctuations of atmospheric air pressure surrounding us
2. It needs a medium to travel through such as air molecules (solid or liquid as well), but it cannout travel when there are no molecules (vacuum)

Question 2

How do humans hear sound?

Answer 2

1. The source of sound creates rapid fluctuations of the atmospheric air pressure (caused by the air molecules bunching together or becoming more widely separated) that surrounds it
2. The atmospheric fluctuations spread outwards from the source, gradually becoming weaker and eventually dye away completely
3. When the prressure variations reach the listener, they act on the listener’s hearing mechanism, causing the eardrum to move in sympathy with the source of the pressure variations
4. The movements of the eardrum are detected and interpreted as sound

Question 3

Explain how different cycles of pressure variations cause different sounds

Answer 3

1. Any meaningful sound consists of cycles of pressure variations
2. Cycles may last for a very short time, such as the short sound caused by a rifle shot
3. Cycles may last for a ** long time**, such as the continuing sound of a person singing a long note
4. It is these cyclical pressure variations that allow us to recognize the sound
5. Random variations of pressure are perceived as ** noise**

Question 4

Pressure wave

Answer 4

A regular pattern of high- and low-pressure regions

Question 5

Travelling wave

Answer 5

When a pressure wave travels away from its source
It’s a way of transmitting energy

not all travelling waves are pressure waves, such as light

Question 6

Sound period

Answer 6

The duration of one cycle which is the time interval between any two corresponding points on consecutive cycles of the pressure wave

Time taken for one cycle to occur

Question 7

A cycle

(or an oscillation)

Answer 7

A complete sequence of motion up to the point at which the motion starts to repeat itself

Question 8

Why is it that in real life, no two cycles of sound are identical?

Answer 8

Because each cycle is slightly weaker than the one before.
However, over the course of few cycles, there will be very little change from one cycle to the next, so the motion can be regarded as periodic

Question 9

Sound Wavelength

Answer 9

The distance between any two adjacent regions of high (or low) pressure

Question 10

Sound Frequency

Answer 10

The number of periods that occur in 1 second
Expressed in Hz (Hertz)

Question 11

Lowest and Highest frequencies humans can hear

Answer 11

Lowest is 20Hz
Upper limit is 20000Hz or 20 kHz but tends to decline with age

Question 11

Pitch

Answer 11

How high or low a sound is
It is a subjective term

Question 12

To measure the speed of sound

Answer 12

measure how long the sound takes to travel a known distance

Question 13

List the number of factors that the speed of sound depend on and is affected by.

Answer 13

temperature, humidity, and atmospheric pressure

Question 14

The speed of sound in air:

Answer 14

around 340 m/s

Question 15

The frequency and the period have an inverse relationship

Answer 15

f=1/T or T=1/f

Question 16

The speed of sound has a joint relationship with the wavelength and the frequency or period of the sound

Answer 16

speed of sound = wavelength / period
speed of sound = frequency * wavelength

Question 17

The phase

Answer 17

the part of a cycle that a particular vibrating system is in at any moment

Question 18

Principle of Superposition

Answer 18

1. When individual sound waves meet, they pass through each other without being changed
2. Where the sound waves are together, the instantaneous air pressure change at any point is simply the sum of the air pressure changes from each individual sound at that point

Question 19

‘completely’ out of phase waves

Answer 19

there is an exact one half cycle difference in the waves

positive peak of one wave corresponds to the negative peak of the other

Question 20

What are examples of situations where phase difference can occur?

Answer 20

1. Between the signals from two microphones places at different distance from a sound source
2. almost any form of electronic sound-processing equipment affects the phase of the signal it is processing so that what comes out is not necessarily in phase with what goes in.

Question 21

Amplitude

size or amplitude of sound

Answer 21

a measure of the amount of change in air pressure that the sound produces

Question 22

Two ways of thinking about amplitude (change in air pressure):

Answer 22

1. The maximum change known as the peak-to-peak value
2. Average change over a long period of time

Question 23

Peak-to-peak amplitude

Answer 23

is the difference between the maximum reduction in air pressure and the maximum increase in air pressure
peak-to-peak amplitude = Pmax - Pmin

Question 24

Peak amplitude

Answer 24

one half of peak-to-peak
peak amplitude= peak-to-peak amplitude/ 2

Question 25

Root mean square

Answer 25

a kind of average, but considers the amount of energy a sound wave can deliver

Question 26

Root-mean-square (r.m.s.) amplitude

Answer 26

the peak amplitude multiplied by a factor of approximately 0.71
r.m.s amplitude = peak amplitude/√2

Question 27

normal atmospheric pressure:

Answer 27

100kPa

Question 28

SPL (Sound Pressure Level)

Answer 28

• non-linear scale
• 0 SPL is definedas the amplitude of the quietest 1 kHz tome that is just at the threshold of human audbility
• Sounds louder than this have +ve SPL and sounds quieter than this have -ve SPL values
• Equal increments/decrements in SPL values correspond to equal multiplications/divisions of actual sound amplitudes
• SPL values are implied/expressed in dB, i.e. decibels