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Physics > Unit 1 - Chapter 1 - Recognising Sound > Flashcards

Unit 1 - Chapter 1 - Recognising Sound Flashcards

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1
Q

What is the threshold of human hearing?

A

The intensity of sound that is at the limit of human hearing.
(1.0 x 10^-2 Wm^-2).
Assigned the sound level of 0 dB.

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2
Q

What is the ratio of decibels to intensity?

A

An increase of 3 decibels is a doubling of intensity.

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3
Q

What are the two regions of a longitudinal wave?

A

Compression (regions of higher pressure)

Rarefactions (regions of lower pressure)

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4
Q

The typical human ear can detect sounds in what range of frequencies?

A

Between 20Hz - 20,000Hz / 20kHz

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5
Q

What is pitch produced by?

A

Fundamental frequency plus the harmonics

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6
Q

What are harmonics?

A

Multiples of the fundamental wave frequency included in the sound.

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7
Q

What it Quality?

A

The factor that gives a musical instrument or voice it’s unique character. (Also called Timbre)

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8
Q

What is the timbre made up of?

A

The harmonics present in the sound
The attack and decay
The vibrato

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9
Q

What it the Attack?

A

How quickly the sound reaches peak amplitude of loudness.

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10
Q

What the decay of a sound?

A

How long the sound takes to die away.

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11
Q

What is a tremolo?

A

A periodic change in amplitude.

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12
Q

What is the frequency of a sound?

A

The number of vibrations of an instrument. (More vibrations = higher frequency)

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13
Q

What is Vibrato?

A

Periodic changes in the pitch of the waveform as the sound continues to be produced.

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14
Q

What is superposition?

A

When two or more waves meet causing their individual displacements to be added together to form a total displacement. After they meet they continue on as if nothing has happened.

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15
Q

What are the requirements for superposition to form a consistent pattern?

A

The waves have to have the:
Same frequency
Same speed
Identical of similar amplitudes

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16
Q

Sound waves of certain pitches that when heard together produce pleasant sensations are called what?

A

Consonant or Harmonic

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17
Q

An separation of an octave is caused by what frequency ratio?

A

2:1 (i.e. one is double the other).

An octave is a particularly pleasing sound

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18
Q

The separation of an interval of a third is a ratio of what frequencies?

A

5:4.

Produces a pleasing sound

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19
Q

What are sound waves?

A

Sound waves rate formed. By vibrations and require a medium to travel through. They are longitudinal waves.

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20
Q

What is amplitude?

A

The maximum displacement from its rest position of the particle in the medium that carries the wave.

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21
Q

What can amplitude be related to?

A

The pressure of air molecules.

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22
Q

Why can’t longitudinal (sound) waves be polarised?

A

Their direction of travel is in the same direction as the oscillation of the particles their travelling through.

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23
Q

What is polarisation?

A

It’s the restriction of the direction of oscillations of a transverse wave to one plane perpendicular to its direction of travel.

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24
Q

How many times faster is light than sound approximately?

A

About 1 Million times faster.

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25
Q

What are the differences between light and sound?

A

Light is transverse (and so can be polarised), sound is longitudinal
Sound needs a medium, light does not.
Most substances easily absorb light
Light is much faster than sound (about 1 million times)
Light is an electromagnetic wave
Light slows down when travelling through transparent surfaces

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26
Q

How would you measure the frequency of a sound wave?

A

A detector turns its into an electrical signal. Then the time period can be measured from an oscilloscope and the frequency then calculated.

27
Q

What is amplitude?

A

The maximum displacement from its rest position of the particle in the medium that carries the wave.

28
Q

What can amplitude be related to?

A

The pressure of air molecules.

29
Q

Why can’t longitudinal (sound) waves be polarised?

A

Their direction of travel is in the same direction as the oscillation of the particles their travelling through.

30
Q

What is polarisation?

A

It’s the restriction of the direction of oscillations of a transverse wave to one plane perpendicular to its direction of travel.

31
Q

How many times faster is light than sound approximately?

A

About 1 Million times faster.

32
Q

What are the differences between light and sound?

A

Light is transverse (and so can be polarised), sound is longitudinal
Sound needs a medium, light does not.
Most substances easily absorb light
Light is much faster than sound (about 1 million times)
Light is an electromagnetic wave
Light slows down when travelling through transparent surfaces

33
Q

How would you measure the frequency of a sound wave?

A

A detector turns its into an electrical signal. Then the time period can be measured from an oscilloscope and the frequency then calculated.

34
Q

How would you measure the speed of sound?

A

You would find the time it takes for sound to travel a certain distance. You need accurate timings so you would electrical pulses (from microphones picking up the sound) to start and stop timers OR using pulses created by an oscilloscope to measure time.

35
Q

What is a node?

A

A point where the displacement is zero.

36
Q

What is a anti-node?

A

A place the displacement is at maximum or minimum.

37
Q

What is the beat frequency of a resultant wave equal to?

A

The beat frequency is the difference between the frequencies. f(b) = f(2) - f(1)

38
Q

When will beats not be heard?

A

When two strings are in tune.

39
Q

How are beats formed?

A

When two notes of similar, but not identical, frequencies are sounded together they will superimpose. This will cause a regular rising and falling of amplitude which is heard as beats.

40
Q

What is the beat period.

A

The time between the two points of minimum (or maximum) frequency.

41
Q

What is loudness? And what does it depend on?

A

The listener’s perception of the sound’s intensity. It depends on the amount of energy transferred to the surroundings by the vibrations.

42
Q

What frequencies are the human ear most sensitive to?

A

Frequencies in the range of 1 - 4 kHz are amplified by the human ear.

43
Q

The energy of a sound is proportional to what?

A

The amplitude squared (i.e. Doubling the amplitude will quadruple the energy).

44
Q

What is intensity proportional to?

A

The inverse square of the distance from the point source.

I = P / 4πr^2

45
Q

What is the fundamental frequency? What will doubling the frequency do?

A

Lowest frequency for which a standing wave will be set up and consist of a single loop. Doubling the frequency halves the wavelength forming two loops.

46
Q

What are the harmonics? What will a string has at each end?

A

A string has a number of frequencies at which it will vibrate, known as the harmonics. There will be a node at either end of the string as they are fixed.

47
Q

What does the pitch a vibrating string depends on? And what is the formula related to this?

A

The mass of the string (more massive = slower vibrations)
The tension of the string (higher tension = higher pitch)
The length of the string (shorter string = higher pitch)

f = (1/2L)*√(T/μ)
f = freq,  L = length,  T = tension,  μ = mass per unit length
48
Q

How are standing waves formed?

A

When two identical waves that are moving in opposite directions superimpose. In string instruments travelling waves are reflected at the strings end and so undergo a phase change of 180°.

49
Q

How does a stretched string instrument produce a noise?

A

It is made to vibrate by plucking or bowing, which creates a standing wave. The vibration of the string is passed to the instrument body which vibrates as a whole and so causes the air to move.

50
Q

How does a violin work?

A

As a violinist bow moves across a string it pulls the sting to the side because of the friction between the hair and string. This causes to tension to increase until is it greater than the friction. The string is then released and vibrates between the bridge and nut or fingered string. As friction between the bow and string grabs it again, the process repeats.

51
Q

How do brass instruments work?

A

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

52
Q

How do open wind instruments work?

A

Air is blow into one end of a pipe and leaves the other end. The vibrations start at the mouthpiece.

53
Q

How do reed wind instruments work?

A

The reed vibrates causing the pipe to open and close quickly, sending pulses of air down the pipe setting up a standing wave.

54
Q

Sounds below 20Hz are called what? And Sounds above 20kHz are called what?

A

Infrasound (less than 20 Hz)

Ultrasound (greater than 20 kHz)

56
Q

What is noise?

A

Noise is variation of sound that reduces the clarity of a signal. Often random and persistent.

57
Q

What does a standing wave form at the open and closed ends of a pipe?

A

Open end - Pressure node (as pressure is constantly atmospheric) and a Displacement Antinode.

Closed end - Pressure Antinode (as pressure varies most here) and a Displacement Node

58
Q

How are sharp edges used in wind instruments?

A

A stream of air is blown at the sharp edge causing air of vibrate above and below the edge so a standing wave forms in the pipe and a sound is produced.

59
Q

Why do large orchestras or choirs need a conductor?

A

Because the speed of sound meant that if they listened to each other, when one person sang by the time the others sing the sounds will be out of time/phase with each other and so produce an unpleasant sound.

60
Q

How is surround sound initially set up?

A

It is assumed that each listener is equidistant from the speakers. Otherwise there will be a different delay from each speaker and the overall effect will be spilt. However this can be corrected by balancing the system.

61
Q

What are a big problem for concert halls?

A

Echo (sound wave reflections) from the walls and ceiling. As when reflected they can superimpose with the initial sound waves and add or subtract to form a new sound, which could be very unpleasant.

62
Q

What are hanging clouds?

A

They are used in concert halls to improve the acoustics and give a musical presence to the audience so they feel part of the concert. They reduce the time it takes for the first echo to reach the audience, which reduces the feeling of isolation.

63
Q

How can you reduce the effects of noise?

A

At the source - bafflers in exhaust systems to muddle noise produced by engines

Absorption - cubicles in office buildings used as soft “acoustical tiles”

Noise masking - Used in headphones. A microphone samples the noise and uses an electrical circuit to generate the same frequency, but completely out of phase so the 2 waves superimpose and cancel each other out. Works best at low frequencies.

64
Q

In a instrument where does the rich sound and timbre come from?

A

Rich sound - several harmonics simultaneously occurring

Timbre - Balance of harmonic’s amplitudes

65
Q

How do you form the second and third harmonics (or first and second overtone)?

A

Second Harmonic / First Overtone - Double the frequency which halves the wavelength, forming 2 loops

Third Harmonic / Second Overtone - Triple the frequency which thirds the wavelength, forming 3 loops

Physics (5 decks)

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