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Acoustics > George Stuff > Flashcards

George Stuff Flashcards

1
Q

What is objective sound?

A

Energy transmitted by means of mechanical disturbances to a transmitting medium which produces no resultant bodily movement of the transmitting medium and leaves the medium at rest after the energy has been transmitted.

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2
Q

What is subjective sound?

A

Audible sensations resulting from the reception of objective sound by our hearing mechanism.

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3
Q

What are the two types of sound and their uses?

A

Direct (commuication, expression of art/music) and Indirect (surroundings - can invoke emotions/impressions).

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4
Q

What is acoustics related to?

A

Anything to do with the generation, transmission, reception and the effects of energy in the form of vibrational waves.

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5
Q

What is a soundscape?

A

Listeners perception of sounds heard as an environment.

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6
Q

What is the difference between sound and light?

A

Information from sound is typically directly from the source, whereas information from light is typically from reflections (ie indirect).

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7
Q

What is noise?

A

Auditory stimulus bearing no useful information. It is subjective sound that we merely hear. We can choose to pay attention to it.

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8
Q

What is acroma?

A

Useful sounds we choose to take info from.

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9
Q

What is acoustic privacy?

A

The state whereby no information about you and your neighbors, including anthing about you or their presences, is communicated by sound.

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10
Q

What role does the imaginary number i play in waves?

A

It essentially shifts the phase of the signal by 90deg.

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11
Q

What is impedance?

A

It is the ratio between the driving force (typically in the form of pressure) and induced velocity. It is the opposition that the system puts up to being vibrated.

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12
Q

What are the four features characterising resonance?

A
  • The velocity response of the system is at a maximum when driven at the natural frequency of the free vibration of the undamped system.
  • Impedance is at a maximum.
  • Velocity and driving force are in phase.
  • Maximum power is delivered to the system.
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13
Q

What is the Maximum Power Theorem

A

In order to transfer the maximum amount of power from one system to another, their impedances must be matched.

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14
Q

What is a plane wave?

A

Any and every acoustic variable has the same value and phase at all points on any plane perpendicular to the direction of propagation of the wave.

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15
Q

What is the Weber-Fechner law?

A

Size of subjective sound is proportional to the log of the size of the objective sound.

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16
Q

What is Steven’s Power Law

A

Loudness is proportional to (sound pressure)^0.6

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17
Q

Why is loudness metric a thing?

A

This is because people do not have a good grasp of SPL, but do have a good grasp of loudness.

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18
Q

What do Sones represent? And what is 1 Sone equal to?

A

The absolute measure of loudness. 1kHz pure tone at SPL=40dB is equal to 1 sone.

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19
Q

Draw equal loudness contours on a graph of SPL vs Freq.

A

See notes. Wiggly stack of equispaced curves decreasing with freq.

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20
Q

Draw A, B, C and Z-weighted curves for SPL vs Freq.

A

See notes. Z is flat. A is most parabolic and most attenuation at lower frequencies. B and C in between A and Z.

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21
Q

What is Leq and what does it represent?

A

Equivalent continuous sound level. It corresponds to the average sound intensity level in the sound environment.

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22
Q

What do we use Lq as a measure for?

A

We use it as a measure of the total amount of work our ears will have done in the environment.

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23
Q

What is the legal maximum Leq?

A

85dB(A)

24
Q

What is the real process of finding Sound Pressure?

A

Involves finding the rms pressure. An exponential weighting is applied to the signals. This weighting is either fast (averaged over 125ms) or slow (averaged over 500ms).

25
Q

What are sound level percentiles? What does Lx represent? What does L(5) and L(90) represent?

A

Lx is the level within the range of variation of the sound level for which is equalled or exceeded for x% of the time. L(5) corresponds to peak levels and L(95) corresponds to noise levels.

26
Q

What is noise dose, why does it exist, and what is its equation?

A
  • A 3dB increase is a doubling of exposure, however it is basically undetectable to the human ear. Noise dose expresses the amount of energy recieved by the ear as a % of that at the legal maximum for an 8hr day.
    ND = 100 x 10^((Leq-85)/10)
27
Q

What are the two methods of sound insulation, and which method is the most common and why?

A

Dissipation and reflection. Dissipation is relatively expensive, so the main means of providing sound insulation is through reflection.

28
Q

Explain why we need different sized drivers in speakers

A

As frequency decreases and radius decreases, the cos(theta) term tends to zero, which means assuming the particle velocity remains constant, this will result in a pressure loss. Thus, different sized drivers are required to maintain the right pressure for lower, middle and upper frequencies.

29
Q

Compare the acoustic impedance of spherical and plane waves

A

Acoustic impedance for spherical waves is smaller than that for plane waves. The smaller the spherical wave and smaller the frequency, the larger the difference.

30
Q

Explain how resonance in tubes happens

A
  • Within a tube, waves are constrained to being plane wave
  • The incident sound waves coming from a distant source will enter easily because the plane and spherical impedances will match
  • However the waves must exit as spherical waves in order to propagate into free space
  • Impedance difference depends on the radius of the bottle
  • The portion of energy that doesn’t exit (because of the impedance difference) is reflected back into the tube
  • Process of multiple reflections up and down causes the resonance
31
Q

Explain why a round trip in a tube must result in a whole number of 360deg shifts in phase for resonance

A
  • Inside the tube many reflections occur
  • Resonance happens when they combine together to produce the highest sound level, which happens when they fully add together
  • This happens when the reflections are shifted in phase by n x 360deg (n=1,2,3…)
32
Q

Explain how a quarter wavelength resonator works

A
  • Between reflection N and reflection N + 1, the sound must travel down the tube and reflect off the closed end back towards the open end and then reflect at the open end to start the journey over again (to produce N + 1).
  • For reflection N+1 to be in phase with N, this process must result in a 360 deg phase shift.
  • When the wave is reflected off the closed end of the tube, it gets reflected without suffering any phase shift, but when it reflects off the open end it experiences a 180deg phase shift.
  • We need N+1 to be in phase with N, so we need an extra 180deg phase shift to add to the open end 180 deg phase shift to give a total of 360deg.
  • This can only be achieved by travelling down and back up the tube
  • When a wave travels a distance of one wavelength (I) its phase shifts by 360deg. As we only want to produce 180deg shift (I/2)
  • The travel up and down the tube (distance of 2D) must therefore be equal to I/2
  • Therefore to produce resonance, I/2 = 2D -> I = 4D
33
Q

What are the 3 main sections of the human hearing system?

A
  • External
  • Middle
  • Inner
34
Q

What does the external ear consist of?

A
  • Pinna

- External auditory canal

35
Q

What does the middle ear consist of?

A
  • Ossicles

- Ear drum

36
Q

What does the inner ear consist of?

A
  • Cochlea

- Utricle, Saccule and semi-circular canals for balance

37
Q

What is the pinna and what is it for?

A
  • Reflecting and funneling sound waves into the auditory canal
  • Gives sense of direction by blocking sounds from behind and strengthening sounds from the front
  • It also has fine detail changes in its frequency response with different directions of incoming sound
38
Q

What is the auditory canal and what is it for?

A
  • Open at one end and closed (ear drum) at the other.
  • Operates as a quarter length resonator, raising the sensitivity of the ear in the resonance frequency
  • Has ear wax (cerumen) which protects ear from drying and infection
39
Q

What is the proper name for the ear drum and what is its purpose?

A
  • Tympanic membrane
  • Seals middle ear from auditory canal
  • Converts sound waves to movement of ossicles
40
Q

What are the three bones in the ossicles called and what is its purpose?

A
  • Malleus
  • Incus
  • Stapes

Purpose of ossicles is to act as a lever to amplify the vibrations delivered to the cochlea

41
Q

What is the tube called that connects the middle ear to the pharynx, and why does it exist?

A

Eustachian tube

- exists to ensure pressure on either side of the tympanic membrane is equal so the membrane can move freely

42
Q

What is the oval window and what is it for?

A
  • Small membrane at the start of the inner ear that is vibrated by the stapes (part of the ossicles)
  • Small compared to ear drum to generate a pressure gain (Pascals Law)
43
Q

What is the cochlea and what is it for?

A
  • Wound tube in inner ear
  • Filled with fluid
  • Middle compartment houses Organ of Corti, which has lines of hair cells on the basilar membrane
  • Responds to a travelling wave with resonance at a position where the mechanical properties of the basilar membrane (ie mass and stiffness) match for the best response for the particular incoming sound frequency
44
Q

Name the three places in the ear where impedance matching/sound amplification occurs

A
  • Closed tube resonance of auditory canal
  • Tympanic membrane and oval window area reduction
  • Lever-type amplification in ossicles
45
Q

Name the two types of hair cells and their purposes

A

Inner:
- Main sound transducers

Outer:
- Tunes the cochlea in response to the type of sound coming in

Cochlea is an active tranducer

46
Q

What are oto-acoustic signals and why do they occur?

A
  • Control signals from the brain cause movement of the hair cells which transmits vibrations back out of the cochlea and into the ear canal into weak objective sound waves which we can hear
47
Q

What are the two forms of hearing loss?

A

1) Conductive hearing loss (ie impediment to part(s) of the transmission chain
2) Sensori-neural hearing loss

48
Q

What are 8 causes of hearing loss

A

1) Very large amplitude sounds which drive the contents of the cochlea beyond the mechanical limits, causing permanent damage.
2) Long exposure of sound which causes fatigue
3) Congenital factors
4) Infections
5) Trauma
6) Fluid imbalance
7) Ageing
8) Oto-toxic drugs

49
Q

What are two early signs of hearing loss?

A

Tinnitus and loss of detectable oto-acoustic emissions

50
Q

What is the loud sound response in the ear? What is its limitation?

A
  • Muscle tightens the ear drum (tympanic membrane) which shifts ossicles away from the ear drum, reducing the transmitted force
  • Its limitation is that it doesnt protect the ear from sudden noises ie gunshots
51
Q

What are the two modes of our hearing system and what do they relate to?

A
  • Wideband (speech)

- Narrowband (music)

52
Q

What is unique about music mode?

A
  • Music mode requires a lot of tuning up

- Thus, it takes a longer time for the ear to process and create subjective sound when in music mode

53
Q

What is integration time?

A
  • It is the time taken to process different objective sounds into subjective sounds.
  • If sounds arrive at the ear while a previous sound is being processed, then they will be added together
54
Q

What are the two ways of defining integration time?

A

1) Minimum time delay required between the arrival of two objective sounds at the ear for them to begin to be processed as separate sounds
2) Maximum time between two sounds arriving at the ear for them to be processed completely into a single subjective sound

55
Q

What is the Haas effect?

A

The ear assigns the direction of the first arriving component to to direction of the entire sound.

Acoustics (3 decks)

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