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Acoustics > Mike > Flashcards

Mike Flashcards

1
Q

What are pressure fluctuations in air caused by? 4 things, identify the most common.

A
  • Vibrating surface (most common)
  • Expansion/contraction of air
  • Turbulent mixing of air
  • Unsteady pressure fluctuations on a surface
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2
Q

What are the four variables that describe a sound field?

A
  • Pressure
  • Velocity
  • Temperature
  • Density
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3
Q

What is the conservation of mass equation given by? What is the linearised equation?

A

d(rho)/dt+d/dx1.(rho.u1)=0

linearised:
d(rho’)/dt+(rho0).du1’/dx1=0

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4
Q

What is the 1D conservation of momentum equation?

What is its linearised equation?

A

rho(du1/dt+u1.du1/dx1) = -dP/dx1

linearised:
(rho0)du1’/dt+dp’/dx1=0

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5
Q

What is the equation that relates pressure and density pertubations, utlising the cons. of mass and momentum eqns?

A

d^2(rho’)/dt^2-d^2p’/dx^2=0

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6
Q

For an isentropic process, how are the pressure perturbations related to the density perturbations. Define all variables/constants

A

p’=c^2.rho’

where c is speed of sound, defined as
c^2=(dp/d(rho))s=const.

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7
Q

Derive 1D wave eqn.

A

see notes

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8
Q

Derive equation that relates speed of sound and kRT

A

see notes

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9
Q

What are the 4 assumptions for the 1D wave eqn

A
  • Perturbations small so linearisation is valid
  • 1D assumption - properties only vary in x1 and t
  • Fluid is inviscid
  • Wave propagation is isentropic
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10
Q

Derive equations for wavelength and time period using a standard solution of the wave eqn.

A

T=1/f

wavelength=c/f

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11
Q

Use the linearised 1D momentum eqn to calculate complex acoustic particle velocity from complex pressure. and then derive the equation for specific acoustic impedance

A

see pg7 (1-8)

u1=+/- p/(rho0.c)

z=p/u1=+/-rho0.c

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12
Q

Use the simple equation 𝐼 = 1/2 Re{𝑝𝑢* } to calculate the time-averaged acoustic intensity for a plane and spherical wave.

A

Plane:
I=+/- 1/(rho0.c) * prms^2
(pg 9; 1-8)

Spherical:
Identical, but different process

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13
Q

What is the definition of an acoustic wave number and its units?

A

k=w/c, units of 1/m

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14
Q

How is a simple source modelled far away?

A

plane wave

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15
Q

Describe how sound radiates from a simple source. Identify all terms in the expression for the complex pressure produced by a simple source of strength Q and state their units.

A

A simple sources radiates sound equallity in all directions. It is an infitesimally small vibrating sphere.

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16
Q

Making the use of time averaged acoustic intensity for a plane and spherical waves, deduce a relationship between SPL and IL

A

They are approximately equal in air. see pg 17

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17
Q

Deduce an expression for the IL of a simple source with sound power W

A

see pg 17

IL = SWL - 11-20log(r/Lref) = SPL

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18
Q

What is the difference between incoherent and coherent sound sources (describe and show mathematically)

A

Coherent sounds have constant phase difference whereas incoherent do not.

Coherent:
P(a+b) = Pa+Pb

Incoherent:
P(a+b)^2=Pa^2+Pb^2

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19
Q

Explain how the sound pressure on a large rigid plane surface differs from the pressure field which would occur in an anechoic environment.

A

In practice, there are objects and surfaces nearby which reflect the sound waves.

A simple source above a rigid plane is equivalent to a source and an image source in the infinite medium. (image source = same amplitude and phase as the original source)

The pressure doubles on a plane surface compared to no surface (coherent addition)

20
Q

Recommend appropriate microphone placement for outdoor sound measurements.

A

Place the microphone on a rigid plate on the ground and subtract 6dB to measurements.

If the microphone is placed on the ground, then there is only one propagation path. Predicting reflected rays is difficult as the ground isnt always rigid.

21
Q

When can a distributed source be modelled as a simple source?

A

At low frequencies

22
Q

Describe the sound field radiated from a baffled piston source

A

At low frequencies the directivity is equal to 1 so the vibrating piston acts as a simple source. As frequency increases, the directivity term significantly influences the radiated sound field

23
Q

What is the equation for the characteristic specific acoustic impedance

A

rho0.c

It is the ratio of acoustic pressure to acoustic particle velocity

24
Q

What are the 5 forms of sound attenuation?

A
  • air absorption
  • ground reflections
  • meteorological effects
  • solid barrier
  • vegetation
25
Q

How is sound absorption defined?

A

ratio of intensity absorbed by material to input intensity

26
Q

How is power transmission defined? How is the sound reduction index defined?

A 
Power transmission (tau): 
Ratio of transmitted intensity through the wall to input intensity

Sound Reduction Index (R) = -10log(tau)

27
Q

How is the sound absorption coefficient calculated?

A

Install a sample of the material in the reverberation room and measure the difference between the reverberation times with and without the sample installed.

28
Q

What are rigid plate boundary conditions?

A

u=0, therefore impedance infinite

29
Q

Describe the mass law

A

the wall is modelled as a series of individual mass ‘elements’ which vibrate independently when exciting by the oscillating fluid this results in a 6dB per octave increase in sound transmission loss).

30
Q

Describe what thickness of porous absorber is required to produce maximum sound absorption for normally incident sound waves

A

absorber should be located so that it occupies the region 1/4 of a wavelength from the rigid surface it is mounted on/close to

31
Q

What are panel absorbers used for?

A

panel absorbers can be used to absorb low frequency sound and note that these are typically ‘tuned’ to absorb a narrow frequency band

32
Q

Describe how the finite size of a panel affects transmission loss

A

resonant modes dominate transmission loss behaviour at low frequencies

33
Q

Draw graph of transmission loss versus frequency and label the 3 regions for a single leaft wall.

A

see pg 14

34
Q

Describe how panel stiffness affects the transmission loss of a single leaf wall. Define the critical frequency, coincidence effect and stiffness controlled region.

A

At high frequencies, the panel stiffness has a greater effect (controls) on the transmission loss than at lower frequencies.

The critical frequency occurs when all of the incident sound travels through the medium. The dip in transmission loss is called the coincidence effect

35
Q

Describe the sound transmission paths through a double leaf wall system

A

vibrating panel excites air in cavity which vibrates adjacent panel, resonance can occur for this system. Also sound can transmit through solid connections between panels

36
Q

Suggest 4methods which can be used to improve the sound transmission loss of a double leaf wall system

A
  • Walls should be designed such that as much disconnection as possible is provided between panels, achieved by reducing number of connection points, using flexible connections, using staggered studs.

Other methods include:

  • Maximise mass of wall panels
  • use different wall thicknesses on either side to avoid both panels having identical critical frequencies
  • Fill cavity between walls with sound absorbing material
37
Q

What is STC and describe the concept behind the STC rating (why the reference curve has the shape it has)

A

STC: Sound Transmission Class

  • It assumes the major noise source which we are designing to minimise is of human speech
  • Has a transmission loss curve which gives us the required level of transmission loss to change a typical speech spectrum into one having an overall shape like that of the NCB curves
  • Each STC curve is associated with a single STC value
  • Since there is relatively low energy at low frequencies, the STC does not take into account sound insulation below frequencies of 125Hz. With powerful home audio systems, STC somewhat inadequte for rating the performance of walls and floor/ceiling structure
38
Q

What is the standard method for assessing the impact insulation of a floor/ceiling system

A

Using a tapping machine which can be used to determine the weighted impact sound level

39
Q

Suggest methods which could be used to reduce impact noise.

A

Covering a floor with a soft lining, or an underlay beneath hard floors.

40
Q

Describe the sound field within a rectangular room (how is it modelled)

A

The sound field within a rectangular room can (in theory) be modelled as a sum of modes. At low frequencies only a couple of modes are significant which can lead to resonances. As frequency increases statistical methods for predicting noise levels are much more accurate.

41
Q

Define (in words) Schroeder’s frequency, and when are statistical methods accurate?

A

The frequency at which a significant number of modes are excited by a single tone. Above this frequency statistical methods are accurate

42
Q

Define reverberation time.

A

Time for sound to decay 60dB

43
Q

What is unique about a diffuse sound field?

A

The SPL is independent of location

44
Q

What is the reverberation radius?

A

It is the radius where the reverberant SPL is equal to the free-field SPL

45
Q

What are the three requirements for a Sabine room

A
  • The major dimensions are the same order
  • The absorption in the room is reasonably evenly distributed over the boundaries
  • All parts of the volume are equally connected (all positions in the space can be seen from all other positions).
46
Q

Describe how sound insulation measurements are made

A

The level of sound insulation between two rooms is defined as the difference in level between a source room and a reciever room.

A diffuse field is generated in the source room, and this sound transmits throug the wall to the reciever room in which the sound field is also diffuse. The rooms are isolated such that flanking noise may be neglected

Acoustics (3 decks)

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