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Acoustics > Fundamentals of Acoustics > Flashcards

Fundamentals of Acoustics Flashcards

1
Q

Noise

A

Excessive or unwanted sound which potentially results in annoyance and/or hearing loss

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2
Q

Sound

A

A pressure variation wave that travels through a medium and is detected by the ear

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3
Q

Longitudinal

A

Particle motion in direction of wave motion

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4
Q

Amplitude

A

Amount of sound pressure (dB)

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5
Q

Frequency

A

Rate of vibration per unit time measured in cycles per second (Hz)

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6
Q

Sound Power Level

A

Total sound power emitted by a source in all directions

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7
Q

Give 3 intuitive facts of sound

A
  • Travels undistorted
  • Travels with some time delay
  • Travels independently if from different sources
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8
Q

Monopole Source

A

A source which radiates equally in all directions

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9
Q

Dipole Source

A

Consists of two monopole sources of equal strength but opposite phase

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10
Q

Locally reacting solid interface

A

The movement at each point of the surface is in a normal direction to the surface and is independent of both the motions of the other points and the angle of incidence

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11
Q

Lossy medium

A

A medium where part of the sound energy will be dissipated to viscous and thermal effects

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12
Q

Shear viscosity coefficient

A

A measure of the diffusion of momentum by molecular collision between regions of the fluid possessing different net velocities

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13
Q

Bulk viscosity coefficient

A

A measure of some conversions of energy between molecular motion, internal molecular states, and structural potential energy states

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14
Q

Speed of sound

A

343m/s

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15
Q

Quadrupole

A

Consists of two out of phase dipole sources

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16
Q

Sound Pressure

A

Difference (scalar) between the pressure caused by a sound wave and the ambient pressure of the media the sound is travelling through (p_total = p_atm + p_acoustic)

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17
Q

Sound Intensity

A

Sound energy flow in Watts per square metre, measured in the direction of the sound wave propagation. Proportional to acoustic pressure squared

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18
Q

Sound Power

A

Total acoustic power (energy per second) radiated by a source in all directions and measured in Watts

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19
Q

Tonal Noise

A

A sound or noise recognised by its regularity. A pure tone has one frequency.

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20
Q

Broadband Noise

A

Noise whose energy is distributed over a wide section of the audible range

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21
Q

Propagation Vector

A

A vector indicating the direction of wave propagation and the phase delay per unit length

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22
Q

Surface of constant phase

A

A wavefront plane of constant amplitude

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23
Q

Specific Acoustic Impedance

A

Ratio of effective sound pressure at a point in an acoustic medium to the effective particle velocity at that point

24
Q

Acoustic Impedance

A

Ratio of sound pressure to sound volume velocity

25
Q

Acoustic Impedance (resistance)

A

Resistance of flow of sound through a medium (real part of acoustic impedance)

26
Q

Acoustic Impedance (Reactance)

A

Imaginary part of acoustic impedance

27
Q

Snells Law

A

A formula used to describe the relationship between angles of incidence and refraction when referring to waves passing through a boundary between media

Sin(theta_i)/c_1 = Sin(theta_t)/c_2

28
Q

Low Pass Filter

A

Signals above cut-off frequency are attenuated. Power transmission outside of this range is approximately 1.

29
Q

High Pass Filter

A

Signals below cut off frequency are attenuated. Power transmission outside of this range is approximately 1.

30
Q

Band Stop Filter

A

Signals inside of band width are attenuated. Power transmission outside of this range is approximately 1.

31
Q

Critical angle

A

Angle of incidence which causes a refraction angle of 90 degrees

32
Q

Threshold of Audibility

A

20Hz

33
Q

Threshold of Pain

A

130dB

34
Q

Equal Loudness

A

A contour of equal loudness over a range of frequencies.

35
Q

Phon Concept

A

Two sounds may have the same sound intensity but may not sound equally loud because the human hearing sensitivity varies with frequency

36
Q

Frequency weightings

A

Correlate objective measurements with human subjective response

37
Q

Sound Reflection

A

when a sound wave reaches the boundary between one medium and another medium, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission across the boundary

38
Q

Scattering

A

Irregular diffraction or reflection of sound waves in many different directions

39
Q

Sound Absorption

A

The proportion of energy absorbed by a material or media encountered by the sound wave

40
Q

Room Modes

A

Decay of sound in a room due to the sound waves interacting with the rooms surfaces

41
Q

Room Modes (Axial)

A

Pairs of parallel walls

42
Q

Room Modes (Tangential)

A

Four room surfaces

43
Q

Room Modes (Oblique)

A

Six Surfaces

44
Q

Reverberation Time

A

Time required for the sound pressure level to decrease by 60dB after the sound source has stopped

45
Q

Sabine Equation

A

Eq relating reverberation time with sound absorption and room volume. T=0.161V/A

46
Q

Masking Noise

A

A noise which is intense enough to render inaudible or unintelligible another sound present

47
Q

Lateral Quadrupole

A

4 monopole sources with alternating phases in the corner of a square (clover leaf pattern)

48
Q

Longitudinal Quadrupole

A

2 opposite phase dipoles lying on a single line
Near field - 4 maxima, 4 minima
Far field - 2 maxima, 2 minima

49
Q

Sound Pressure Level (SPL) calculation

A

L_p = 20log(p/p_0)

50
Q

Sound Power Level (SWL) calculation

A

L_w = 10log(W/W_0)

51
Q

Loudness

A

Impression of magnitude we hear with our ears

52
Q

Adding decibel values together

A

L_total = 10*log( 10^(dB1/10) + 10^(dB2/10) + … )

53
Q

Omni-directional sound

A

Source emits sound equally in all directions

54
Q

Directional sound

A

Power emitted in each direction is different

55
Q

What is the absorption coefficient

A

alpha = (absorbed energy) / (incident energy)

56
Q

Image Method

A

Rays are perfectly reflected at boundaries. When a ray is reflected, it spawns a secondary source “behind” the boundary surface, located on a line perpendicular to the wall, at the same distance from it as the original source

A single reflection is a first order source, more than one reflection represents a higher order source

57
Q

Rayleigh reflection coefficient

A

Defines the angle of transmission through an oblique incidence

Acoustics (3 decks)

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