Fundamentals of Acoustics

Question 1

Noise

Answer 1

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

Question 2

Sound

Answer 2

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

Question 3

Longitudinal

Answer 3

Particle motion in direction of wave motion

Question 4

Amplitude

Answer 4

Amount of sound pressure (dB)

Question 5

Frequency

Answer 5

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

Question 6

Sound Power Level

Answer 6

Total sound power emitted by a source in all directions

Question 7

Give 3 intuitive facts of sound

Answer 7

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

Question 8

Monopole Source

Answer 8

A source which radiates equally in all directions

Question 9

Dipole Source

Answer 9

Consists of two monopole sources of equal strength but opposite phase

Question 10

Locally reacting solid interface

Answer 10

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

Question 11

Lossy medium

Answer 11

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

Question 12

Shear viscosity coefficient

Answer 12

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

Question 13

Bulk viscosity coefficient

Answer 13

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

Question 14

Speed of sound

Answer 14

343m/s

Question 15

Quadrupole

Answer 15

Consists of two out of phase dipole sources

Question 16

Sound Pressure

Answer 16

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)

Question 17

Sound Intensity

Answer 17

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

Question 18

Sound Power

Answer 18

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

Question 19

Tonal Noise

Answer 19

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

Question 20

Broadband Noise

Answer 20

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

Question 21

Propagation Vector

Answer 21

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

Question 22

Surface of constant phase

Answer 22

A wavefront plane of constant amplitude

Question 23

Specific Acoustic Impedance

Answer 23

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

Question 24

Acoustic Impedance

Answer 24

Ratio of sound pressure to sound volume velocity

Question 25

Acoustic Impedance (resistance)

Answer 25

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

Question 26

Acoustic Impedance (Reactance)

Answer 26

Imaginary part of acoustic impedance

Question 27

Snells Law

Answer 27

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

Question 28

Low Pass Filter

Answer 28

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

Question 29

High Pass Filter

Answer 29

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

Question 30

Band Stop Filter

Answer 30

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

Question 31

Critical angle

Answer 31

Angle of incidence which causes a refraction angle of 90 degrees

Question 32

Threshold of Audibility

Answer 32

20Hz

Question 33

Threshold of Pain

Answer 33

130dB

Question 34

Equal Loudness

Answer 34

A contour of equal loudness over a range of frequencies.

Question 35

Phon Concept

Answer 35

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

Question 36

Frequency weightings

Answer 36

Correlate objective measurements with human subjective response

Question 37

Sound Reflection

Answer 37

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

Question 38

Scattering

Answer 38

Irregular diffraction or reflection of sound waves in many different directions

Question 39

Sound Absorption

Answer 39

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

Question 40

Room Modes

Answer 40

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

Question 41

Room Modes (Axial)

Answer 41

Pairs of parallel walls

Question 42

Room Modes (Tangential)

Answer 42

Four room surfaces

Question 43

Room Modes (Oblique)

Answer 43

Six Surfaces

Question 44

Reverberation Time

Answer 44

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

Question 45

Sabine Equation

Answer 45

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

Question 46

Masking Noise

Answer 46

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

Question 47

Lateral Quadrupole

Answer 47

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

Question 48

Longitudinal Quadrupole

Answer 48

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

Question 49

Sound Pressure Level (SPL) calculation

Answer 49

L_p = 20log(p/p_0)

Question 50

Sound Power Level (SWL) calculation

Answer 50

L_w = 10log(W/W_0)

Question 51

Loudness

Answer 51

Impression of magnitude we hear with our ears

Question 52

Adding decibel values together

Answer 52

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

Question 53

Omni-directional sound

Answer 53

Source emits sound equally in all directions

Question 54

Directional sound

Answer 54

Power emitted in each direction is different

Question 55

What is the absorption coefficient

Answer 55

alpha = (absorbed energy) / (incident energy)

Question 56

Image Method

Answer 56

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

Question 57

Rayleigh reflection coefficient

Answer 57

Defines the angle of transmission through an oblique incidence