p.19
Practical Sources of Sound
What characterizes a dipole source of sound?
Structural vibration and oscillating pressure on stationary or moving surfaces.
p.19
Practical Sources of Sound
What is a quadrupole source of sound?
Derived from two tightly coupled dipoles, producing oscillating pressure in the absence of structural surfaces.
p.4
Mass and Momentum Conservation
What does the 1D momentum equation represent?
The relationship between mass flow and momentum in a fluid.
p.4
Mass and Momentum Conservation
What is the significance of the net rate of mass influx in mass conservation?
It represents the mass entering or leaving a control volume.
p.6
Speed of Sound in Gases
What does the wave speed 'c' derive from?
The Latin word 'celeritas' meaning 'velocity'.
p.10
Mathematical Derivation of Acoustic Principles
What is the 3D wave equation derived from?
By eliminating velocity components (u, v, w) from the mass and momentum equations and isentropic thermodynamics.
p.19
Practical Sources of Sound
What type of noise do blades produce during steady rotation?
Dipole noise due to unsteady forces.
What does Tij represent in the context of acoustic analogy?
Tij represents the flow in a real situation where it cannot be separated from sound.
p.10
Mathematical Derivation of Acoustic Principles
When is it useful to simplify the formulation of the wave equation?
When sound is constant along one direction, usually the azimuth angle.
p.6
Speed of Sound in Gases
What is the general solution for the 1D wave equation?
p = f(x ± c₀t), where f is an arbitrary function.
p.27
Mathematical Derivation of Acoustic Principles
What mathematical concept is being derived in section 2.3A?
The Lighthill stress tensor.
p.27
Mathematical Derivation of Acoustic Principles
What does the equation involving 'p', 'u', and 'c' represent?
The relationship between pressure, velocity, and speed of sound in a medium.
p.21
Practical Sources of Sound
How does the noise level of elevated structures compare between steel and concrete?
Lightweight steel structures are noisier than concrete with ballasted trackbeds or concrete decks.
p.22
Practical Sources of Sound
What is the main source of excitation for rail vibration?
Rail roughness and corrugation patches (50 mm or more, tens of microns deep).
p.18
Monopole and Dipole Sources
What is the compromise made when enclosing a loudspeaker?
A small vent is left to prevent the diaphragm vibration from becoming too heavy to drive.
p.17
Monopole and Dipole Sources
How does a real source differ from a theoretical dipole?
Real sources have finite size, leading to time differences in sound reception.
p.4
Mass and Momentum Conservation
What does the term ∂p/∂x represent in the 1D momentum equation?
The net force (per unit volume) acting to accelerate a particle.
p.23
Practical Sources of Sound
What technology is replacing percussive piling?
Low-noise rotary machines.
p.19
Practical Sources of Sound
What is a monopole source of sound?
A vibrating piston in 1D, large rigid body vibration in 3D, or unsteady combustion.
p.9
Acoustic Energy Conservation
What is acoustic energy defined as?
The difference of fluid energy when there is sound and when there isn’t.
p.30
Monopole and Dipole Sources
What does the total power flux available depend on according to Ffowcs Williams?
It is related to the factor ܷ^3.
p.5
Mass and Momentum Conservation
What are the three parts into which all equations are expanded?
Steady-flow part, first order part, and second-order terms.
p.16
Monopole and Dipole Sources
How does the sound field behave near a dipole source?
The sound field exhibits specific patterns due to the interaction of the two monopole sources.
p.20
Practical Sources of Sound
What are the main sources of noise in vehicles?
Engine vibration, exhaust noise, inlet noise, cooling fan noise, and tyre noise.
p.20
Practical Sources of Sound
What causes engine vibration in vehicles?
Explosive combustion and mechanical impaction.
p.6
Speed of Sound in Gases
What happens when a time-reversed record is generated at the boundary of a pond?
It generates a reverse water wave focusing inward to the center of the pond.
p.21
Practical Sources of Sound
What components of a train's propulsion system are dominant at low speeds?
Traction motors, cooling fans, and reduction gears.
p.21
Practical Sources of Sound
Which auxiliary equipment is dominant at very low speeds and when stationary?
Compressors, motor generators, brakes, and ventilation systems.
p.12
Homogeneous Wave Equation
What is the wave propagation equation derived from?
Combining mass and momentum conservation laws with isentropic thermodynamics.
p.1
Thermodynamics and Sound Propagation
What is the relationship between pressure disturbance in sound waves and thermodynamics?
It involves changes in density and temperature, unlike incompressible flow approximations.
What did Curle (1955) extend in the Lighthill formulation?
The inclusion of a stationary surface.
p.14
Practical Sources of Sound
What is derived from the new state equation in combustion?
Enthalpy (energy) addition.
p.5
Mass and Momentum Conservation
What is assumed about the second-order terms in the equations?
They can be ignored when compared with the first order.
p.17
Monopole and Dipole Sources
What is an example of a dipole source with unequal reception?
A pair of sources with strengths (+5, -3) can be represented as dipole (+4, -4) + monopole (+1, +1).
p.9
Acoustic Energy Conservation
What does the total acoustic energy density depend on?
It depends on the perturbation pressure and volume.
p.20
Practical Sources of Sound
What is one method for highway noise control?
Using porous road surfacing.
p.19
Practical Sources of Sound
When is a quadrupole an efficient noise source?
Only at very high speed or strength.
p.33
Practical Sources of Sound
What is the blade-passing frequency (BPF)?
It is calculated as the number of blades multiplied by the rotational speed.
p.6
Speed of Sound in Gases
Who corrected Newton's assumption about sound speed differences?
Laplace, a century later.
p.30
Monopole and Dipole Sources
What happens to a monopole when the surface is rigid?
It degenerates into a dipole called thickness noise.
p.9
Acoustic Energy Conservation
What are the two types of energy present in acoustic energy?
Kinetic energy and potential energy.
p.16
Monopole and Dipole Sources
What is the relationship between monopoles and dipoles?
A dipole can be viewed as two monopoles tightly coupled together.
p.22
Practical Sources of Sound
What maintenance practice can help reduce rail vibration?
Regular maintenance by grinding train on rail tracks.
p.9
Mathematical Derivation of Acoustic Principles
What is the first step in deriving the energy equation?
Multiplying the linearized momentum equation by the perturbation velocity.
p.26
Mathematical Derivation of Acoustic Principles
What condition is described by '0, 0, , 1 when , 0 otherwise'?
It indicates specific conditions under which certain equations apply.
p.12
Homogeneous Wave Equation
When is momentum conservation questioned?
When an external unsteady force is applied on a stationary surface.
p.26
Speed of Sound in Gases
What does the term 'incompressible' refer to in the context of sound?
It refers to a fluid or medium where density changes are negligible.
p.13
Monopole and Dipole Sources
What is a monopole source characterized by?
Externally introduced mass and applied force per unit volume in space.
p.24
Aeroacoustics and Jet Noise
What was the approach to suppressing jet noise before Lighthill's work?
A matter of trial and error.
p.12
Homogeneous Wave Equation
What effect does irreversible heat transfer have on sound?
It either consumes or generates sound.
p.11
Acoustic Energy Conservation
What happens to sound power when the wave number is small?
The sound power is very small.
p.13
Mass and Momentum Conservation
What is the role of external forces in the context of monopole and dipole sources?
They are accounted for per unit volume in the equations governing sound propagation.
What is the essence of the acoustic analogy?
It relates the Lighthill equation to sound generation and propagation.
p.21
Practical Sources of Sound
What type of vibration and noise is produced by subway operations?
Ground-borne vibration and noise.
p.19
Practical Sources of Sound
What is the radiation efficiency of a dipole source?
Moderate radiation efficiency.
p.6
Speed of Sound in Gases
What is acoustic focusing used for?
To generate powerful ultrasound to smash a kidney stone.
p.18
Monopole and Dipole Sources
What happens when the back of a loudspeaker is enclosed?
The receiver sees only the front end, making it a monopole-like powerful radiator.
p.16
Monopole and Dipole Sources
What is the significance of the vertical line of symmetry in dipole sources?
Sound generated by the two sources cancels exactly on this line, which is perpendicular to the axis joining the two sources.
p.27
Mathematical Derivation of Acoustic Principles
What is the significance of the term 'Lighthill stress tensor'?
It describes the distribution of momentum in a fluid and its relation to sound generation.
p.21
Practical Sources of Sound
What types of noise are generated from interior structures of trains?
Structure-borne wheel/rail vibrations, air-borne transmission, and interior installations.
p.32
Aeroacoustics and Jet Noise
What occurs when a vortex passes over the trailing edge of an airfoil?
The trailing edge sheds a vortex into the wake, canceling the effect of the passing vortex.
p.15
Monopole and Dipole Sources
What is the key factor that creates sound in a monopole source?
Only the variation of volume flow creates sound.
p.26
Mathematical Derivation of Acoustic Principles
What does Eq1 and Eq2 represent in the derivation of acoustic analogy?
They represent equations related to the behavior of sound sources.
p.12
Homogeneous Wave Equation
How can mass conservation be violated in sound propagation?
By sphere pulsation if the sphere itself is included in the air medium.
p.24
Aeroacoustics and Jet Noise
What is the title of Lighthill's seminal paper?
On sound generated aerodynamically: I. General theory.
p.1
Thermodynamics and Sound Propagation
What happens to entropy during sound propagation?
In sound propagation, dQ = 0 (adiabatic), leading to isentropic conditions.
What is the significance of the FWH equation?
It is the most general of all acoustic analogies.
p.1
Thermodynamics and Sound Propagation
How does temperature change in sound waves?
Temperature changes due to mechanical compression and expansion by surrounding particles, not by heat exchange.
p.2
Thermodynamics and Sound Propagation
What does the internal energy of an ideal gas depend on?
It is a function of temperature only, u = u(T).
p.1
Thermodynamics and Sound Propagation
What is the effect of fluid viscosity on sound propagation?
Effects are generally small, and disturbances are reversible, leading to isentropic conditions.
p.20
Practical Sources of Sound
What is the significance of crank angle in combustion?
It relates to pressure changes during the combustion process.
What is the role of the observer in the context of jet noise?
The observer's position affects the perception of noise.
p.34
Aeroacoustics and Jet Noise
Does structural vibration always create monopole sound?
No, structural vibration does not always create monopole sound.
p.34
Aeroacoustics and Jet Noise
What is ‘trailing edge noise’?
Noise generated at the trailing edge of an airfoil due to turbulent flow.
p.34
Aeroacoustics and Jet Noise
Why is rotational fan noise limited to multiples of the blade-passing frequency?
Because the noise is generated by discrete events at the blade passing.
p.31
Aeroacoustics and Jet Noise
What is the Cambridge-MIT initiative focused on?
Developing 'silent aircraft' that are no louder than background street noise.
p.6
Speed of Sound in Gases
What does time-reversal in wave propagation allow?
It allows a recorded wave to be played back in reverse time order.
p.4
Mass and Momentum Conservation
What does mass conservation consider in fluid dynamics?
The change of mass contained in the space between sections 1 to 2.
p.33
Practical Sources of Sound
How can a disturbance be characterized in sound sources?
It usually comes in impulsive form and can be decomposed into harmonic functions.
p.18
Monopole and Dipole Sources
What is a real source 'dipole' in sound sources?
It is the residual of incomplete cancellation of a monopole, hence much weaker than a pure monopole.
p.30
Monopole and Dipole Sources
What is the relationship between Ffowcs Williams' findings and the parameter ܯ?
For ܯ >> 1, ܲ ~ ܯ^3 due to the (1 - ݎܯ) factor.
p.12
Homogeneous Wave Equation
What is sound defined as?
The propagation of small pressure perturbations accompanied by changes in acoustic particle velocity, temperature, and density in a linear, isentropic manner.
p.17
Monopole and Dipole Sources
What forms a dipole-like source?
When a rigid body vibrates without changing its total volume.
p.10
Mathematical Derivation of Acoustic Principles
In which coordinates can the Laplacian be translated for simplification?
Cylindrical (x, r, θ) or spherical coordinates (r, θ, φ).
p.1
Thermodynamics and Sound Propagation
What does the second law of thermodynamics state?
The change in entropy (ds) is greater than or equal to the external heat input (dQ) divided by temperature (T).
p.17
Monopole and Dipole Sources
What happens when sound reception differs in a dipole-like source?
It transforms part of the dipole-like source into a monopole.
What was the further extension made by Ffowcs Williams and Hawkings (1969)?
The FWH equation for bodies in arbitrary motion.
p.11
Mathematical Derivation of Acoustic Principles
What does the solution of the 1D wave equation satisfy?
The solution is a sink, which is normally unrealistic but could be generated by a 'time-reversal' technique.
p.14
Practical Sources of Sound
What happens when cold air is squeezed by hot air in combustion?
It leads to changes in pressure and specific volume.
p.2
Thermodynamics and Sound Propagation
What is the significance of the ideal gas approximation?
It is good for low pressure ranges; real-gas effects should be accounted for otherwise.
p.14
Practical Sources of Sound
What equations are combined in the analysis of combustion?
Linearized mass and momentum equations.
How does the far field of sound compare to the near field?
The far field is similar to a plane wave, while the near field vanishes in the far field.
p.5
Mass and Momentum Conservation
What is the significance of the first-order equation?
It represents the primary dynamics of the system.
What is the relationship between time scale and frequency in jet noise?
They are inversely related.
p.13
Monopole and Dipole Sources
What does the term 'force dipole' refer to?
A type of dipole source characterized by applied forces.
p.34
Aeroacoustics and Jet Noise
How do you rank the importance of monopole, dipole, and quadrupole noise for M<1?
Monopole noise is most important, followed by dipole, then quadrupole.
p.16
Monopole and Dipole Sources
What is a dipole in the context of sound sources?
A dipole is the limit of two tightly coupled monopoles when the distance approaches zero.
p.16
Monopole and Dipole Sources
What happens to the force when considering a dipole?
The force remains constant as the distance approaches zero.
p.4
Mass and Momentum Conservation
What is the Lagrangian derivative used for in fluid dynamics?
To describe the rate of change following a particle moving at speed u.
p.33
Practical Sources of Sound
What happens to the components of the noise spectrum related to BPF?
They coincide with BPF and its harmonics are radiated, while the rest cancel out.
p.4
Mass and Momentum Conservation
What is the Eulerian derivative used for in fluid dynamics?
To compare the velocity between different particles in a flow field.
What is the significance of quadrupoles in acoustic analogy?
Quadrupoles create sound in a space filled with still fluid.
p.32
Aeroacoustics and Jet Noise
What factors influence the amplitude and waveform of lift in vortex-airfoil interaction?
The distance between the vortex and the blade.
p.4
Mass and Momentum Conservation
Why do Newton's laws apply to the Lagrangian reference frame?
Because they describe the motion of individual particles effectively.
p.14
Practical Sources of Sound
What does the source term in the combustion equation represent?
The change of specific volume by heat addition at constant pressure.
p.24
Aeroacoustics and Jet Noise
What are the main topics covered in section 2.3 of Aeroacoustics?
Acoustic analogy, power laws and jet noise, fan noise mechanisms.
What does the variable c0 represent in the acoustic analogy?
It represents the speed of sound in still fluid.
p.15
Monopole and Dipole Sources
What does the volume integral of pulse equal in the context of monopole sources?
The volume integral of pulse is unity.
p.5
Mass and Momentum Conservation
What does the full mass equation represent?
It is a representation of the conservation of mass.
p.15
Monopole and Dipole Sources
How is sound pressure distributed in relation to a monopole source?
Sound pressure distribution varies as a function of direction (angle).
p.13
Mass and Momentum Conservation
How is a mass source affected according to the text?
Through volume displacement, not external mass.
p.13
Mathematical Derivation of Acoustic Principles
What is the significance of the volume ratio 'b' in the context of sound sources?
It represents the external matter occupying a small but oscillatory fraction of volume.
p.2
Thermodynamics and Sound Propagation
What is the Boltzmann constant?
k_B = 1.38 × 10^-23 J/(K·molecule).
p.34
Aeroacoustics and Jet Noise
How much is Tij in a uniform flow?
Tij is constant in a uniform flow.
p.32
Aeroacoustics and Jet Noise
What happens to lift when a vortex is right above the leading edge of an airfoil?
An impulsive fluctuating lift is generated.
p.7
Speed of Sound in Gases
What is the relationship between pressure and particle velocity in a traveling wave?
They are related through the momentum equation.
p.14
Practical Sources of Sound
What is the effect of combustion under constant time-mean pressure?
It causes local, unsteady expansion of hot gas.
p.12
Homogeneous Wave Equation
What happens when the assumptions of wave propagation are violated?
Sound is generated instead of being merely propagated.
p.15
Monopole and Dipole Sources
What is the directivity characteristic of a monopole source?
A monopole has a uniform directivity.
p.32
Aeroacoustics and Jet Noise
How can a wake be described in the context of vortex interactions?
As a street of small vortices.
p.13
Monopole and Dipole Sources
What are the two types of sound sources discussed in Chapter 2?
Monopole and dipole sources.
p.11
Monopole and Dipole Sources
What is the relationship between the radius of a vibrating sphere and the radiated field?
The radiated field can be obtained from the radial velocity of the sphere.
p.13
Mass and Momentum Conservation
What does the continuity equation account for in fluid dynamics?
The main fluid's behavior in relation to external forces.
p.14
Practical Sources of Sound
What is the significance of linearization in the combustion equations?
It allows for simplification of the total variables.
p.25
Practical Sources of Sound
What did Michael Green study at Trinity Cambridge?
He got a scholarship to study at 15 but was allowed in at 17.
p.5
Mass and Momentum Conservation
What does the full momentum equation account for?
It accounts for the conservation of momentum in the system.
p.2
Thermodynamics and Sound Propagation
What additional energy is involved for diatomic gases?
Energy involved in the rotational degree of freedom.
p.15
Monopole and Dipole Sources
What is a monopole in the context of sound sources?
A monopole occurs when the radius of an oscillating sphere approaches zero while the volume outflow caused by pulsation is held constant.
p.9
Acoustic Energy Conservation
How is kinetic energy expressed in the context of acoustic energy?
As ½ r₀ u² per unit volume in 1D.
p.18
Monopole and Dipole Sources
What is the situation when a powered breaker shakes a piece of rock?
It creates a mixture of dipole and monopole-like sources.
What equation is referenced in the context of acoustic analogy?
The exact Navier-Stokes equation.
p.7
Speed of Sound in Gases
What does the constant ρ₀ represent?
The average density, which is 1.225 kg/m³.
p.9
Mathematical Derivation of Acoustic Principles
What does the energy equation relate to in terms of fluid dynamics?
It relates to the linearized mass conservation.
p.20
Practical Sources of Sound
How does tyre noise occur?
Through tyre vibration and air trapping.
p.26
Mathematical Derivation of Acoustic Principles
What is the significance of the equations involving 'u', 'p', and 'r'?
They describe the relationships between velocity, pressure, and density in sound propagation.
p.26
Mathematical Derivation of Acoustic Principles
What is the general form of the equations presented in the derivation?
They are differential equations that describe sound propagation.
p.34
Aeroacoustics and Jet Noise
How does flow generate dipole sound?
By creating pressure fluctuations due to changes in flow direction.
p.1
Thermodynamics and Sound Propagation
What is the significance of entropy in energy quality?
Lower entropy (s) indicates better order and higher energy quality.
p.15
Monopole and Dipole Sources
Can the expression for a monopole source be applied to non-harmonic processes?
Yes, the expression is valid for non-harmonic source processes as well.
p.17
Monopole and Dipole Sources
What is the significance of the size of a dipole compared to wavelength?
A large size dipole compared to wavelength contains monopole characteristics.
p.25
Aeroacoustics and Jet Noise
What was the focus of the work done by the ministry of aviation in 1949?
To see if war-time jets can be used for civilian transport.
p.22
Practical Sources of Sound
What is a recommended method to deal with rail vibration noise according to new European research?
Very low, close, line-side barriers.
p.20
Practical Sources of Sound
What components contribute to vehicle noise?
Cooling fan, air intake, engine, transmission, and tyres.
p.22
Practical Sources of Sound
Why is wheel vibration considered important?
Because it contributes to overall noise levels.
p.2
Thermodynamics and Sound Propagation
How is temperature related to molecular kinetic energy?
Temperature is essentially the molecular kinetic energy.
p.34
Aeroacoustics and Jet Noise
What is meant by ‘thickness noise’?
Noise generated by the thickness of an airfoil as it moves through the air.
p.34
Aeroacoustics and Jet Noise
What extra effects are considered in the Ffowcs Williams Hawkings equation?
It includes effects of moving surfaces and unsteady flow.
p.7
Speed of Sound in Gases
What is the significance of the constant ρ₀ in the equation p = ρ′c²?
It represents the density variation.
p.12
Homogeneous Wave Equation
What causes linearity to be violated in sound propagation?
When jet flow creates massive oscillations of velocity.
p.8
Practical Sources of Sound
What values are used for the atmospheric conditions in the calculations?
p atm = 10^5 Pa, r0 = 1.225 kg/m³, T atm = 288 K.
p.26
Mathematical Derivation of Acoustic Principles
What does the notation '¶' signify in the equations?
It signifies partial derivatives with respect to time or space.
p.34
Aeroacoustics and Jet Noise
What kind of aerodynamic sound radiates as monopole?
Sounds generated by uniform pressure changes.
p.34
Aeroacoustics and Jet Noise
How do you rank the importance of monopole, dipole, and quadrupole noise for M>1?
Dipole noise becomes more significant than monopole, with quadrupole being the least.
p.13
Mathematical Derivation of Acoustic Principles
What equations are combined to describe the behavior of sound sources?
The continuity equation and the momentum equation.
p.34
Aeroacoustics and Jet Noise
What is the dominant source term in Lighthill’s stress tensor?
The rate of change of momentum in the flow.
p.34
Aeroacoustics and Jet Noise
Why is the frequency content at an observer position different from the force fluctuation on the surface of a moving blade?
Due to propagation effects and the influence of the surrounding flow field.
