What is the main noise source in construction?
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Percussive piling.
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What is the main noise source in construction?
Percussive piling.
What does 'Ñ' represent in the equations?
The divergence operator.
Who studied the interaction between a single convecting vortex and an airfoil?
Michael S. Howe.
What characterizes a dipole source of sound?
Structural vibration and oscillating pressure on stationary or moving surfaces.
What is a quadrupole source of sound?
Derived from two tightly coupled dipoles, producing oscillating pressure in the absence of structural surfaces.
What does the 1D momentum equation represent?
The relationship between mass flow and momentum in a fluid.
What is the projected reality year for the 'silent aircraft' initiative?
What is the main focus of Chapter 2 in the context of sound?
Sources of Sound.
What is the significance of the net rate of mass influx in mass conservation?
It represents the mass entering or leaving a control volume.
What does the wave speed 'c' derive from?
The Latin word 'celeritas' meaning 'velocity'.
What is the 3D wave equation derived from?
By eliminating velocity components (u, v, w) from the mass and momentum equations and isentropic thermodynamics.
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.
When is it useful to simplify the formulation of the wave equation?
When sound is constant along one direction, usually the azimuth angle.
What is the general solution for the 1D wave equation?
p = f(x ± c₀t), where f is an arbitrary function.
What mathematical concept is being derived in section 2.3A?
The Lighthill stress tensor.
What does the equation involving 'p', 'u', and 'c' represent?
The relationship between pressure, velocity, and speed of sound in a medium.
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.
What is the main source of excitation for rail vibration?
Rail roughness and corrugation patches (50 mm or more, tens of microns deep).
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.
How does a real source differ from a theoretical dipole?
Real sources have finite size, leading to time differences in sound reception.
What does the term ∂p/∂x represent in the 1D momentum equation?
The net force (per unit volume) acting to accelerate a particle.
What year was Lighthill's seminal paper on aerodynamically generated sound published?
What technology is replacing percussive piling?
Low-noise rotary machines.
What is a monopole source of sound?
A vibrating piston in 1D, large rigid body vibration in 3D, or unsteady combustion.
What vibrations are dominant at normal train speeds?
Wheel/rail vibrations.
What does a sharp impulse in sound contain?
All frequencies.
What is the peak pressure variation generated in the duct?
± 10 Pa (or 111 dB).
What is acoustic energy defined as?
The difference of fluid energy when there is sound and when there isn’t.
What is the focus of Chapter 2 in the context of sound?
Sources of Sound.
What mathematical operator is used in the 3D wave equation?
The Laplacian (Ѳ).
What does the total power flux available depend on according to Ffowcs Williams?
It is related to the factor ܷ^3.
What are the three parts into which all equations are expanded?
Steady-flow part, first order part, and second-order terms.
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.
What are the main sources of noise in vehicles?
Engine vibration, exhaust noise, inlet noise, cooling fan noise, and tyre noise.
Who worked on aeroacoustics theory at the University of Manchester?
James Lighthill.
What causes engine vibration in vehicles?
Explosive combustion and mechanical impaction.
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.
What components of a train's propulsion system are dominant at low speeds?
Traction motors, cooling fans, and reduction gears.
Which auxiliary equipment is dominant at very low speeds and when stationary?
Compressors, motor generators, brakes, and ventilation systems.
Who was the first appointed professor of Classics and Mathematics at Cambridge?
Isaac Barrow in 1664.
What is the frequency of the wave in the duct?
500 Hz.
What is the wave propagation equation derived from?
Combining mass and momentum conservation laws with isentropic thermodynamics.
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.
What is derived from the new state equation in combustion?
Enthalpy (energy) addition.
What is assumed about the second-order terms in the equations?
They can be ignored when compared with the first order.
What is the linear amplitude of particle velocity calculated?
0.024 m/s.
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).
What position did Stephen Hawking hold at Cambridge?
Lucasian Professor.
How much noise reduction can bogie skirts provide?
1-2 dBA at most.
What does the total acoustic energy density depend on?
It depends on the perturbation pressure and volume.
What is one method for highway noise control?
Using porous road surfacing.
Who was the head of the Royal Aeronautical Establishment at Farnborough?
James Lighthill.
When is a quadrupole an efficient noise source?
Only at very high speed or strength.
What is the blade-passing frequency (BPF)?
It is calculated as the number of blades multiplied by the rotational speed.
Who corrected Newton's assumption about sound speed differences?
Laplace, a century later.
What happens to a monopole when the surface is rigid?
It degenerates into a dipole called thickness noise.
What are the two types of energy present in acoustic energy?
Kinetic energy and potential energy.
What is the relationship between monopoles and dipoles?
A dipole can be viewed as two monopoles tightly coupled together.
What maintenance practice can help reduce rail vibration?
Regular maintenance by grinding train on rail tracks.
What year was George Gabriel Stokes appointed?
What type of waves are described in Chapter 2?
Spherical waves.
What is the first step in deriving the energy equation?
Multiplying the linearized momentum equation by the perturbation velocity.
What condition is described by '0, 0, , 1 when , 0 otherwise'?
It indicates specific conditions under which certain equations apply.
When is momentum conservation questioned?
When an external unsteady force is applied on a stationary surface.
What does the term 'incompressible' refer to in the context of sound?
It refers to a fluid or medium where density changes are negligible.
What is a monopole source characterized by?
Externally introduced mass and applied force per unit volume in space.
What was the approach to suppressing jet noise before Lighthill's work?
A matter of trial and error.
What effect does irreversible heat transfer have on sound?
It either consumes or generates sound.
What is the value of the specific heat ratio (γ) used in the calculations?
1.4.
What happens to sound power when the wave number is small?
The sound power is very small.
Who taught James Lighthill during his studies?
Paul Dirac.
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.
Since when has the Cambridge-MIT 'silent aircraft' initiative been in progress?
Since 2006.
What type of vibration and noise is produced by subway operations?
Ground-borne vibration and noise.
What is the radiation efficiency of a dipole source?
Moderate radiation efficiency.
What is acoustic focusing used for?
To generate powerful ultrasound to smash a kidney stone.
What do all elastic materials support?
Wave motions.
What happens when the back of a loudspeaker is enclosed?
The receiver sees only the front end, making it a monopole-like powerful radiator.
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.
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.
What types of noise are generated from interior structures of trains?
Structure-borne wheel/rail vibrations, air-borne transmission, and interior installations.
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.
What is the key factor that creates sound in a monopole source?
Only the variation of volume flow creates sound.
What does Eq1 and Eq2 represent in the derivation of acoustic analogy?
They represent equations related to the behavior of sound sources.
How can mass conservation be violated in sound propagation?
By sphere pulsation if the sphere itself is included in the air medium.
What is the title of Lighthill's seminal paper?
On sound generated aerodynamically: I. General theory.
What does the steady-flow part of the equation do?
It balances itself.
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.
How does temperature change in sound waves?
Temperature changes due to mechanical compression and expansion by surrounding particles, not by heat exchange.
What does the internal energy of an ideal gas depend on?
It is a function of temperature only, u = u(T).
What is the effect of fluid viscosity on sound propagation?
Effects are generally small, and disturbances are reversible, leading to isentropic conditions.
What type of noise is associated with dipole sources?
Dipole noise.
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.
Does structural vibration always create monopole sound?
No, structural vibration does not always create monopole sound.
What is ‘trailing edge noise’?
Noise generated at the trailing edge of an airfoil due to turbulent flow.
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.
What is the Cambridge-MIT initiative focused on?
Developing 'silent aircraft' that are no louder than background street noise.
What does time-reversal in wave propagation allow?
It allows a recorded wave to be played back in reverse time order.
What does mass conservation consider in fluid dynamics?
The change of mass contained in the space between sections 1 to 2.
How can a disturbance be characterized in sound sources?
It usually comes in impulsive form and can be decomposed into harmonic functions.
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.
What is the relationship between Ffowcs Williams' findings and the parameter ܯ?
For ܯ >> 1, ܲ ~ ܯ^3 due to the (1 - ݎܯ) factor.
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.
What forms a dipole-like source?
When a rigid body vibrates without changing its total volume.
Which notable physicist was appointed in 1669?
Isaac Newton.
What is the focus of Chapter 2 in the context of sound?
Sources of Sound.
In which coordinates can the Laplacian be translated for simplification?
Cylindrical (x, r, θ) or spherical coordinates (r, θ, φ).
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).
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.
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.
What is the state equation for air approximated as an ideal gas?
pV = nRT.
What happens when cold air is squeezed by hot air in combustion?
It leads to changes in pressure and specific volume.
What is the significance of the ideal gas approximation?
It is good for low pressure ranges; real-gas effects should be accounted for otherwise.
What equations are combined in the analysis of combustion?
Linearized mass and momentum equations.
What type of noise is associated with monopole sources?
Monopole noise.
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.
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.
What does the term 'force dipole' refer to?
A type of dipole source characterized by applied forces.
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.
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.
What happens to the force when considering a dipole?
The force remains constant as the distance approaches zero.
What is the Lagrangian derivative used for in fluid dynamics?
To describe the rate of change following a particle moving at speed u.
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.
What is the Eulerian derivative used for in fluid dynamics?
To compare the velocity between different particles in a flow field.
What is the rms value for pressure variation calculated?
7.07 Pa.
What is the significance of quadrupoles in acoustic analogy?
Quadrupoles create sound in a space filled with still fluid.
What is the formula for waves traveling in the negative x direction?
′p = ρ₀ - ∂u/∂t.
What factors influence the amplitude and waveform of lift in vortex-airfoil interaction?
The distance between the vortex and the blade.
Why do Newton's laws apply to the Lagrangian reference frame?
Because they describe the motion of individual particles effectively.
What does the source term in the combustion equation represent?
The change of specific volume by heat addition at constant pressure.
What is the dominant source term in the context of the acoustic analogy?
Reynolds stress.
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.
What does the volume integral of pulse equal in the context of monopole sources?
The volume integral of pulse is unity.
What does the full mass equation represent?
It is a representation of the conservation of mass.
How is sound pressure distributed in relation to a monopole source?
Sound pressure distribution varies as a function of direction (angle).
How is a mass source affected according to the text?
Through volume displacement, not external mass.
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.
What is the Boltzmann constant?
k_B = 1.38 × 10^-23 J/(K·molecule).
How much is Tij in a uniform flow?
Tij is constant in a uniform flow.
What happens to lift when a vortex is right above the leading edge of an airfoil?
An impulsive fluctuating lift is generated.
What is the relationship between pressure and particle velocity in a traveling wave?
They are related through the momentum equation.
What is the effect of combustion under constant time-mean pressure?
It causes local, unsteady expansion of hot gas.
What happens when the assumptions of wave propagation are violated?
Sound is generated instead of being merely propagated.
What is the rms value for temperature variation calculated?
0.0058 K.
What is the typical noise level at 25m from a passenger train running at 160 km/h?
91-94 dBA.
What is the directivity characteristic of a monopole source?
A monopole has a uniform directivity.
How can a wake be described in the context of vortex interactions?
As a street of small vortices.
What are the two types of sound sources discussed in Chapter 2?
Monopole and dipole sources.
What law describes the sound energy in relation to distance?
The inverse-square law.
What is the condition defined by the steady-flow part of the equation?
∂ρ/∂t = 0.
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.
What does the continuity equation account for in fluid dynamics?
The main fluid's behavior in relation to external forces.
What is the significance of linearization in the combustion equations?
It allows for simplification of the total variables.
What did Michael Green study at Trinity Cambridge?
He got a scholarship to study at 15 but was allowed in at 17.
What does the full momentum equation account for?
It accounts for the conservation of momentum in the system.
What additional energy is involved for diatomic gases?
Energy involved in the rotational degree of freedom.
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.
Who published a seminal paper on jet noise suppression?
M. J. Lighthill.
How is kinetic energy expressed in the context of acoustic energy?
As ½ r₀ u² per unit volume in 1D.
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.
What does the constant ρ₀ represent?
The average density, which is 1.225 kg/m³.
What noise level was measured for the Hong Kong Airport Express at 135 km/h?
82 dBA at 25m.
What does the energy equation relate to in terms of fluid dynamics?
It relates to the linearized mass conservation.
How does tyre noise occur?
Through tyre vibration and air trapping.
What is the term used for the air impedance for sound?
ρ₀c₀.
What is the significance of the equations involving 'u', 'p', and 'r'?
They describe the relationships between velocity, pressure, and density in sound propagation.
What is the contact area of the wheel typically?
10x10 mm.
What is the general form of the equations presented in the derivation?
They are differential equations that describe sound propagation.
What method can be used to mitigate wheel vibration?
Use wheel damping.
How does flow generate dipole sound?
By creating pressure fluctuations due to changes in flow direction.
What is the significance of entropy in energy quality?
Lower entropy (s) indicates better order and higher energy quality.
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.
What is the significance of the size of a dipole compared to wavelength?
A large size dipole compared to wavelength contains monopole characteristics.
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.
What is a recommended method to deal with rail vibration noise according to new European research?
Very low, close, line-side barriers.
How is the general wave equation modified for waves traveling in the n direction?
′p = ρn - ∂u/∂t.
What components contribute to vehicle noise?
Cooling fan, air intake, engine, transmission, and tyres.
Why is wheel vibration considered important?
Because it contributes to overall noise levels.
How is temperature related to molecular kinetic energy?
Temperature is essentially the molecular kinetic energy.
What is the relationship between pressure, temperature, and molecular velocity?
p ∝ ρv^2 ∝ T.
What is meant by ‘thickness noise’?
Noise generated by the thickness of an airfoil as it moves through the air.
What extra effects are considered in the Ffowcs Williams Hawkings equation?
It includes effects of moving surfaces and unsteady flow.
What is the significance of the constant ρ₀ in the equation p = ρ′c²?
It represents the density variation.
What is the maximum air particle displacement calculated?
7.6 μm.
What causes linearity to be violated in sound propagation?
When jet flow creates massive oscillations of velocity.
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.
What does the notation '¶' signify in the equations?
It signifies partial derivatives with respect to time or space.
What is the value of the gas constant (R) used in the calculations?
287 J/kgK.
What kind of aerodynamic sound radiates as monopole?
Sounds generated by uniform pressure changes.
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.
What is the relationship between enthalpy and internal energy?
h = u + pV.
What is the value of the specific heat ratio (γ) for air?
γ = 7/5 = 1.4.
What equations are combined to describe the behavior of sound sources?
The continuity equation and the momentum equation.
What is the dominant source term in Lighthill’s stress tensor?
The rate of change of momentum in the flow.
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.
