Unit One – Sources of Noise

Question 1

Involute gear tooth shape

Answer 1

a gear tooth shape that provides constant rotational velocity during rotation of the gear.

Question 2

Reversal of sliding velocity

Answer 2

the change in direction of sliding velocity between contact surfaces along the line of action of meshed gears.

Question 3

Tooth-meshing frequency

Answer 3

the frequency that controls tonal noise harmonic frequencies for two meshed gears, related to the number of teeth in the smaller pinion gear and its angular speed in RPM.

Question 4

Transmission errors

Answer 4

the combination of variations in the number of teeth in contact and the geometry of the gear teeth from perfect involute profiles which results in a departure from a constant angular velocity.

Question 5

Asynchronous alternating current (AC) electric motors

Answer 5

an electric motor also known as an induction motor configured so that AC current is applied to the windings in the fixed stator to create a rotating magnetic field which induces a current in the rotor windings. These motors experience ‘slip’ which is the difference in speeds between the speed of the rotor and the rotating magnetic field created by the stator.

Question 6

Synchronous alternating current (AC) electric motors

Answer 6

an electric motor configured so that in addition to applying AC current to the stator, AC current is applied to the rotor via armature windings through brushes or slip rings. These motors experience no ‘slip’ because the stator and rotor fields align and maintain position.

Question 7

Direct current (DC) electric motors

Answer 7

an electric motor configured so that DC current is applied to the stator to produce a stationary magnetic field, and to the rotor through a commutator that reverses polarity to keep its magnetic field aligned with the stator magnetic field.

Question 8

Magnetostriction

Answer 8

changes in the core shape in a transformer caused by the alternating electrical field which results in the creation of noise.

Question 9

Indirect injection diesel engine combustion

Answer 9

diesel engine combustion that starts in a prechamber in the cylinder head of the engine, causing a slower rate of combustion.

Question 10

Direct injection diesel engine combustion

Answer 10

diesel engine combustion that occurs through direct fuel injection into the combustion chamber leading to a faster rate of combustion and more noise.

Question 11

Engine block transfer function

Answer 11

the difference between the sound pressure level inside the cylinder of an internal combustion engine and the sound pressure level of the noise radiated by the engine block as measured at a distance of one meter from the surface of the block.

Question 12

Piston slap

Answer 12

the impact of the piston in a combustion engine cylinder against the cylinder wall as the piston is forced from one side of the cylinder to the other as it passes through the top of the stroke.

Question 13

Lighthill’s stress tensor

Answer 13

a term added to the wave equation for sound to simulate flow noise added to a system due to movement of fluid over a stationary structure.

Question 14

Causality condition

Answer 14

the characterization describing the delayed effect of noise transmitted from the source to the receiver until the sound pressure generated at the source reaches the receiver.

Question 15

Reynolds stresses

Answer 15

a component of the total shear stress in a fluid which accounts for turbulent fluctuations in the fluid, based on the ratio of the inertial to viscous stresses.

Question 16

Mach number

Answer 16

the magnitude of velocity (v) m/s in a fluid flow, divided by the speed of sound (c) m/s in the fluid, 343 m/s for air.

Question 17

Choked jet flow

Answer 17

the condition of flow in a nozzle when the velocity of the air flow in the nozzle reaches the speed of sound, i.e. becomes sonic, occurring when the pressure inside the nozzle is greater than 1.89 times the ambient pressure outside the jet.

Question 18

Flow tones

Answer 18

tonal noise created by airflow transverse to an opening caused by pressure fluctuations at the trailing edge of the opening coming into phase with the flow oscillations at the leading edge. The tone may be amplified if there is a cavity under the opening.

Question 19

Boundary layer displacement thickness

Answer 19

the distance away from a flat surface which the inviscid (low to no viscosity) flow is spaced by the boundary layer flow.

Question 20

Turbulent boundary layer wavenumber spectrum

Answer 20

the Fourier transform of wall pressures caused by turbulent flow over a surface, taken over space at a fixed time.

Question 21

Convective ridge

Answer 21

the point in the turbulent layer boundary flow where the amplitude of the pressure at the wall peaks.

Question 22

Convective wavenumber

Answer 22

the acoustic wavenumber where the spectrum of the turbulent boundary layer peaks, coincident with the peak in the boundary layer wall pressure as flow passes over a surface.

Question 23

Axial-flow fans

Answer 23

a fan where airflow through the fan is in the direction of the axis of rotation of the fan blades, with no change in direction of the airflow through the fan.

Question 24

Centrifugal fans

Answer 24

a fan where air is drawn into the fan wheel in the direction parallel to the axis of rotation and thrown out in the radial direction perpendicular to the axis of rotation.

Question 25

Blade volume-displacement or ‘thickness’ noise

Answer 25

fan noise created when a fan blade displaces air.

Question 26

Harmonic analyses of wake fields

Answer 26

the use of a Fourier series expansion to estimate the magnitudes of unsteady lifts and radiated noise resulting from blade rotation through the wakes left behind obstructions upstream of the rotating blade.

Question 27

Blade number mismatching

Answer 27

ensuring the number of blades of an axial flow fan are not an integer multiple of the number of obstructions upstream of the fan blades.

Question 28

Turbulence ingestion noise

Answer 28

broadband noise caused by turbulence generated upstream of the fan rotor as it passes into the rotor and impacts the blades.

Question 29

In-line pumps

Answer 29

pumps that move fluid through piston motion in cylinders placed parallel to the rotating shaft, caused by attachment to an angled yoke plate.

Question 30

Vane pumps

Answer 30

pumps that move fluid through movable vanes mounted on a rotor that is offset in a bore so that on one side, the vanes move outward to collect fluid and on the other side, the vanes retract, and fluid is pushed out of the bore.

Question 31

Gear pumps

Answer 31

pumps that move fluid through counterrotating gears that pull fluid into the pump through the meshing gears and expel it on the other side as the gears unmesh.

Question 32

Cavitation

Answer 32

collapse of vapor bubbles in a fluid that have been formed due to localized low static pressure regions caused by turbulence in the flow stream.

Question 33

Fan laws

Answer 33

ratios relating fan speed, fan size, volume flow and static pressure between two operating conditions, used to estimate performance for operating conditions other than a known performance.

Question 34

Critical frequency of a rotating component

Answer 34

when the rotational frequency of a rotor-shaft system coincides with the shaft’s natural frequency.

Question 35

Static balancing

Answer 35

adding or removing mass to a component until the center of gravity of the component falls on the axis of rotation so that the component remains at any given rotational position when placed there and released.

Question 36

Dynamic balancing

Answer 36

adding masses in pairs about the rotational axis so that the static balance is not changed but the rotational balance is aligned with the principal axis of rotation.

Question 37

Principal axes of rotation

Answer 37

an axis of rotation where the moments generated by centrifugal forces during rotation sum to zero.

Question 38

Modal balancing

Answer 38

balancing of a flexible rotating component by adding masses distributed to counter the mode shape at each frequency of resonance.