Unit One – Sources of Noise Flashcards
Involute gear tooth shape
a gear tooth shape that provides constant rotational velocity during rotation of the gear.
Reversal of sliding velocity
the change in direction of sliding velocity between contact surfaces along the line of action of meshed gears.
Tooth-meshing frequency
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.
Transmission errors
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.
Asynchronous alternating current (AC) electric motors
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.
Synchronous alternating current (AC) electric motors
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.
Direct current (DC) electric motors
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.
Magnetostriction
changes in the core shape in a transformer caused by the alternating electrical field which results in the creation of noise.
Indirect injection diesel engine combustion
diesel engine combustion that starts in a prechamber in the cylinder head of the engine, causing a slower rate of combustion.
Direct injection diesel engine combustion
diesel engine combustion that occurs through direct fuel injection into the combustion chamber leading to a faster rate of combustion and more noise.
Engine block transfer function
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.
Piston slap
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.
Lighthill’s stress tensor
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.
Causality condition
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.
Reynolds stresses
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.
Mach number
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.
Choked jet flow
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.
Flow tones
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.
Boundary layer displacement thickness
the distance away from a flat surface which the inviscid (low to no viscosity) flow is spaced by the boundary layer flow.
Turbulent boundary layer wavenumber spectrum
the Fourier transform of wall pressures caused by turbulent flow over a surface, taken over space at a fixed time.
Convective ridge
the point in the turbulent layer boundary flow where the amplitude of the pressure at the wall peaks.
Convective wavenumber
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.
Axial-flow fans
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.
Centrifugal fans
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.
