Unit Four: Noise and Vibration Treatments

Question 1

Static deflection of the resilient mounting system

Answer 1

the amount a resilient isolator deflects under the dead load of the equipment or structure supported by the isolator system.

Question 2

Internal resonances in the resilient mount

Answer 2

resonances caused inside the isolator mount at frequencies with small wavelengths which decrease the performance of the mounting system.

Question 3

Coupled rigid-body modes in resilient mounting systems

Answer 3

modes of vibration coupled due to the six degrees of freedom (three translational and three rotational) of a resiliently mounted device.

Question 4

Shape factor for mounts

Answer 4

the ratio of the area of the loaded surface to the total force-free area.

Question 5

Distributed Isolation Material (DIM)

Answer 5

a rubber isolation pad that has grooves cut or formed into the surface, allowing more room for expansion during deformation under load, reducing the spring constant and allowing more deflection and better isolation.

Question 6

Snubbers

Answer 6

devices used to limit the movement of equipment mounted on isolators, to prevent excessive deflections that could damage the mounts or unseat the equipment from the mounts.

Question 7

Passive dynamic absorbers

Answer 7

a simple oscillator attached to a vibrating device to reduce vibration at a selected frequency.

Question 8

Logarithmic increment

Answer 8

the natural logarithm of the ratio of the decaying amplitude of a single degree of freedom system over one period of oscillation. Used to measure material damping.

Question 9

Free-layer damping treatment

Answer 9

a layer of highly damped viscoelastic material glued to the outer surface of a structure where the outer surface of the damping layer is left free.

Question 10

Constrained-layer damping treatment

Answer 10

a layer of highly damped viscoelastic material glued to the outer surface of a structure where the outer layer of the damping material is constrained by gluing a plate to the outer surface.

Question 11

Shear parameter

Answer 11

a variable relating the shear modulus of a damping layer to the stiffness of a constraining layer, the height of the damping layer and the constraining layer, and the bending wavenumber of the structure.

Question 12

Transition temperature

Answer 12

the temperature below which a rubber-like material is in a glasslike state.

Question 13

Transition frequency

Answer 13

the frequency above which a rubber-like material behaves as a glasslike material.

Question 14

Reduced frequency parameter

Answer 14

frequency scaled by adjusting the modulus of elasticity and damping loss factor plotted as a function of frequency at different temperatures until the curves overlap.

Question 15

Waveguide absorber

Answer 15

a device attached to a vibrating structure to provide broadband damping at low frequencies by adding enough damping so that the absorber appears to be infinitely long.

Question 16

Trap-and-damp approach

Answer 16

channeling structureborne vibration energy into insensitive areas, containing it by means of impedance discontinuities, and applying damping to the area of containment to dissipate the energy.

Question 17

Small or close-fitting enclosure

Answer 17

an enclosure where the air space between the source and the inside surface of the enclosure is small compared to the acoustic wavelength.

Question 18

Large enclosures

Answer 18

an enclosure where the airspace is nearly equal or larger than the acoustic wavelength so that resonances may exust between the surfaces of the source and enclosure.

Question 19

Free standing enclosures

Answer 19

enclosures that have no direct mechanical connection to the noise source.

Question 20

Equipment-mounted enclosures

Answer 20

enclosures that are mounted to the noise source.

Question 21

Partial enclosures

Answer 21

an enclosure containing panels that surround the source with an open area greater than 10%.

Question 22

Cladding treatments

Answer 22

a treatment in which the source of airborne noise is wrapped with an absorptive layer, e.g. fiberglass, covered by an impervious layer.

Question 23

Transfer impedance matrix method

Answer 23

a method of modelling the effect of a muffler or silencer in a system which breaks the system into separate elements modelled using mass density, area, particle velocity and pressure which can be assembled into products of matrices and used to solve for the acoustic performance of the muffler.

Question 24

Electro-mechanical analog circuits

Answer 24

a method of modelling the effect of a muffler or silencer in a system which uses voltage as the electrical analog to acoustic pressure and current as the electrical analog to acoustic volume velocity to create an equivalent circuit for mufflers.

Question 25

Half-wavelength side branches

Answer 25

a noise path separate from the main passage of a noise path that is half a wavelength longer than the main path, resulting in sound waves re-entering the main passage and destructively interfering with the noise in the main passage.

Question 26

Silencer self-noise

Answer 26

noise caused by airflow through the restrictive air passages in a silencer.

Question 27

Feedforward active control

Answer 27

a method of active noise control which uses input from a primary source noise or vibration reference sensor to generate a signal for a secondary source, controlled such that the combination of the two sources results in a reduction of the source noise as measured at the location of an error sensor.

Question 28

Feedback active control

Answer 28

a method of active noise control which uses only an error sensor located near a secondary source, which is used to drive a signal intended to reduce the noise level from a primary source as received at the error sensor.

Question 29

z-transforms

Answer 29

a method of converting a time varying digital signal to a Fourier transform representation that can be used to produce a digitally filtered output.

Question 30

Finite Impulse Response (FIR) filters

Answer 30

a digital filter that has zero impulse response after a finite number of terms of the series used to represent the filtered signal.

Question 31

Infinite Impulse Response (IIR) filters

Answer 31

a digital filter that produces an impulse response that never decays to zero no matter the number of terms of the series used to represent the filtered signal.

Question 32

Optimal digital filter

Answer 32

a digital filter that minimizes the error signal between the filter output and a desired output.

Question 33

Adaptive digital filter

Answer 33

a digital filter that adjusts its filter coefficients dependent on the source input to maintain a minimized error signal.

Question 34

Nyquist stability criterion

Answer 34

a method of determining the stability of feedback control based on contour plots in the complex plane.

Question 35

Nyquist plots

Answer 35

complex plane contour plots used to determine stability of feedback control.

Question 36

Modal spillover

Answer 36

transfer of energy into higher-order uncontrolled modes by an active control system due to a limited number of sensors.

Question 37

Piezoelectric (PZT) and polyvinylidene (PVDT) patches

Answer 37

patches used as sensors or actuators in the active control of vibration in a vibrating structure.

Question 38

Modal rearrangement

Answer 38

changing the phase of vibrational modes to reduce coupling between a panel and a cavity, thereby obtaining reductions in acoustic modes within the cavity.