Unit Two: Outdoor Noise and Structural Acoustics

Question 1

Excess attenuation

Answer 1

attenuation over distance between a source and a receptor due to effects other than spherical divergence and atmospheric absorption, such as refraction, atmospheric turbulence and interaction with the ground and other obstacles.

Question 2

Atmospheric absorption

Answer 2

attenuation of sound energy in air as a result of viscous losses due to friction between air molecules and relaxation processes in which sound energy is momentarily absorbed in the air molecules and reradiated at a later instant, causing partial interference with the sound wave.

Question 3

Atmospheric absorption coefficient

Answer 3

an empirical quantity used to calculate attenuation due to atmospheric absorption effects over a given distance from a source as a function of frequency, temperature and molar concentration of water vapor.

Question 4

Flow resistance model

Answer 4

an equation, based on measurements, that relates flow resistance and frequency to ground impedance.

Question 5

Scattering by atmospheric turbulence

Answer 5

for outdoor sound propagation, the scattering of sound by fluctuations created by atmospheric turbulence.

Question 6

Fast-Field Program

Answer 6

a modeling technique used to predict propagation of sound outdoors involving dividing the atmosphere into layers, using pressure and particle velocity for each layer and integrating using Fast Fourier transform algorithms to arrive at a ratio of pressure at a distance to pressure at the source.

Question 7

Parabolic equation

Answer 7

a modeling technique used to predict propagation of sound outdoors involving the two dimensional Helmholtz equation containing only horizontal and vertical direction values and iterated in the x direction to map the pressure over the area of interest.

Question 8

Torsional wave phase speed

Answer 8

the speed of vibration transmission in a component as a result of a twisting motion of the component perpendicular to the direction of wave propagation.

Question 9

Dispersion equations

Answer 9

equations used to relate frequency to phase speed for propagation of vibrations through a structure.

Question 10

Group speed

Answer 10

the rate at which energy propagates in a structure, different from the phase speed when the wave is dispersive and contains energy at more than one frequency.

Question 11

Timoshenko beam theory

Answer 11

a theory that utilizes a shear strain correction factor for beams to account for transverse shear through the thickness of the beam to allow more accurate prediction of phase speed at higher frequencies.

Question 12

Mindlin plate theory

Answer 12

a theory for bending waves in plates in which higher-order effects are included e.g. transverse shear and rotary inertia for cross-sections of the plate. This theory is more accurate at higher frequencies than classical plate theory.

Question 13

Structural modal density

Answer 13

the modal density of vibrational waves in a solid structure at any given frequency, similar to the modal density of sound waves contained within a reverberant room.

Question 14

Mean-Value Theory

Answer 14

a method of estimating the mean value of the drive-point impedance of a structure as a function of frequency, by using the drive-point impedances at resonance and anti-resonance.

Question 15

Equal partition of energy

Answer 15

for broadband excitation, energy is evenly divided among resonant modes in the excited system.

Question 16

Fluid loading of structures

Answer 16

the effect that a heavy fluid on the surface of a structure has on the vibration response of the structure.

Question 17

Coupling between bending and longitudinal waves by curvature

Answer 17

for curved structures at low frequencies, the curvature will couple longitudinal and bending waves which results in higher speeds for the bending-like waves in the structure.

Question 18

Ring frequency

Answer 18

the frequency above which the curvature of a structure has little effect on the bending wave phase speed, in other words the structure appears flat to the bending waves.

Question 19

Pass bands

Answer 19

bands of frequencies in which wave propagation will occur in structures that are reinforced with evenly spaced ribs.

Question 20

Stop bands

Answer 20

bands of frequencies in which wave propagation will be blocked in structures that are reinforced with evenly spaced ribs.

Question 21

Acoustically slow

Answer 21

the frequencies below the critical frequency of a plate where there is no acoustic radiation because the bending waves have phase speeds lower than the acoustic phase speed.

Question 22

Acoustically fast

Answer 22

the frequencies above the critical frequency of a plate where there is acoustic radiation because the bending waves have phase speeds above the acoustic phase speed.

Question 23

Trace wavenumber matching

Answer 23

when the wavenumber for bending waves in a structure equals the acoustic wavenumber traced on to the surface of the structure.

Question 24

Surface radiating mode

Answer 24

the condition when the disturbing frequency in the plate is above the critical frequency, the radiation is from the entire surface of the plate.

Question 25

Edge radiating mode

Answer 25

when the plate is acoustically fast in one direction but not the other, the radiation will come from the edges of the plate in the acoustically fast direction.

Question 26

Corner radiating mode

Answer 26

when the plate is acoustically slow in both directions, the radiation will come only from the corners of the plate.

Question 27

Radiation from the structural nearfield

Answer 27

the region close to the source where nearfield radiated noise is dominant and the force response cannot be significantly reduced by adding damping.

Question 28

Flanking paths

Answer 28

a flanking path is any transmission path that bypasses a partition or other dividing barrier set up to block a noise or vibration from one area to another.

Question 29

Transmission loss for a composite partition

Answer 29

the transmission loss of a partition such as a wall, floor or ceiling which is non-homogeneous, such as a wall with a door and a window.

Question 30

Mass law

Answer 30

the theoretical characteristic performance of a partition that causes a transmission loss increase of 6 dB per doubling of surface mass density and 6 dB per doubling of frequency.

Question 31

Coincidence dip

Answer 31

a dip in the transmission loss curve of a partition at the critical frequency of the coincidence between the incident trace wavenumber for obliquely incident waves and the wavenumber for free bending waves.

Question 32

Frequency of coincidence

Answer 32

the frequency where the transmission loss for a partition is lowest for a given angle of incidence of the sound wave onto the partition.

Question 33

Mass-spring-mass resonance

Answer 33

the effect caused when air trapped between the two leaves of a partition acts like a spring, coupling the two leaves which act like masses.

Question 34

Partition cavity resonances

Answer 34

for a double leaf partition, the resonance that occurs in the cavity between the two leaves..

Question 35

Break-in noise

Answer 35

occurs as sound from outside an HVAC duct passes through the duct material, enters the ducting, and propagates through the ducting system.

Question 36

Break-out noise

Answer 36

occurs when sound from inside an HVAC duct passes through the duct material and into the airspace surrounding the ducting system, where it can propagate through that airspace.