Unit Two: Outdoor Noise and Structural Acoustics Flashcards
Excess attenuation
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.
Atmospheric absorption
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.
Atmospheric absorption coefficient
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.
Flow resistance model
an equation, based on measurements, that relates flow resistance and frequency to ground impedance.
Scattering by atmospheric turbulence
for outdoor sound propagation, the scattering of sound by fluctuations created by atmospheric turbulence.
Fast-Field Program
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.
Parabolic equation
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.
Torsional wave phase speed
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.
Dispersion equations
equations used to relate frequency to phase speed for propagation of vibrations through a structure.
Group speed
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.
Timoshenko beam theory
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.
Mindlin plate theory
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.
Structural modal density
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.
Mean-Value Theory
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.
Equal partition of energy
for broadband excitation, energy is evenly divided among resonant modes in the excited system.
Fluid loading of structures
the effect that a heavy fluid on the surface of a structure has on the vibration response of the structure.
Coupling between bending and longitudinal waves by curvature
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.
Ring frequency
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.
Pass bands
bands of frequencies in which wave propagation will occur in structures that are reinforced with evenly spaced ribs.
Stop bands
bands of frequencies in which wave propagation will be blocked in structures that are reinforced with evenly spaced ribs.
Acoustically slow
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.
Acoustically fast
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.
Trace wavenumber matching
when the wavenumber for bending waves in a structure equals the acoustic wavenumber traced on to the surface of the structure.
Surface radiating mode
the condition when the disturbing frequency in the plate is above the critical frequency, the radiation is from the entire surface of the plate.
Edge radiating mode
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.
Corner radiating mode
when the plate is acoustically slow in both directions, the radiation will come only from the corners of the plate.
Radiation from the structural nearfield
the region close to the source where nearfield radiated noise is dominant and the force response cannot be significantly reduced by adding damping.
Flanking paths
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.
Transmission loss for a composite partition
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.
Mass law
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.
Coincidence dip
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.
Frequency of coincidence
the frequency where the transmission loss for a partition is lowest for a given angle of incidence of the sound wave onto the partition.
Mass-spring-mass resonance
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.
Partition cavity resonances
for a double leaf partition, the resonance that occurs in the cavity between the two leaves..
Break-in noise
occurs as sound from outside an HVAC duct passes through the duct material, enters the ducting, and propagates through the ducting system.
Break-out noise
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.
