Sound Transmission Flashcards
Sound transmission
Sound fades away over time and distance
- frictional resistance
- damping of vibration
- free vs. Forced vibration
When sound is propagated in free, unbounded medium, intensity decreases in a lawful way - this is called the __________.
Inverse square law
Spherical waves
___________ form a spherical wave called _______.
Compressions
Wavefronts
Wavefronts move __________ from the source
Outward
Spherical wave fronts become progressively _________ and travel from _________ –> _________.
Larger
Middle
Outside
Inverse square law
From middle –> outward, energy is dissipated over a larger and larger surface area.
Inverse square law
If distance is _________, the Intensity is decreased by a factor of ____.
If distance is doubled, SPL increases by _____dB
Doubled
4
6
Inverse square law only holds in free, unbounded mediums with 0 obstacles. Otherwise, the sound would be :
Reflected
Refracted
Diffracted
Absorbed
Obstacles offer large acoustic ____________ .
Reflection in H20:
_____% of incident energy penetrates water’s surface but ____% is reflected back by the H20.
Impedance
- 1%
- 9%
Reflection of sound waves
If energy is retained in a medium the inverse square law ______ hold.
Doesn’t
Is the decrease in intensity < or > than the inverse square law would predict?
Less than: energy is retained in the medium
Reflection from plane surfaces
Angles of reflected rays = angles of __________ to the perpendicular
Incident
Under what circumstances will a ray be reflected back on itself toward the source?
@ 0 degrees
——>|
|
Regardless of whether the obstacle surface is _____, ______, or ______, the angles of the reflected rays to the perpendicular = to the angles of _______ rays to the perpendicular
Plane
Convex
Concave
Incident
Echoes, reverberation, and reverberation time:
Reflected waves are called _______ or ____________.
Echoes
Reverberating waves
Reverberant rooms vs. Anechoic rooms:
Reverberant = hard surfaces to maximize reflection
Anechoic = absorbing surfaces to minimize reflexion
Reverberation time = the time required for ________ _______ to ______ by 60dB (T60)
Sound energy
Decay
Interference
Name 2 kinds and what effects they have:
Constructive: reinforcing effects
Destructive: cancelling effects
Standing waves occur when 2 __________ waves incident and _______ at same frequency and amplitude traveling in ___________ direction or along a medium
Progressive waves
Reflect
Opposite
When 2 waves are moving in & out of phase over time, each wave is traveling but the resultant wave is ________.
Stationary
Refraction:
When wave encounters _________ which results in large impedance, the wave is _______ with _____ change in speed of propagation.
Obstacle
Reflected
No
Of wave moves to anther medium or encounters change in the medium, the speed of propagation ______ and the rays are ______.
Changes
Bent
When sound travels against the wind, wave fronts are __________ and bent ________.
Refracted
upward
When sound travels with the wind, wave fronts are refracted ________.
“It goes where the wind goes”
Upward
Sound travels fast with the wind T/F?
True
Sound travel in early a.m. Vs midday
Early morning:
Warmer air is higher
Warmer air = less density
Less density = increased speed
Wavefronts:
refracted downwards
Reflected upward etc.
What happens midday?
Warmer is lower
Wavefronts:
Refracted upward into atmosphere
Diffraction
Same as when H20 wave encounters a raft
Wave bends around obstacle and reforms
When moving left-right and wave encounters barrier:
Some energy reflected back
Some bend around obstacle
Then reform and continue as plane wave front
When wave fronts encounter opening in wall:
Some energy is reflected back
Parts of wave pass through and reform as plane waves
Absorption
Opposition to sound transmission exist at any boundary where impedances differ.
Some sound energy will be absorbed by the new medium
Intensity of reflected wave will be less than intensity of incident wave
Absorption coefficient
Magnitude of absorption given by absorption coeff.
Is the proportion of energy in incident wave absorbed by material
Ia/Ii - Ia = energy absorbed, Ii = energy of incident wave
Absorption coefficient from air –> water
- 1% absorbed
99. 9% reflected
Sound treated rooms (2)
Anechoic : high absorption coefficient - fibre class wedges/absorbing material
Sound isolated rooms: designed to reduce sound transmission - reasonably high absorption coeff.
Absorption is ______ proportion to reflection
This, as absorption coefficient increases, reflecting and reverberation time ______.
Inversely
Decreases
Interference
The further away from the ___________ the _______ the sound becomes.
Sound source
Softer
