Sound Transmission

Question 1

Sound transmission

Answer 1

Sound fades away over time and distance

• frictional resistance
• damping of vibration
• free vs. Forced vibration

Question 2

When sound is propagated in free, unbounded medium, intensity decreases in a lawful way - this is called the __________.

Answer 2

Inverse square law

Question 3

Spherical waves

___________ form a spherical wave called _______.

Answer 3

Compressions

Wavefronts

Question 4

Wavefronts move __________ from the source

Answer 4

Outward

Question 5

Spherical wave fronts become progressively _________ and travel from _________ –> _________.

Answer 5

Larger
Middle
Outside

Question 6

Inverse square law

Answer 6

From middle –> outward, energy is dissipated over a larger and larger surface area.

Question 7

Inverse square law

If distance is _________, the Intensity is decreased by a factor of ____.

If distance is doubled, SPL increases by _____dB

Answer 7

Doubled
4
6

Question 9

Inverse square law only holds in free, unbounded mediums with 0 obstacles. Otherwise, the sound would be :

Answer 9

Reflected
Refracted
Diffracted
Absorbed

Question 10

Obstacles offer large acoustic ____________ .

Reflection in H20:
_____% of incident energy penetrates water’s surface but ____% is reflected back by the H20.

Answer 10

Impedance

0. 1%
1. 9%

Question 11

Reflection of sound waves

If energy is retained in a medium the inverse square law ______ hold.

Answer 11

Doesn’t

Question 12

Is the decrease in intensity < or > than the inverse square law would predict?

Answer 12

Less than: energy is retained in the medium

Question 13

Reflection from plane surfaces

Angles of reflected rays = angles of __________ to the perpendicular

Answer 13

Incident

Question 14

Under what circumstances will a ray be reflected back on itself toward the source?

Answer 14

@ 0 degrees

——>|
|

Question 15

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

Answer 15

Plane
Convex
Concave
Incident

Question 16

Echoes, reverberation, and reverberation time:

Reflected waves are called _______ or ____________.

Answer 16

Echoes

Reverberating waves

Question 17

Reverberant rooms vs. Anechoic rooms:

Answer 17

Reverberant = hard surfaces to maximize reflection

Anechoic = absorbing surfaces to minimize reflexion

Question 18

Reverberation time = the time required for ________ _______ to ______ by 60dB (T60)

Answer 18

Sound energy

Decay

Question 19

Interference

Name 2 kinds and what effects they have:

Answer 19

Constructive: reinforcing effects

Destructive: cancelling effects

Question 20

Standing waves occur when 2 __________ waves incident and _______ at same frequency and amplitude traveling in ___________ direction or along a medium

Answer 20

Progressive waves
Reflect
Opposite

Question 21

When 2 waves are moving in & out of phase over time, each wave is traveling but the resultant wave is ________.

Answer 21

Stationary

Question 22

Refraction:

When wave encounters _________ which results in large impedance, the wave is _______ with _____ change in speed of propagation.

Answer 22

Obstacle
Reflected
No

Question 23

Of wave moves to anther medium or encounters change in the medium, the speed of propagation ______ and the rays are ______.

Answer 23

Changes

Bent

Question 24

When sound travels against the wind, wave fronts are __________ and bent ________.

Answer 24

Refracted

upward

Question 25

When sound travels with the wind, wave fronts are refracted ________.

“It goes where the wind goes”

Answer 25

Upward

Question 26

Sound travels fast with the wind T/F?

Answer 26

True

Question 27

Sound travel in early a.m. Vs midday

Answer 27

Early morning:
Warmer air is higher
Warmer air = less density
Less density = increased speed

Wavefronts:
refracted downwards
Reflected upward etc.

Question 28

What happens midday?

Answer 28

Warmer is lower
Wavefronts:
Refracted upward into atmosphere

Question 29

Diffraction

Answer 29

Same as when H20 wave encounters a raft

Wave bends around obstacle and reforms

Question 30

When moving left-right and wave encounters barrier:

Answer 30

Some energy reflected back

Some bend around obstacle
Then reform and continue as plane wave front

Question 31

When wave fronts encounter opening in wall:

Answer 31

Some energy is reflected back

Parts of wave pass through and reform as plane waves

Question 32

Absorption

Answer 32

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

Question 33

Absorption coefficient

Answer 33

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

Question 34

Absorption coefficient from air –> water

Answer 34

0. 1% absorbed

99. 9% reflected

Question 35

Sound treated rooms (2)

Answer 35

Anechoic : high absorption coefficient 
- fibre class wedges/absorbing material

Sound isolated rooms: designed to reduce sound transmission - reasonably high absorption coeff.

Question 36

Absorption is ______ proportion to reflection

This, as absorption coefficient increases, reflecting and reverberation time ______.

Answer 36

Inversely

Decreases

Question 37

Interference

The further away from the ___________ the _______ the sound becomes.

Answer 37

Sound source

Softer