Reflection, Diffraction, & Absorption - 10/13

what is sound reflection?

when sound waves bounce off of a surface (echo or reverberation)

the angle of the reflected path to the perpendicular is equal to…

the angle of the incident path to the perpendicular

when a sound is reflected from air to air, there is a [low/high] impedance mismatch and a [low/high] energy transfer

low impedance mismatch, high energy transfer

if continuing to travel in the same medium, when reflected off of an obstacle, a sound wave’s speed of propagation [will/will not] change

will not

does inverse square law hold when a sound is reflected in the same medium? why?

no. reflections add to the intensity by creating constructive interference with the original waves

the decrease in intensity of a reflected sound wave is [more/less] than the inverse square law would predict

less. energy is retained in medium

under what circumstance will a ray be reflected back on itself toward the source?

when the ray of incidence comes in at 0 degrees

this is an example of a [convex/concave] surface. 

convex

when sound reflects off of a convex surface, it becomes [more/less] intense. why?

less intense because the sound diverges in many different directions → causes the sound to spread out more

this is an example of a [convex/concave] surface. 

concave

when sound reflects off of a concave surface, it becomes [more/less] intense. why?

more intense because the reflected waves converge, and sound energy is collected - intensity is greatest at focal point

regardless of whether the obstacle surface is plane, convex, or concave, the angles of reflected rays to the perpendicular equal….

the angles of the incident rays to the perpendicular

if the roof of a building is removed, will there be [more/less] echos? why?

less because there is less area for sound to reflect off of

what are reverberant rooms? give an example

rooms with hard surfaces to maximize reflections, like a church or a house with no furniture = more echos

what are anechoic rooms? give an example

rooms for absorbing surfaces to minimize reflections, like a sound booth = less echos

what is T₆₀?

reverberation time = the time it takes for a sound to decay by 60dB 

we want T₆₀ to be [longer/shorter]

shorter = less echo-y

a classroom will have a [longer/shorter] T₆₀ value

shorter

a church or large, open space will have a [longer/shorter] T₆₀ value

longer

empty rooms are very echo-y because….

there is nothing to absorb sounds

what is sound diffraction?

when sound waves bend around an object

this is an example of….

diffraction

what is sound absorption?

when sound is absorbed (converted to other forms of energy) to reduce echos in a room

if an object’s impedance is infinite, the intensity of the reflected wave will equal the intensity of the incident wave (I_r = I_i. why?

if there is infinite opposition, nothing will be absorbed by the object

if impedance is not infinite, [some/all/none] of the sound energy will be absorbed by the new medium

some

if impedance is not infinite, how do the intensity of the reflected and incident waves compare?

the intensity of the reflected wave will be less than the intensity of the incident wave (I_r < I_i) because some energy is absorbed by the new medium

the magnitude of absorption is given by the absorption coefficient ____. what does it represent?

alpha (α). represents the proportion of energy in the incident wave that is absorbed by material

alpha (α) =

α = I_a / I_i = energy absorbed/energy of incident wave

the absorption coefficient of a material that absorbs all energy will be closer to….

1

[anechoic/reverberant] rooms will have a high absorption coefficient (close to 1)

anechoic

the absorption coefficient of a material that absorbs no energy will be closer to….

0

[anechoic/reverberant] rooms will have a low absorption coefficient (close to 0)

reverberant

if SPL _i = 80 dB, and 0.1% is absorbed, what is the SPL of the sound wave retained in the medium?

79.996 dB SPL is retained in the medium

dB = 10Log(0.999) = -0.004

dB SPL = 80 - 0.004 = 79.996

a material’s absorption coefficient [does/does not] change with intensity

does not change with intensity

a material’s absorption coefficient [does/does not] change with frequency

does change with frequency

does a material’s absorption coefficient vary with the intensity of the incident wave (I_i)?b

no because a material has a fixed absorption coefficient.

α = 0.001, how much of the energy is absorbed regardless of the intensity of the incident wave?

0.1%

absorption and reflection have a [direct/inverse] relationship

inverse

as the absorption coefficient increases, reflection and reverberation time [increase/decrease]

decrease

if you use high quality sound absorbing materials, T₆₀ value will go [up/down]

down

every material has an α, which varies with _____

frequency

total absorption in a room depends on what two things

absorption and room volume 

in a large, bare room with an open window, with the a=1 for the window, will the sound reflect?

no. the window has 100% absorption, so there is no surface for the sound to reflect off of

will materials have an α of 1? will open spaces?

materials will never have an α of 1 (100% absorption), but yes, open spaces like windows or doors will have an α of 1

what is the relationship between T₆₀ and V (volume of a room)? how is the sound energy affected?

directly proportional. T₆₀∝V. If the volume of a room increases, then T₆₀ increases. Sound energy is retained in the room for a longer period of time

what is the relationship between T₆₀ and A (area of an opening)? how is the sound energy affected?

inversely proportional. T₆₀∝ 1/A. if the area of an opening increases, then T₆₀ decreases. sound energy escapes the room more quickly

how can you lower (improve) a T₆₀ value?

by having more openings (windows/doors)

what is total absorption (A’)?

the sum of the total absorptions of individual surfaces.

A’ = S₁ α₁ + S₂ α₂ +…. + S_n α_n

T₆₀ varies with ___ and ___

Volume of a room and frequency of a sound

what is the doppler effect?

as a sound gets closer to a person, its pitch increases. as a sound gets further from a person, its pitch decreases. 

why does the doppler effect happen?

as a sound gets closer to a person, its wavelength decreases, causing frequency to increase.

as a sound gets further from a person, its wavelength increases, causing frequency to decrease.