PH 105

Dr. Cecilia VogelLecture 4

OUTLINE Room Acoustics

concert hall properties

Reverberation Time Reverberation time

V, volume of room A, the absorption area = sum of

aS for all surfaces a from table for material, S = surface

area

Na for all seats a from table for seat, N = # of seats

aV a from table for air, V = volume

K is a constant: 0.161 for mks units

VRT K

A

Absorption example A 10 mX10mX4m room currently has a

RT of 1.0 s for sounds at 500Hz. What change will be made by covering five 1.2mX0.5m glass windows with heavy drapery?

The current absorption, A, can be found V

RT KA

A =

Example cont’d Lose the absorption of glass, add the

absorption of drapery: the absorption for each is aS, a from

___________, S =

A = 64.4m2 A =

VRT K

ART

Properties of Concert Halls “Intimacy”

means _____________ between direct and early sound is

sounds like “Liveness”

level of adds to

Properties of Concert Halls “Spaciousness”

has to do with sound from a)

b)

Properties of Concert Halls “Clarity”

more important in keep reverb

“Warmth” level of drapery, carpet

“Loudness” total

Properties of Concert Stage “Balance”

keep appropriate relative level of instruments

“Blend” mix of sound from

“Ensemble” can performers hear

Whispering Gallery A curved reflecting surface can focus sound

it sounds like it comes from

There are spots in the science bldg where you hear a voice think it’s but actually The sound was reflected and focused, making an

Diffraction When a wave meets an obstacle

that doesn’t totally or one that

it can diffract or to get

video

Diffraction SometimesNote that a wave will not diffract,

if its wavelength is

Ex: Visible light has wavelengths ____________ than a meter,

do not see through a doorway, or around a tree

Ex: Audible sound has wavelengths ______a meter

you can hear through doorway, or around a tree.

Ex: in lab, to see _________ without too much __________, must use ultrasound

“ultra” meaning high high freq corresponds to

SuperpositionWhen two disturbances (or waves)are at the same place at the same time,total disturbance is the sum of the two.

watch impulsive waves

InterferenceBecause of superposition,Waves, when they meet

can add or interfere constructively so the total is if they happen to have the same phase

periodic waves, when they meetcan cancel or interfere destructively so the total is if they happen to have opposite phase

BeatsTwo waves with slightly different frequency (period) go in and out of phase

InterferenceWaves from two source,will have places where they interfere constructively

what does it sound like with sound?what does it look like with light?

and other places where they interfere destructively

what does it sound like with sound?what does it look like with light?

video

SummaryUsing RTConcert hall qualities definedDiffraction is the bending of wave around an obstacle or through an opening

if obstacle or opening is similar in size to wavelength of the wave

Interference is the addition of waves at point where they meet

constructive interference means they add to larger wave (louder)destructive interference means they add to smaller wave (quieter)