Transcript Lecture 20 - UCF Physics

Lecture 20
Bouncing Around
November 5, 2004
Last Time
I played a sound.
It had a frequency of about 3000 Hz.
The wavelength was therefore
 l=velocity/frequency = 340 (m/s)/3000=0.13m
– This distance is about 10 cm, or about 6
inches.
– As you moved your head through this
distance you heard maxima and minima as
you went from CONSTRUCTIVE
INTERFERENCE to DESTRUCTIVE
INTERFERENCE.
The cause of this was reflected sound
adding to the original sound. This is what
we might call a “room effect”
Yesterday’s Demo
Move yer head
WALL
Different Distances
Schematic
DIRECT SOUND
Head Movement
REFLECTED SOUND
Today
We discuss
– How sound propagates.
– How rooms work.
Remaining topics,
– Electricity and Magnetism and the use ob
both for
Microphones and Speakers.
How the Brain hears music – psychophysics
Miscellany
Consider this Table
Two surfaces
IS THIS A ROUGH SURFACE???
1 nm = 10-12 meters
=0.000000000001 m
Consider a Wall
How smooth is it?
Smooth is in the feel of the feeler!
Smooth or Rough are Relative terms.
We define:
– SMOOTH – Variations occur on a scale much
smaller than a wavelength of the sound we
are considering.
– ROUGH – The variations in the surface are
comparable to the size of the wavelength.
SMOOTH
SPECULAR
ROUGH
DIFFUSE
SOFT Walls
A soft wall (like rubber or cork) will yield when
you push on it.
Sound (music) pressure pushes on the wall.
IF the wall deforms, than a force (pA) times a
distance (the deformation), means that the wave
does WORK.
The sound therefore loses some energy when it
hits such a wall.
The reflection isn’t as strong as one from an “unyielding” wall.
Consider an outdoor concert
Musicians on stage
People in the audience
No Walls or Ceilings
Only reflections possible are from
structures in back of the musicians.
Useful aspects of reflection
Think about the reverse!
The old Greek Amphitheater
Closer Audience
“Band Shell”
Care in a band-shell
The focus can’t be too good because then
all of the performers need to be at the
same place.
Since they can’t be, a vertical wall might
be better.
Real Band shells look right but really do
NOT properly focus. ON PURPOSE!
What does “focus” mean
Sound waves hit a surface which can be called a
mirror.
The mirror surface can be curved so that rays of
sound from different directions can be made to
come together at the same place.
– Like a lens
In a concert hall, too much focusing can also
mean that there is only ONE good seat in the
house!
EXAMPLE: The Ellipse
A & B = foci
Whispering Gallery
Note – This Wren design was actually a spherical surface
that doesn’t really focus that well. It probably comes close to
a portion of an ellipse.
APPROXIMATION ??
Parabolic Reflector
Parabolic
Receiver
Another One
An interesting application
With this device, you
can magnify faint or
distant sounds with a
clarity you never
thought possible.
You can listen to
bird calls in the
forest!
(Advertising Pitch).
What about REAL Rooms???
In a Real Room
What about
the walls?
Smooth
– How Smooth?
WALL
Rough
– How Rough?
Diffraction
Sound can “bend” around objects.
Sound can change its properties
depending upon the size of the
wavelength compared to objects.
The Diffraction effect can be understood
via one of the early theories of waves.
A Bad Photo .. sorry
ploop
Huygen's Principle 1678
Polaroid Photo
Huygen's Principle
vt
Every point on the front
of a wave (wave front)
acts as a source of
spherical waves.
The next position of the
wave front will be the
surface that is tangent
to all of the other parts
of the surface created
in the same way.
The spherical wave
travels at the speed of
sound.
Another View
A Slit (Window)
Diffraction Through a SMALL
Opening (comparable to l)
DIFFRACTION
An Edge
Sound
Travels in straight Lines.
Travels in crooked lines.
Can be focused.
Can be absorbed by a surface
Can be diffracted
Can interfere “with itself”
Is dependent on the properties of the
room.
What else?
Small objects will scatter or diffract sound so it
can be heard in non-straight lines.
– Around edges, etc.
Small objects do very little to long wavelength
sounds (low tones). They are like the
eEverready Battery … they keep going and
going and going …..
Higher frequency sounds will be deflected or
absorbed more than low frequency sounds.
– Band Example
What about REAL Rooms???
In a Real Room
What about
the walls?
Smooth
– How Smooth?
WALL
Rough
– How Rough?
We discussed Reflections
What Do You Think?
Or a school performance hall
Professional Concert Hall
(mucho Dolleros )
Types of Surfaces
Crowell Concert Hall
Soft Walls
People???
Create a SUDDEN Sound
loudness
time
Listen & Record with a microphone
Real Example: Royal Festival Hall
Room Reflections
Listen to the Room!
Lets start a musical tone and listen to
the auditorium with a sound recorder.
How about the return to silence?
There is a steady musical sound in the
auditorium.
The symphony is over.
The music suddenly stops. It takes a
certain time for the sound level to get to a
very small level.
The time it takes for the auditorium sound
to drop to 1/1,000,000th of the steady level
is called the REVERBERATION TIME.
The Return to Peace
Reverberation Time
Sound in a Room can be modeled
Leakage
Sound level is
reduced
each second.
Sound Filled Room
More Absorbing
Absorbing Materials
A Formula NOT to be Remembered

0.16 x Volume of the room
reverberat ion time  
 " effective" area of all absorbing surfaces



Let’s try a calculation – Living Room @
500 Hz (Book states this wrong)
3m
4m
5m
Ceiling Area = 4 x 5 = 20 m2
Effective = 0,1 x 20 = 2m2
Another Example
300 x 0.1
same
Reverberation Times Desired
Lecture 20A
Completion of #20
11/22
Last time
Between the source and the ear
– reflections
– interference (constructive and destructive)
– diffraction
bend around obstacles
spread out from openings comparable to the
wavelength of the tone
We started to discuss the properties of
rooms … walls, ceiling, floor, etc.
A Slit (Window)
Diffraction Through a SMALL
Opening (comparable to l)
DIFFRACTION
An Edge
So, Sound …
Travels in straight Lines.
Travels in crooked lines.
Can be focused. (Band shell, whispering gallery)
Can be absorbed by a surface
Can be reflected by a surface
Can be diffracted
Can interfere “with itself” and with other sounds.
Is dependent on the properties of the room.
What else?
Small objects will scatter or diffract sound so it
can be heard in non-straight lines.
– Around edges, etc.
Small objects do very little to long wavelength
sounds (low tones). They are like the Eveready
Battery … they keep going and going and going
…..
Higher frequency sounds will be deflected or
absorbed more than low frequency sounds.
– Band Example
What about REAL Rooms???
Back to the Real Room
Let’s get serious about
the walls!
WALL
We discussed Reflections
What Do You Think?
Or a school performance hall
Professional Concert Hall
(mucho dolleros )
Types of Surfaces
Crowell Concert Hall
Soft Walls
People???
Create a SUDDEN Sound
loudness
time
Listen & Record with a microphone
Real Example: Royal Festival Hall
Room Reflections
Listen to the Room!
Lets start a musical tone and listen to
the auditorium with a sound recorder.
How about the return to silence?
There is a steady musical sound in the
auditorium.
The symphony is over.
The music suddenly stops. It takes a
certain time for the sound level to get to a
very small level.
The time it takes for the auditorium sound
to drop to 1/1,000,000th of the steady level
is called the REVERBERATION TIME.
The Return to Peace
Reverberation Time
Sound in a Room can be “modeled”
Leakage
Sound level is
reduced
each second.
Sound Filled Room
More Absorbing
Absorbing Materials
A Formula NOT to be Remembered

0.16 x Volume of the room
reverberat ion time  
 " effective" area of all absorbing surfaces



Let’s try a calculation – Living Room @
500 Hz (Book states this wrong)
3m
4m
5m
Ceiling Area = 4 x 5 = 20 m2
Effective = 0.1 x 20 = 2m2
Another Example
300 x 0.1
same
Reverberation Times Desired