Transcript Room Acoustics

Room Acoustics
Bouncing Around
October 27, 2006
Music and Other Sounds
Come from a source.
The source is not isolated, it is in an
environment.
The environment can affect what the
listener will hear:
– Ambient noise level
– Properties of the wall, ceiling, etc.
– Other sources producing sound at the same
time.
Consider a Pulse of Sound
WALL
Sound Changes
Different Travel Distances
The wall
Sends a “delayed” reflection of the sound
to the ear.
– A matter of distance.
The reflection may be synchronized with
the source so that they may “interfere”
The reflection may, be hindered by the
absorption of the sound energy by the
wall.
Example - Interference
“Wall
Wall
Wavelengths in music
Note different wavelengths and
compare with the size of a room.
Wavelength will be an important
variable in a room.
Surfaces (Walls, floors, etc.)
Rough or Smooth
Hard or soft
Location with respect to listener
Characteristics depend on the sound
being detected.
Two surfaces
IS THIS A ROUGH SURFACE???
1 nm = 10-12 meters
=0.000000000001 m
Again, 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.
– And possibly the ground
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
What about REAL Rooms???
In a Real Room
What about
the walls?
Smooth
– How Smooth?
WALL
Rough
– How Rough?
Transmission
properties?
Another Factor RESONANCE
Resonance Examples
Speakers?
At home with Shostakovich
If you can see it, you can hear it! Wherever you see your speaker
reflected in the mirror, that's a point of reflection that should receive
absorptive, or in some cases, diffusive acoustic treatment.
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 Eveready
Battery … they keep going and going and going
…..
Higher frequency sounds will be deflected or
absorbed more than low frequency sounds.
We discussed Reflections
What Do You Think?
Or a school performance hall
Professional Concert Hall
(mucho Dolleros )
Surfaces
Soft Walls
People???
Note
Modern halls are adjustable for
– The piece being played
– The size of the audience
Create a SUDDEN Sound
loudness
time
Listen & Record with a microphone
Real Example: Royal Festival Hall
Room Reflections
Room full of sound!
Room Full Of Sound
Cut a small Window into
the wall
EACH SECOND THE SAME FRACTION OF SOUND WILL
LEAK FROM THE ROOM LEADING TO WHAT IS CALLED
EXPONENTIAL DECAY.
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
Reverberation time is the time the sound takes to be reduced to one
millionth of its initial level.
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
For Music
Rooms must be carefully designed. The
“engineering” contains a lot of “Kentucky
Windage”.
Different kinds of music require different
acoustical designs.
In the right room, you hear what the
composer intended you to hear.
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