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Room AcousticsRoom Acoustics
Bouncing ARoundBouncing ARound
Travels in straight Lines.Travels in straight Lines.
Travels in crooked lines.Travels in crooked lines.
Can be focused.Can be focused.
Can be absorbed by a surfaceCan be absorbed by a surface
Can be diffractedCan be diffracted
Can interfere “with itself”Can interfere “with itself”
Is dependent on the properties of the room.Is dependent on the properties of the room.
sound ABsoRptionsound ABsoRption
Acoustic absorption or sound
absorption refers to the process by which a
material, structure, or object takes in sound
energy when sound waves are encountered,
as opposed to reflecting the energy. Part of
the absorbed energy is transformed
into heat and part is transmitted through the
absorbing body. The energy transformed into
heat is said to have been 'lost'.
Sound can “bend” around objects.Sound can “bend” around objects.
Sound can change its properties dependingSound can change its properties depending
upon the size of the wavelength comparedupon the size of the wavelength compared
to objects.to objects.
The Diffraction effect can be understoodThe Diffraction effect can be understood
via one of the early theories of waves.via one of the early theories of waves.
Listen to a tone and move yourListen to a tone and move your
head from side to side.head from side to side.
What do you hear?What do you hear?
Why do you hear it??Why do you hear it??
movE youR hEAdmovE youR hEAd
considER A WAllconsidER A WAll
How smooth is it?How smooth is it?
Smooth is in the feel of the feeler!Smooth is in the feel of the feeler!
Smooth or Rough are Relative terms.Smooth or Rough are Relative terms.
We define:We define:
– SMOOTH – Variations occur on a scale muchSMOOTH – Variations occur on a scale much
smaller than a wavelength of the sound we aresmaller than a wavelength of the sound we are
– ROUGH – The variations in the surface areROUGH – The variations in the surface are
comparable to the size of the wavelength.comparable to the size of the wavelength.
soft WAllssoft WAlls
A soft wall (like rubber or cork) will yield whenA soft wall (like rubber or cork) will yield when
you push on it.you push on it.
Sound (music) pressure pushes on the wall.Sound (music) pressure pushes on the wall.
IF the wall deforms, than a force (pA) times aIF the wall deforms, than a force (pA) times a
distance (the deformation), means that the wavedistance (the deformation), means that the wave
does WORK.does WORK.
The sound therefore loses some energy when itThe sound therefore loses some energy when it
hits such a wall.hits such a wall.
The reflection isn’t as strong as one from an “un-The reflection isn’t as strong as one from an “un-
yielding” wall.yielding” wall.
considER An outdooRconsidER An outdooR
Musicians on stageMusicians on stage
People in the audiencePeople in the audience
No Walls or CeilingsNo Walls or Ceilings
Only reflections possible are fromOnly reflections possible are from
structures in back of the musicians.structures in back of the musicians.
usEful AspEcts ofusEful AspEcts of
Think about the reverse!
The old GreekThe old Greek
WhAT does “foCus” meAnWhAT does “foCus” meAn
Sound waves hit a surface which can be called aSound waves hit a surface which can be called a
The mirror surface can be curved so that rays ofThe mirror surface can be curved so that rays of
sound from different directions can be made tosound from different directions can be made to
come together at the same place.come together at the same place.
– Like a lensLike a lens
In a concert hall, too much focusing can alsoIn a concert hall, too much focusing can also
mean that there is only ONE good seat in themean that there is only ONE good seat in the
eXAmple: The ellipseeXAmple: The ellipse
A & B = foci
WhisperinG GAlleryWhisperinG 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.
An interesting ApplicAtionAn interesting ApplicAtion
With this device, you can
magnify faint or distant
sounds with a clarity
you never thought
WhAt About reAlWhAt About reAl
in A reAl roomin A reAl room
What about theWhat about the
– How Smooth?How Smooth?
– How Rough?How Rough?
WhAt else?WhAt else?
Small objects will scatter or diffract sound so itSmall objects will scatter or diffract sound so it
can be heard in non-straight lines.can be heard in non-straight lines.
– Around edges, etc.Around edges, etc.
Small objects do very little to long wavelengthSmall objects do very little to long wavelength
sounds (low tones). They are like the eEverreadysounds (low tones). They are like the eEverready
Battery … they keep going and going and goingBattery … they keep going and going and going
Higher frequency sounds will be deflected orHigher frequency sounds will be deflected or
absorbed more than low frequency sounds.absorbed more than low frequency sounds.
We discussed reflectionsWe discussed reflections
A School Performance HallA School Performance Hall
Professional Concert HallProfessional Concert Hall
Lets start a musical tone and listen toLets start a musical tone and listen to
the auditorium with a sound recorder.the auditorium with a sound recorder.
How about tHe return toHow about tHe return to
There is a steady musical sound in theThere is a steady musical sound in the
The symphony is over.The symphony is over.
The music suddenly stops. It takes aThe music suddenly stops. It takes a
certain time for the sound level to get to acertain time for the sound level to get to a
very small level.very small level.
The time it takes for the auditorium soundThe time it takes for the auditorium sound
to drop to 1/1,000,000to drop to 1/1,000,000thth
of the steady level isof the steady level is
called thecalled the REVERREVERBERATION TIMEBERATION TIME..
tHe return to PeacetHe return to Peace
Sabine’s FormulaSabine’s Formula
A resonant absorber is a vibrational
system that “runs” on sound pressure. As
vibrational science will tell us a resonant
absorber is a mass vibrating against a
spring. The mass is the cabinet and front wall
or diaphragm. The spring is the air inside the
cavity of the resonant absorber.
If you change the vibrating mass and
stiffness of the spring, you can control and
tune the resonant absorber to the resonant
frequency of choice. The internal mass or
cabinet depth determines design frequency.
The spring or internal air and cavity are used
for achieving rate of absorption above the
unit’s designed for resonant frequency.
There are two types of resonant absorbers:
Helmholtz and Diaphragmatic or membrane.