PH 105

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PH 105
Acoustics of Small Rooms, Recording Studios
Dr. James van Howe Lecture 21
Beatles, 1969 Apple recording session
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Announcements
Wednesday class, Feb. 20 Meet on stage in
Centennial Hall at 230 pm
-Enter through Bergendof, top floor stage door.
Buddy up with a musician if confused. -If you
go through main doors you will trigger an alarm
and be in big trouble!
Not that you would, but please do not move
anything on stage as it has been precisely set up
for rehearsal/performance
-Stay tuned for Fridays class. We may meet in
Centennial again.
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Name the Artist

• They Might be Giants
• Guster
• Cake
• School House Rock

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From what Beatles Album does this song come?

• White Album
• Sgt. Peppers Lonely Hearts Club Band
• Abbey Road
• Revolver

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Name the Artist

• The Roots
• Black eyed Peas
• Fugees
• Wyclef Jean

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Name the vocalist

• Robert Plant
• Jack White
• Bon Scott
• Chris Cornell

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True or False
Long Narrow concert halls tend to have better
sound because of the dominant first reflection
from the ceiling
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Which material would you expect to have an
absorption coefficient of 0.06?

• Painted Concrete
• Heavy Drapery
• Carpet
• Acoustical tile

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Concert Hall Review Sample Problem
What is for the concert hall below
(shoebox design) at 500 Hz?
Walls plaster on lath a0.06 Ceiling Acoustic
Tile a0.83 Floor Carpet on pad a0.57
10 m
15 m
25 m
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Sample Problem cont.
I just designed a very dry hall a good lecture
hall bad for music
If I changed my ceiling to plaster rather than
acoustical tile,
Much better hall, good for music and speaking!
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Sample Problem cont.
What if I take into account the absorption of the
air, the seats, and people in seats?
To take into account the air, we modify the
reverb time equation
m is a constant but only comes into play for
sounds 2,000 Hz
Can give upwards of 100 m2 extra drain area for
orchestral halls
Adding seats and people adds an effective area of
the drain as well
People in upholstered seats give an additional
0.56 m2 per seat at 500 Hz
If I have 200 seats in my auditorium, Ill have
an extra drain area of ?
Less reverb but still o.k. for music
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Reverb time and size of room
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Frequency dependence of reverb
-Reverb of bass very important (want longer times
than high-frequency)
-Good thing building materials stink at
absorption of bass
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Other important aspects

• Spatial impression
• Auditory source width (ASW) sound appears coming
  from a wider source than seen visually why early
  sidewall reflections important
• Listener Envelopment (LEV) impression that
  reverb comes in all directions (diffuse)
• Early decay time
• Rate of decay time often more important than
  total reverb time, a short initial rapid decay
  followed by a long decay still tricks the ear
  into thinking too short a reverb and a dry hall

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Early decay problems
60 dB
Sound Pressure Level
Should be Treverb
time
Even though total reverb time (time to decrease
by 60 dB pressure) long, initial decay very short
and tricks the ear
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What makes a good concert hall?
Study of 22 European concert halls

• Long reverberation up to 2.0 s however 2.0s
  less pleasing
• Narrow halls
• -early reflection from side walls very important
  (concert halls acoustically pleasing are often
  shoebox design)
• -drawback is you cant see performance as well
  often sacrifice sound for ability to see
  performance (fan-out design)
• High binaural dissimilarity (one ear hears
  something slightly different than other)
• Less definition sound energy in the first 50 ms
  lower in good halls

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Things to avoid in auditorium design

• Echoes
• -usually culprit is the rear wall
• Flutter Echoes
• -rapid series of echoes from two hard
  parallel surfaces
• Sound focusing
• -curved walls (concave) focus sound causing
  shadows in some spots and loud spots in other
  locations
• Sound shadows
• -often occur under balconies because
  reflections are blocked
• Background noise
• -Heating and cooling systems usually culprit

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Small Rooms
Reverberation time typically very small, and not
a figure-of-merit since volume of room so small
How do you up the reverb in a small room?
Make the walls hard (non-absorbent lots of
reflection)
But this leads to a shower room or dorm room
resonances
This very acoustically non-absorbent box that you
live in, is a nice resonant cavity Think
standing waves on a string, but in three
dimensions
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Shower Modes
Resonant modes of a box
H
Shower strengthens the specific frequencies given
by fl,m,n and kills the others
W
L
Example Tahiti Sand Shower Stall from Home Depot
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Why are Shower Modes Bad?
-The room acts as a filter that takes away
frequencies you might want!
-Not so good for your home theatre or recording
studio
Why doesnt this happen in a Concert Hall?
-It does happen in rectangular halls, but since
the volume of the hall is so big it really isnt
noticeable
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Ways to avoid strong room resonances

• For rectangular room, make or choose one whose
  ratio of dimensions is non-integer
• Make the room irregular shape (avoid the
  rectangle)
• -crucial for recording studio
• Electronically compensate
• Spread sound with sound diffusers
• -crucial for recording studios

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Non-integer ratios
A room whose dimensions (L x W x H) give a ratio
of 532 has a more continuous frequency response
than 123
This is because for integer ratios, many fl,m,n
are the same number (degenerate), making these
peaks stronger in the response curve
A cube has a ratio of 111 is the worst possible
choice
If you want your home theatre to sound the best,
put it in a room whose dimensions are way off
from integer ratios
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Irregularly shaped rooms, walls, surfaces
-Lack of parallel surfaces reduces constructive
and destructive interference that leads to box
resonances -Irregular surfaces help smooth out
spectral response -Also eliminates flutter echoes
Recording Studio Schematic Utusan Malaysia
Irregularly shaped reflecting surfaces
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Electronically compensate
Intelligent sound field tuning system for home
theater systems
IEEE Transactions on Consumer Electronics 51, pg
635 639, 2005.
System responds to acoustics of room by
adding/subtracting the appropriate sound field,
like very fancy noise canceling headphones
Can make the room sound less like a box without
changing the structure of the room!
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Sound diffusion
-Same reasoning behind irregular shape of room
-Want sound to spread in many directions to avoid
resonance
-Additional effect is to allow sound to
reverberate and mix
Sound diffusers are angular shapes or convex
often rough surfaces
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Hi-fidelity Sound Criteria

• The reproduced sound should contain nearly all
  frequency components of original sound
• Reproduced sound should be free of distortion
• Reproduced sound should have loudness and dynamic
  range comparable to original
• -Room really shouldnt be as loud as
  concert hall 55 dB dynamic range good for a room
  70 db good for hall
• Spatial sound pattern of original sound should be
  reproduced
• -Often most difficult to satisfy
• Reverb characteristics of original sound should
  be preserved
• -Dont want to add reverb from room to
  reverb in recording

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Spatial Sound Reproduction
Multi-channel sound reproducing systems
Top how it is recorded
Monophonic
Monaural
Binaural
Stereophonic
Surround
Bottom how it is reproduced
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Multi-channel

• Monophonic Not to be underestimated, but not
  many good systems exist
• Monaural Like telephone no where near hi-fi
• Binaural Microphones in dummy head reproduces
  realistic directionality
• Unfortunately only for one position in room
• Stereophonic very realistic if sitting close to
  the median plane
• Surround sound 5 speakers THX format left,
  right, center, left-surround (left side wall),
  right-surround, plus subwoofer
• Center used for voice tracks, since voice
  intended to be centrally localized voice track
  is behind screen in movie theatre

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Speaker Placement, Sound Images
Median plane
If sources are at identical sound level and
symmetrically placed, the sound image is at A
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What if we turn up Level on Left Speaker?
Sound image moves to left
Direction you perceive is
Ex. Left speaker is 100 units, Right is 10 units,
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Speaker Placement
Three arrangements for a rectangular room ratio
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Larger sweet spot area in this geometry
Shaded area near median plane shows sweet spot
Speakers in corner give optimal radiation spread
and enhance bass resonance
Want the whole angle between speakers to be 40-90
degrees
-Less than 40 degrees gives monophonic
sound -More than 90 leaves hole in the center
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Recording Studios

• Irregular shape to increase sound mixture, smooth
  out room resonance, eliminates flutter echoes
• Uses sound diffusers to blend sound
• Isolation systems to rid background noise
• Rule of three Musician should be three times as
  close to her microphone than anyone elses
  (eliminates background)

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Control Room
Dead end less reflections
More reflections
Mixer needs to hear sounds from studio clearly,
the ITD (initial delay time) between reflections
is too short to hear a nice blend of sound
Making control room in the above shape lengthens
ITD from 5 ms studio to 20 ms
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Homework
No more for the rest of the course!
Use this opportunity to study and come ask
questions this week
See you in Centennial on Wednesday