ACOUSTICS

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ACOUSTICS
Stein Reynolds Chapter 17 The Fundamentals of
Architectural Acoustics
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General Concepts

• Sound- desired and designed for
• Noise- unwanted and designed out
• We live and work in environments of increasing
  levels of noise-

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• 3 Common elements of all acoustic systems
• Source
• Can be made to be louder or quieter
• Transmission path
• Can be designed to transmit more or less sound
• Receiver
• Listeners reception may also be altered

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Sound

• Is simply an audible signal
• Is a physical wave or a mechanical vibration or a
  series of pressure variation is an elastic medium
• Airborne sound - medium is air
• Structure born sound - building materials

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• Humans hear sounds in the 20-20,000 Hz range
• Most sensitive to sounds in the 125-6000Hz range
• Many animals can hear and make sounds in ranges
  higher than this- some lower
• Sounds lower than 20Hz are usually sensed as
  vibrations

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Sound Intensity

• Sound waves carry energy that can be used to do
  work- like move an eardrum, sonic boom
• The amount of energy transported per second by a
  sound wave is the power of the wave- measured in
  j/s or watts
• The eardrum is moved by air pressure
• We are more interested in this pressure as force
  density and pressure level than in sound
  intensity as a power density.

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Sound Intensity

• Sound waves generated in free space, spread out
  and pass through surfaces of increasingly large
  areas
• Sound Intensity (I) is sound power (P) passing
  perpendiculary through area (A)
• IP/A measured in W/m2

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Sound Intensity

• Sound reaches a listeners ear it is interpreted
  as loud or soft, depending on the intensity of
  the wave.
• Doubling intensity does not double loudness.

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Sound Intensities and Decibels

• Decibels are unitless values
• Its greatest value is to compare sound
  intensities- typically by computing and comparing
  the ratio of the intensities.
• The ear responds to sound pressure not intensity.
  BUT-Sound Pressure Level (SPL) is numerically
  equal to Intensity Levels (IL)
• Pressure levels as a ratio of sound pressure to a
  base level, is expressed in decibels

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Sound Pressure Levels (SPL)

• SPL is the end result
• It is the resultant sound or noise that in an
  enclosed space resulting from a source in that
  space as affected by the characteristics of the
  space and the position of the listener.

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Sound Intensity

• 1-3dBs in barely detectable
• Example - 2 stereo systems
• 90dB and 93dB
• ? 3dB 2x intensity
• Perceived - 70dB is perceived as twice as loud as
  60dB
• 80dB is perceived as twice as loud as 70dB

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Sound Pressure and Sound Pressure Level (SPL)

• Pressure levels as a ratio of sound pressure to a
  base level, is expressed in decibels
• Corresponds to the threshold of hearing
• 120-130dB pain limit occurs at all frequencies
• 0dB threshold occurs only at the 1000Hz level
• Ear in most sensitive in the 3000-4000Hz level at
  which point the threshold is -5dB

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• Sound Masking
• 2 or more separate sources of sound perceived
  together it is difficult to clearly perceive one
  due to the presence of the other
• Defined by the number of decibels by which the
  threshold of audible sound is raised by the
  presence of the other sound
• Low frequency will more effectively mask a high
  frequency than the reverse
• Can be used as a noise control technique

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• Directivity
• High frequency direct line travel, reflective
  surfaces give clues to listener as to origin of
  sound- low frequencies are hard to pin point

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Noise Criteria

• 2 issues concerning noise
• Psychological /practical
• Noise levels that cause annoyance and disturbance
  of sleep, relaxation, work and other daily
  activities
• Physiological
• Physical impacts of noise such as hearing loss

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Noise Criteria

• Psychological /Practical-annoyance
• 1. Proportional to loudness
• 2. Greater at high frequencies
• 3. Greater at intermittent levels than constant
• 4. Greater for pure tone than broad band noise
• 5. Greater for moving rather than fixed sources
• 6. Much greater for information bearing noise
• - speech interference levels (SIL)-office/commerc
  ial design

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Noise Criterion Curves

• Contours represent the maximum continuous
  background noise that will be considered
  acceptable in the environment specified

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Room Criteria Curves

• Evaluate the acceptable mechanical system
  background noise

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Sound in Enclosed Space

• Sound generated in an enclosed space radiates out
  - strikes large surfaces and room boundaries
  where it is reflected, transmitted and or
  absorbed-sound intensity is attenuated by
  distance-
• Greater reflection causes intermittent sounds to
  mix in room- possibly less intelligible speech,
  better music mix
• Room can be noisy (reverberation) or
• Dead (absorption)

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Design Considerations

• 2 things happen with the reflection of sound in a
  room
• 1- the noise level (volume) is greater than it
  would be in an open field
• 2- there is a delay factor- after the sound
  source has stopped some sound will persist in the
  space- reverberation

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reverberation and echos

• reverberation is similar to an echo but there are
  significant differences-
• Echo- discreet reflection of a sound- usually
  delayed 1/10th of a second or more
• an entire word can be return intact
• canyon wall
• Reverberation- continuous reflection over a short
  time span
• organ note dying out slowly in a cathedral

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Reverberation

• Reverberation-slow fading of sound in an enclosed
  space
• Reverberation Time- the amount of elapsed time
  before complete silence after a 60dB sound has
  stopped
• As space becomes larger TR tends to increase
• absorption increases TR decreases
• different room functions require different TR

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Reverberation

• Speech typically requires short TR
• Music typically requires longer TR

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• Absorptive treatment in a room will affect the
  reverberant noise level within that room but have
  little effect on the noise level in the adjoining
  spaces
• Absorbent material can also be carpets, drapery,
  sprayed on acoustic plaster

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Absorption Recommendations

• Absorption techniques are good
• 1- to change room reverberation techniques
• 2- In spaces with distributed noise sources such
  as offices, schools, restaurants, and machine
  shops
• 3- In spaces with hard surfaces and little
  absorbent content
• 4- where listeners are in the reverberant field

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Absorptive Materials

• Acoustic Tile
• standard sizes, various colors, finishes, some
  fire rated,
• mineral fiber or faced fiberglass
• NRC absorption rating
• 0.45- 0.75 mineral fiber tiles
• 0.95 fiberglass

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Noise Reduction

• 3 Components
• 1- Reduce noise generation at source
• -and installation proper equipment selection
• 2- Reduce noise transmission point to point
• - proper selection and installation of
  construction material
• 3- Reduce noise at the receiver
• - meet noise criteria level through appropriate
  acoustical treatment
• and possibly-Combination of all

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Specific noise control

• A specific, concentrated source of noise should
  be controlled by isolation
• (mech room enclosure)

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Impact Noise

• Erratic noise caused by footfalls, dropped
  objects vibrations of mechanical equipment
• Impact Isolation Class (IIC)
• Code required for adjacent dwellings
• Improved with carpet, resilient tile floor,
  suspended ceilings, concrete slab floated on
  compressed glass fiberboard laid on the
  structural floor

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Sound Transmission and Isolation

• Other than controlling sound within a space, it
  is important control the transmission of sound
  between 2 spaces
• Transmission Loss (TL)- the amount of noise
  reduced as the sound travels through the wall.
  Ratio expressed in dBs of acoustic energy
  reradiated on receiving room side of wall.

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Sound Transmission and Isolation

• TL-lab controlled value for walls material
  ability to act as acoustic barrier
• Noise Reduction (NR)- accounts for the TL and the
  receiving rooms acoustic character.
• (reverbs and absorption)

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Sound Transmission and Isolation

• Sound Transmission Class (STC) is often used in
  place of the TL rating
• STC- the index of a partitions resistance to the
  passage of sound
• The STC of a wall can be compromised when it only
  extends up to a light suspended ceiling, it is
  not sealed well at penetrations or if the barrier
  is penetrated by a common air duct

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Sound Transmission and Isolation

• Doubling the walls mass is one method of
  increasing TL but other methods are more viable
  such as
• - staggered studs and resilient channels

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Sound Isolation Descriptors

• Hearing conditions in a room can be altered by
  changing either the barrier characteristics and
  or the background sound level
• - barrier effectiveness expressed in STC
• - ambient noise level is frequency dependent it
  is approximated by NC (HVAC)
• The perceived isolation value of a barrier may be
  enhanced by raising the background , masking,
  noise level in the receiving room

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Overall Approach

• Isolate specific concentrated noise generators
• Design for room Noise Criteria
• Control Absorption and Reverberation in room
• Control the transmission of sound between 2
  spaces
• Certain rooms design require a more detailed
  control of reverberation times, location of
  reflections and listener positions (direct and
  reflected sound paths) - auditoriums, music
  halls, open office landscapes- this can be
  manually roughed in but should be modeled for a
  more complete analysis and design