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ACOUSTICS

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We live and work in environments of increasing levels of noise ... 4. Greater for pure tone than broad band noise. 5. Greater for moving rather than fixed sources ... – PowerPoint PPT presentation

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Title: ACOUSTICS


1
ACOUSTICS
Stein Reynolds Chapter 17 The Fundamentals of
Architectural Acoustics
2
General Concepts
  • Sound- desired and designed for
  • Noise- unwanted and designed out
  • We live and work in environments of increasing
    levels of noise-

3
  • 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

4
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

5
  • 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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7
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.

8
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

9

10
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.

11
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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14
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.

15
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

16
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

17
  • 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

18
  • Directivity
  • High frequency direct line travel, reflective
    surfaces give clues to listener as to origin of
    sound- low frequencies are hard to pin point

19
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

20
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

21
Noise Criterion Curves
  • Contours represent the maximum continuous
    background noise that will be considered
    acceptable in the environment specified

22
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23
Room Criteria Curves
  • Evaluate the acceptable mechanical system
    background noise

24
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25
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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27
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

28
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

29
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

30
Reverberation
  • Speech typically requires short TR
  • Music typically requires longer TR

31
  • 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

32
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

33
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

34
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

35
Specific noise control
  • A specific, concentrated source of noise should
    be controlled by isolation
  • (mech room enclosure)

36
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

37
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.

38
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)

39
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

40
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

41
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

42
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
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