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

  • 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

  • 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

  • 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

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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.

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


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.

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.

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
  • 80dB is perceived as twice as loud as 70dB

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

  • 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

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

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

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

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
  • 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
  • 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

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

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

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

  • 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
  • Absorbent material can also be carpets, drapery,
    sprayed on acoustic plaster

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
  • 3- In spaces with hard surfaces and little
    absorbent content
  • 4- where listeners are in the reverberant field

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

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

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

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

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.

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)

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

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

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

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
  • 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