Abigail Stefaniw

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Room Acoustics for Classrooms measurement
techniques

• Abigail Stefaniw

University of Georgia Classroom Acoustics
Seminar
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Classroom Acoustics Standard

• Draft ANSI standard
• 0.4 0.6 RT
• 35 dB(A) level
• Specifies Measurement Procedures
• Possibly included in International Building Code

ACOUSTICAL PERFORMANCE CRITERIA, DESIGN
REQUIREMENTS AND GUIDELINES FOR SCHOOLS
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Properties of Sound Waves
Amplitude
1 wavelength
A
time
Time 1/f
Frequency of wavelengths/second (in Hertz)
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Wavelength

• High frequencies mean small wavelengths
• Low frequencies mean large wavelengths
• Things affect sound most if they are larger than
  the wavelength

If bgtgt wavelength solid acts as barrier
b
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Sound Pressure

• Sound pressure is measured or heard at a point
• At any given point, sound pressure varies from
  about 10-6 Pa to 105 Pa
• The weakest sound that the average ear can detect
  is 20 µPa.
• The ear can tolerate sound roughly 1 million
  times greater than 20 µPa (i.e. 20 Pa).

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Decibels

• Because of the great range of pressure within the
  range of human hearing ( 0.0002 to 100,000 Pa)
  decibels were developed.
• decibel level (dB) 10 x log (power ratio)
• For sound, the power ratio Pressure2/Reference
  Pressure2
• where Reference Pressure threshold of hearing
  0.000020 Pa 20 micro Pa

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Sound Pressure Level
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LOUDNESS AND WEIGHTING

• At certain frequencies, some sounds at the same
  (dB) level seem louder than others.
• Fletcher-Munson did a survey using pure tones,
  which resulted in Loudness Curves.

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deciBels and dB(A) levels

• Fletcher-Munson produced rationale for A-, B-,
  and C-weighting.
• the frequency range of speech is our most
  sensitive range.

• dB(A) gives the frequencies humans hear as louder
  more weight.
• So, if the noise contains mostly low frequencies,
  the dB(A) will be less than the unweighted dB(C).

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Reverberation Time

• Length of Time a sound takes to decay 60 dB.
• Developed by Sabine when studying a lecture hall
  at Harvard.
• RT 0.05V/A
• A each surfaces
• area absorption

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Eyring Equation

• Developed to improve accuracy for smaller rooms.
• Absorption treated slightly differently

• RT

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Measurement Methods

• GOAL find the response of the room
• to an impulsive sound

• METHODS
• Recorded noise burst
• Starting gun
• Thick balloon

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Starting Gun Method

• Simple, easily transportable, consistently loud.
• Gives a impulse noise with energy mostly in the
  middle frequencies, but thats what we need.

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Extech Sound Level Meters

• Accurate, detachable microphone
• Built-in storage and computer interface.
• So, how noisy is THIS room?

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HVAC concerns

• Main source of noise in unoccupied rooms.
• In-room units
• Central units
• Measure both while it is actively blowing air and
  while its passive.

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Speech Intelligibility Tests

• Modified Rhyme Test (MRT)
• Standardized
• Hearing Comfort Survey
• Answer three questions after each MRT test

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Classroom Acoustics Goals

• High Speech Intelligibility
• Requires proper Reverberation Time,
• Low volume, high sound absorption
• Requires low background noise level.
• High Hearing Comfort
• Requires proper overall geometry
• Indicated by detailed acoustical metrics

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Classroom Geometries
Classroom 2 Volume 330m3
Classroom 3 Volume 330m3
Classroom 1 Volume 330m3
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Intelligibility Test Results
1 2 3
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Trapezoidal Geometries
B
A
C
E
D
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Hearing Comfort Survey

• 1. Ear strain How much did you have to guess,
  or fill in from context?
• -3 -2 -1 0 1 2 3
• too much ? average ? nothing
• 2. Processing strain How hard are you
  concentrating to understand words?
• -3 -2 -1 0 1 2 3
• difficult ? average ? no concentration
• 3. General strain How pleasant and comfortable
  is the sound environment?
• -3 -2 -1 0 1 2 3
• unpleasant ? average ? very pleasant

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Hearing Comfort Results
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Research Conclusions

• Rooms C and D, with LEF from 26-28 are in the
  optimal range for Hearing Comfort, but the range
  width needs confirmation with many rooms with
  Lateral Energy Fractions around 22-32
• Acoustical Comfort and Ease of Hearing are not
  the same thing, but they seem to overlap. The
  nature of the relationship has yet to be
  determined.
• Ease of Hearing is definitely more refined in
  scale, and describes a higher quality range than
  speech intelligibility.

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Acoustical Comfort and Hearing Comfort
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Information to be Analyzed

• Noise Levels in dB(A), unoccupied
• Plans or Geometry drawings of rooms
• with materials noted, photos if possible
• Rooms Response to Impulse Noise
• Find Reverberation Time
• Speech Intelligibility Test results
• Hearing Comfort Survey results