Title: Abigail Stefaniw
1Room Acoustics for Classrooms measurement
techniques
University of Georgia Classroom Acoustics
Seminar
2Classroom 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
3Properties of Sound Waves
Amplitude
1 wavelength
A
time
Time 1/f
Frequency of wavelengths/second (in Hertz)
4Wavelength
- 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
5Sound 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).
6Decibels
- 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
7Sound Pressure Level
8LOUDNESS 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.
9deciBels 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).
10Reverberation 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
11Eyring Equation
- Developed to improve accuracy for smaller rooms.
- Absorption treated slightly differently
12Measurement Methods
- GOAL find the response of the room
- to an impulsive sound
- METHODS
- Recorded noise burst
- Starting gun
- Thick balloon
13Starting Gun Method
- Simple, easily transportable, consistently loud.
- Gives a impulse noise with energy mostly in the
middle frequencies, but thats what we need.
14Extech Sound Level Meters
- Accurate, detachable microphone
- Built-in storage and computer interface.
- So, how noisy is THIS room?
15HVAC 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.
16Speech Intelligibility Tests
- Modified Rhyme Test (MRT)
- Standardized
- Hearing Comfort Survey
- Answer three questions after each MRT test
17Classroom 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
18Classroom Geometries
Classroom 2 Volume 330m3
Classroom 3 Volume 330m3
Classroom 1 Volume 330m3
19Intelligibility Test Results
1 2 3
20Trapezoidal Geometries
B
A
C
E
D
21Hearing 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
22Hearing Comfort Results
23Research 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.
24Acoustical Comfort and Hearing Comfort
25Information 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