Transcript Room Acoustics for Classrooms: measurement techniques

Room Acoustics for Classrooms:
measurement techniques
Abigail Stefaniw
University of Georgia
Classroom Acoustics Seminar
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
Properties of Sound Waves
Amplitude
1 wavelength
A
time
Time = 1/f
Frequency = # of wavelengths/second (in Hertz)
Wavelength
High frequencies mean small wavelengths
 Low frequencies mean large wavelengths
 Things affect sound most if they are larger
than the wavelength

b
If b>> wavelength
solid acts as barrier
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).
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
Sound Pressure Level
SOURCE
Pressure (Pa) Level (dB)
threshold of
hearing
real quiet
library
speech
heavy truck
orchestra
jet engine
0.00002
0
0.0006
0.006
0.06
00001.0
00010.0
00500.0
30
50
70
94
100
128
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.”
100
80
dB(A)
dB(B)
dBC
60
Lp 40
20
0
-20
50
100
200
500 1000 3000 5000 6000 10000
Hz
deciBels and dB(A) levels
Fletcher-Munson
produced rationale for
A-, B-, and Cweighting.
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).
Reverberation Time
Length of Time a sound takes to decay 60 dB.
Developed by Sabine when studying a lecture hall
at Harvard.
RT = 0.05*V/A
A = each surface’s
area * absorption
Measurement Methods
GOAL: find the response of the room
to an impulsive sound
METHODS:
Recorded noise
burst
Starting gun
Thick balloon
Starting Gun Method
Simple, easily transportable, consistently loud.
Gives a impulse noise with energy mostly in the
middle frequencies, but that’s what we need.
Extech Sound Level Meters
Accurate, detachable microphone
Built-in storage and computer
interface.
So, how noisy is THIS room?
HVAC concerns
Main source of noise
in unoccupied rooms.
In-room units
Central units
Measure both while it
is actively blowing air
and while it’s passive.
Speech Intelligibility Tests
Modified Rhyme Test (MRT)
Standardized
Hearing Comfort Survey
Answer three questions after each MRT
test
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
Classroom Geometries
Classroom 2
Volume = 330m3
Classroom 3
Volume = 330m3
Classroom 1
Volume = 330m3
Intelligibility Test Results
1
2
3
MRT Data Spread
Average Intelligibility
100
Percentage Correct
90
80
70
60
50
1A
2A
3A
1B
2B
Room
3B
1C
2C
3C
Trapezoidal Geometries
A
D
B
C
E
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
Hearing Comfort Results
0.12
0.1
C
Relative Comfort
0.08
0.06
0.04
D
0.02
0
-0.02
105
110E
115
120
125
130
-0.04
B
-0.06
A
-0.08
Wall Angle
135
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.
Acoustical Comfort and Hearing Comfort
Acoustical Comfort
Requires high speech
intelligibility, Clarity, and
pleasant tonal spectrums
Ease of Hearing
Depends on Early
Energy Patterns
All Classrooms
(speech communication)
• Speech Intelligibility
• depends on RT, dBA
Information to be Analyzed
Noise Levels in dB(A), unoccupied
Plans or Geometry drawings of rooms
with materials noted, photos if possible
Room’s Response to Impulse Noise
Find Reverberation Time
Speech Intelligibility Test results
Hearing Comfort Survey results