Abigail Stefaniw

Room Acoustics for Classrooms: measurement techniques

University of Georgia Classroom Acoustics Seminar

Classroom Acoustics Standard

Draft ANSI standard0.4 – 0.6 RT35 dB(A) level

Specifies Measurement ProceduresPossibly included in International Building Code

ACOUSTICAL PERFORMANCE CRITERIA, DESIGN REQUIREMENTS AND GUIDELINES FOR SCHOOLS

Properties of Sound Waves

Amplitude

time

1 wavelength

Time = 1/f

A

Frequency = # of wavelengths/second (in Hertz)

Wavelength

b If b>> wavelengthsolid acts as barrier

High frequencies mean small wavelengthsLow frequencies mean large wavelengthsThings affect sound most if they are larger

than the wavelength

Sound Pressure

Sound pressure is measured or heard at a pointAt 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

0.00002 0

real quiet 0.0006 30 library 0.006 50 speech 0.06 70 heavy truck 00001.0 94 orchestra 00010.0 100 jet engine 00500.0 128

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.”

-20

0

20

40

60

80

100

50 100 200 500 1000 3000 5000 6000 10000

Hz

Lp

dB(A)

dB(B)dBC

deciBels and dB(A) levels

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).

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

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

METHODS:

Recorded noise burst

Starting gun

Thick balloon

GOAL: find the response of the room

to an impulsive sound

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 SurveyAnswer three questions after each MRT test

Classroom Acoustics Goals

High Speech IntelligibilityRequires proper Reverberation Time,

• Low volume, high sound absorption

Requires low background noise level.

High Hearing ComfortRequires proper overall geometry

Indicated by detailed acoustical metrics

Classroom Geometries

Classroom 1Volume = 330m3

Classroom 2Volume = 330m3

Classroom 3Volume = 330m3

1 2 3

Intelligibility Test Results

Trapezoidal Geometries

BA C

D E

Hearing Comfort Survey1. 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

Research ConclusionsRooms 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

Ease of HearingDepends on Early Energy Patterns

• Speech Intelligibility• depends on RT, dBA

Acoustical ComfortRequires high speech intelligibility, Clarity, and pleasant tonal spectrums

All Classrooms

(speech communication)

Information to be Analyzed

Noise Levels in dB(A), unoccupiedPlans or Geometry drawings of rooms

with materials noted, photos if possible

Room’s Response to Impulse NoiseFind Reverberation Time

Speech Intelligibility Test resultsHearing Comfort Survey results