# Abigail Stefaniw Room Acoustics for Classrooms: measurement techniques University of Georgia Classroom Acoustics Seminar.

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Abigail Stefaniw Room Acoustics for Classrooms: measurement techniques 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 time 1 wavelength Time = 1/f A Frequency = # of wavelengths/second (in Hertz)

Wavelength b If b>> wavelength solid acts as barrier High frequencies mean small wavelengths Low frequencies mean large wavelengths Things affect sound most if they are larger than the wavelength

Sound Pressure Sound pressure is measured or heard at a point At any given point, sound pressure varies from about 10 -6 Pa to 10 5 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 = Pressure 2 /Reference Pressure 2 where Reference Pressure = threshold of hearing 0.000020 Pa = 20 micro Pa

Sound Pressure Level

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.”

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

Eyring Equation Developed to improve accuracy for smaller rooms. Absorption treated slightly differently RT=

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 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 1 Volume = 330m 3 Classroom 2 Volume = 330m 3 Classroom 3 Volume = 330m 3

1 2 3 Intelligibility Test Results

Trapezoidal Geometries B AC D E

Hearing Comfort Survey 1. Ear strain : How much did you have to guess, or fill in from context? -3-2-1 0123 too much  average  nothing 2. Processing strain: How hard are you concentrating to understand words? -3-2-1 0123 difficult  average  no concentration 3. General strain : How pleasant and comfortable is the sound environment? -3-2-1 0123 unpleasant  average  very pleasant

Hearing Comfort Results

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 Ease of Hearing Depends on Early Energy Patterns Speech Intelligibility depends on RT, dBA Acoustical Comfort Requires high speech intelligibility, Clarity, and pleasant tonal spectrums All Classrooms (speech communication)

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

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