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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.

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Room Acoustics for Classrooms: measurement techniques

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

  2. 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

  3. Properties of Sound Waves Amplitude 1 wavelength A time Time = 1/f Frequency = # of wavelengths/second (in Hertz)

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

  5. 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).

  6. 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

  7. Sound Pressure Level

  8. 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.”

  9. 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.

  10. 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

  11. Measurement Methods • METHODS: • Recorded noise burst • Starting gun • Thick balloon • GOAL: find the response of the room • to an impulsive sound

  12. 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.

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

  14. 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.

  15. Speech Intelligibility Tests • Modified Rhyme Test (MRT) • Standardized • Hearing Comfort Survey • Answer three questions after each MRT test

  16. 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

  17. Classroom Geometries Classroom 2 Volume = 330m3 Classroom 3 Volume = 330m3 Classroom 1 Volume = 330m3

  18. Intelligibility Test Results 1 2 3

  19. Trapezoidal Geometries B A C E D

  20. 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

  21. Hearing Comfort Results

  22. 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.

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

  24. 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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