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Classroom Acoustics January 13, 2014
Aural RehabilitationUniversity of Mississippi
David S. Woolworth
Oxford Acoustics
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Overview
● Decibels, Background Noise, and Speech Intelligibility● Room Acoustics● Identifying and Solving Problems in the Field● Summary● Standards Resources● Relationship of Classroom Acoustics and Hearing Aids/Hearing
Impaired Systems
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Decibels and Waveforms
● How does a sound go from pressure variations in the air to become a single number in decibels?
What is the National Standard for Background Noise in Classrooms?
● 35dBA
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Background Noise Sources● Outdoor sources
– Transportation (trains, planes, autos)– Delivery/removal– playground
● Indoor sources– Footfall/impact/rain– Other classrooms (students teachers recreation,
music)● Mechanical and Plumbing
– Ventilation systems– Restrooms
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Mechanical Background Noise● Examine the layout of the building- place noise sources
away from sensitive areas
● HVAC ● Type of system● Use of lined duct, plenums, and silencers● VAV
● Plumbing● Isolation of piping and fixtures● PVC vs iron pipe● Quiet fixtures● Location
● Electrical-Transformers and ballasts
● Elevators
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Signal to Noise Ratio
●For persons with average hearing, a good signal to noise ratio (voice to background noise) is 15 dB.
●For hearing impaired persons a larger ratio is required that depends on their individual hearing abilities.
●Let's take a listen to speech in noisy environments
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Noise and HearingPERCEIVED LOUDNESS CURVES
NOISE CRITERIA (NC) CURVES
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How does sound travel in a classroom?
●Sound travels in a sphere outward from the source●Sound level drops over distance●The boundaries of the classroom reflect and absorb sound●The sound arrives at the listener
●A teacher speaks with an average voice of about 65dB at 3'●Toward the back of a typical classroom sound can drop to ~50dB.
●For a typical student, to get an appropriate signal to noise ratio, you need the background sound to be 35dBA maximum, lower for hearing impaired students.
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How does sound travel in a classroom?
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Room Acoustics
One of the ways we describe room acoustics is Reverberation Time, or the persistence of sound.
RT60 (or RT) is the time that it takes for a sound impulse to dissipate to inaudible.
RT is determined by the size and type of finishes of a room.Bigger rooms have longer reverberation times
Sound absorptive finishes reduce reverberation
The National Classroom Standard requires that an unoccupied classroom has a reverberation time of 0.6 seconds. Hearing
impaired classrooms should meet this or be lower.
Let's listen to some reverberation times.
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Reverberation Time
Edits DSW based on Current design practices and
incorporation of sound systems
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Room Acoustics
What is the reverberation time of our occupied classroom?
Let's figure it out!
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Identifying and Solving Problems at the workplace
Identification of weak points or path for intrusive sounds
Windows and doors (seals)Poorly constructed wallsLightweight construction
Wall heightCeiling tiles
Penetrations for mechanical or electrical
Some clues can be garnered from the nature of the sound
Also AVOID ADJACENCIES TO LOUD SOUND SOURCES
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Controlling Exterior Noise
Exterior Noise
●Not under the control of user: planes, trains, autos, industry
●Partially controllable: trash hauling, site ingress/egress, and sports/playgrounds
●Controllable: Building service and equipment, through location, silencers, or scheduling. Can affect the interior as well as the community.●
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Doors and WindowsDoors and windows are one of the most common places for sound to enter a room.
Doors should be heavy with full perimeter seals
Doors should not have vents in them, and windows in them should be thick, preferably double pane.
Two doors can improve isolation as well
Windows should have good seals and double pane is preferable.
Air conditioners in windows are an open sound path (they are also loud!); the same for fan coil units that connect to the ouitside.
YOU SHOULD NOT SEE DAYLIGHT AROUND THE PERIMETER OF DOORS OR WINDOWS.
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Doors and Windows
Lets take a walk around the room to look at the doors and windows.
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Walls
Walls need to meet a certain sound stopping power to really be effective. If you can hear voices through the wall:
1) The construction may be too lightweight, requiring additional construction.
2) There may be cracks or penetrations of pipes, ducts or power outlets.
These penetrations can be sealed with silicone caulk (non-hardening).
Outlets and larger holes can have a box built around them or use a mastic (putty pad).
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Walls (what do we have?)
STC
CAC (enhanced with gyp)16”+ plenum
SourceReceiver
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Ceiling tiles
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NRC is roughly the absorption of speech energyand helps reduce unwanted reflections;
classrooms need 0.7 minimum (typical 0.5 to 0.99)
CAC is an indicator of sound blocking capability range 0-40.
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HVAC Elements
MECHANICAL CURB DUCT PENETRATION
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HVAC Elements
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Fan Coil
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Well Isolated RTU Example
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AHU's and VAV's
● AHU (air handling unit) should be treated as other machinery and placed in an area away from sensitive spaces or properly treated for isolation paying attention to low frequencies
● VAV (variable air volume) boxes in the ducting system should be placed out of sensitive areas and should have a liner or distance to avoid noise at nearby supply registers from fan or turbulence noise. (i.e. Place VAV in hallway outside of classroom)
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Crosstalk
● Avoid registers that are connected through a straight line of duct
● Add in duct liner in short runs between registers in different spaces
● Individual feeds from main trunk preferable (offset)
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Sound Level Apps for the smart phone
To determine the true sound level, the app meters vary from phone to phone, manufacturer to manufacturer.
HOWEVER, it could do a reasonable job of measuring background sound to your voice sound to determine signal to noise ratio.
If you need to determine your exact background levels, the local police may have a sound level meter.
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Room Acoustics Finishes
Sound absorptive finishes are desirable for your classroom. They can include:
1) Ceiling tiles (NRC 0.7 or greater)2) Carpeting3) Sound absorbing panels (compressed fiberglass, cellulose, cotton, Tectum, and more).4) the presence of a lot of other surfaces that are complex shapes (i.e. bookshelves)
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Which will lower the sound more?
FUZZ? BOX?
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SUMMARY
Let's watch another cool NASA video.
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The Relationship of Acoustics and Noise
BackgroundNoise
Room Acoustics
Speech Intelligibility
(or other Acoustic
Goal)
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StandardsANSI 12.60 American National Standards Institute Classroom
Acoustics Standard
International (Green) Building Standard I(G)BC to include acoustic requirements for classrooms utilized by hearing impaired students.
LEED (Leadership in Energy Efficient Design) standards for schools include mandatory and voluntary classroom acoustics
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Hearing aids and Assisted Listening
Hearing aids do not improve room acoustics; they amplify any problems
Assisted listening such as FM, infrared, or hearing loop are more effective as they give a high(er) level direct signal and cut back on the
room.
Amplifiers and loudspeakers may be helpful with good acoustics; otherwise it makes the problem worse.
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Thank you
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Hearing loss based on noise exposure and ageadjustments to standard curves based on time
PROGRESSIVE HEARING LOSS
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