Classroom Acoustics January 13, Aural Rehabilitation University of Mississippi. David S. Woolworth Oxford Acoustics

Transcription

1 Classroom Acoustics January 13, 2014 Aural Rehabilitation University of Mississippi David S. Woolworth Oxford Acoustics

2 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

3 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

4 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

5 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

6 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

7 Noise and Hearing PERCEIVED LOUDNESS CURVES NOISE CRITERIA (NC) CURVES

8 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.

9 How does sound travel in a classroom?.

10 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.

11 Reverberation Time Edits DSW based on Current design practices and incorporation of sound systems

12 Room Acoustics What is the reverberation time of our occupied classroom? Let's figure it out!

13 Identifying and Solving Problems at the workplace Identification of weak points or path for intrusive sounds Windows and doors (seals) Poorly constructed walls Lightweight construction Wall height Ceiling 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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15 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.

16 Doors and Windows Doors 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.

17 Doors and Windows Lets take a walk around the room to look at the doors and windows.

18 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 (nonhardening). Outlets and larger holes can have a box built around them or use a mastic (putty pad).

19 Walls (what do we have?) STC CAC (enhanced with gyp) 16 + plenum Receiver Source

20 Ceiling tiles ` NRC is roughly the absorption of speech energy and 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.

21 HVAC Elements MECHANICAL CURB DUCT PENETRATION

22 HVAC Elements

23 Fan Coil

24 Well Isolated RTU Example

25 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)

26 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)

27 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.

28 Room Acoustics Finishes Sound absorptive finishes are desirable for your classroom. They can include: 1) Ceiling tiles (NRC 0.7 or greater) 2) Carpeting 3) 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)

29 Which will lower the sound more? FUZZ? BOX?

30 SUMMARY Let's watch another cool NASA video.

31 The Relationship of Acoustics and Noise Background Noise Speech Intelligibility (or other Acoustic Goal) Room Acoustics

32 Standards ANSI 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

33 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.

34 Thank you

35 Hearing loss based on noise exposure and age adjustments to standard curves based on time PROGRESSIVE HEARING LOSS