Classroom Acoustics January 13, 2014

Aural RehabilitationUniversity of Mississippi

David S. Woolworth

Oxford Acoustics

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

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

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

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

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

Noise and HearingPERCEIVED LOUDNESS CURVES

NOISE CRITERIA (NC) CURVES

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.

How does sound travel in a classroom?

.

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.

Reverberation Time

Edits DSW based on Current design practices and

incorporation of sound systems

Room Acoustics

What is the reverberation time of our occupied classroom?

Let's figure it out!

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

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.●

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.

Doors and Windows

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

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).

Walls (what do we have?)

STC

CAC (enhanced with gyp)16”+ plenum

SourceReceiver

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.

HVAC Elements

MECHANICAL CURB DUCT PENETRATION

HVAC Elements

Fan Coil

Well Isolated RTU Example

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)

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)

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.

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)

Which will lower the sound more?

FUZZ? BOX?

SUMMARY

Let's watch another cool NASA video.

The Relationship of Acoustics and Noise

BackgroundNoise

Room Acoustics

Speech Intelligibility

(or other Acoustic

Goal)

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

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.

Thank you

Hearing loss based on noise exposure and ageadjustments to standard curves based on time

PROGRESSIVE HEARING LOSS

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