Acoustics and Classrooms

BARD MANUFACTURING COMPANYREGISTERED AIA PRESENTER

VAUGHAN SMITH

Acoustics and Classrooms

• SOURCES OF NOISE • ACOUSTICAL TERMS AND DEFINITIONS• ANSI STANDARD• OCCUPIED AND UNOCCUPIED CLASSROOMS• HVAC AND SOUND IN CLASSROOMS

Acoustics and Classrooms

• LEARNING OBJECTIVES:• SOURCES ON NOISE AFFECTING CLASSROOM LEARNING• ACOUSTICAL TERMS AND DEFINITIONS• DESIGN RECOMMENDATIONS• MEASUREMENT OF SOUND IN CLASSROOMS• ACOUSTICAL STANDARDS

Background noise

Noise in the classroom

• Outside:

• Vehicles, Voices (highway, playground)

• Inside:

•Students• Equipment: computers, projections, mini refrigerator?

• Reverberation

• Neighboring classrooms and hallways

• Lighting

• HVAC

Acoustical Terms

• Noise: unwanted sound

• Sound Power

• Sound Pressure

• Reverberation

• NC: Noise Criteria

• dB

• dBA

Sound Power vs. Sound Pressure

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Sound Pressure is dependent on the acoustical environment.The factors involved include:• Effects of near by reflecting surfaces• Receiver distance• Type of space• Amount and location of absorption in the space• The location in the space• The presence of barriers in the space• Intrusion of ambient sounds

Sound Power vs. Sound Pressure

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Typical HVAC manufacturer provides equipment sound power data in dB per octave band

Sound Power is a measure of total energy per unit time emitted by the source in all directions

Sound Power vs. Sound Pressure

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dBA

Sound Power vs. Sound Pressure

• Sound Power: 100 watt light bulb, relative power rating Noise Criteria

• Sound Pressure: Lumens or brightness at any location in room is similar to Pressure decibel dBA

Typical HVAC manufacturer provides equipment sound power data in dB per octave band

Noise Criteria (NC) A one-number measure of background noise, created by measuring the sound pressure at the loudest points in the environment

Decibel (dB)

• A unit of measure of the loudness of a sound. Louder sounds have a larger decibel value.

• A weighting (dBA): a measure of sound pressure level designed to reflect the response of the human ear, which is less sensitive to low and high frequencies

dBA levels for various noises

http://acousticalsociety.org/about_acoustics/acoustics_of_classrooms

Key TermsReverberation:

http://acousticalsociety.org/about_acoustics/acoustics_of_classrooms

•Want early reflections• No echoes

FACTORS• Size of room• Shape of room• Surfaces

Good Sound Treatments• Acoustical ceiling tile• Sound absorbing walls• Soft space elements• Carpet• Early reflection of teacher’s

voice

Background noise in classrooms

• Signal to noise ratio (SNR)

difference in dBA between the speech (signal) and noise (background)

• Recommended: 15 dBA difference,

• Teacher’s voice: 60 dBA

• Background noise should be 45 dBA or less

STC

Wikipedia:

“ Sound Transmission Class (or STC) is an integer rating of how well a buildingpartition attenuates airborne sound. In the USA, it is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (see ASTMInternational Classification E413 and E90).”

The Unoccupied Classroom

ANSI S12. 60 2002 unoccupied classrooms

• Classrooms less than 10,000 cu ft• 35 dBA background noise• .6 sec. reverberation time• STC 50 (for construction)

Why 35 dBA background noise ?

•Signal to noise ratio 15 dBA

•Teachers voice at 50 dBA can be heard clearly with background of 35 dBA

•Human voice 50-70 dBA

Teacher voice: 55-57 dBA with proper room acoustics

SNR Signal to noise ratio? 10 – 15 dBA

40-47

35 dBA background noise

• Acoustical analysis of site

• Acoustically isolated from hallway, adjacent rooms

• considerable upgrade of building and grounds design

• Acoustical Engineer likely required

Noise in

occupied classrooms!!

So: how loud is it ?

Studies of Occupied classrooms

• Multiple studies indicate occupied sounds levels during quiet periods average 48 dBA or more

• Noise levels during general activities frequently exceeded 60 dBA

LAUSD site analysis

LEED Schools:

Why is HVAC singled out for noise?

Systems can be designed for reduced noise in classrooms

AHU located in hallway ceiling

Trane EDU-SLB026-EN June 01, 2011

Roof mounted equipment

Vertical unit: LEED installation

HVAC manufacturers have information!

HVAC Sound Test Lab

Summary: students learn in occupied classrooms

Some acoustic recommendations concerning HVAC

• Reverberation is determined by the room design and acoustics

• ‘Soft rooms’ reduce background noise

• Supply and return ductwork properly designed (may not be required)

• Isolate equipment noise inside and outside the classroom

Questions or Comments?