cae 334/502 lecture 1a from spring 2014

8/12/2019 CAE 334/502 Lecture 1a from Spring 2014

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CAE 334/542Lecture 1a

Motivation to Study AcousticsSimple Acoustic Waves

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This TimeLecture 1a

Review of Syllabus Motivation for Taking This Course

What is Sound?

Simple Waves (Pure Tones)

Properties of a Wave

Phasors

Superposition

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Wave References

Chapter 2 of Longs Architectural Acoustics (LAA)

Chapter 1 of Allan D. Pierce,Acoustics: AnIntroduction to Its Physical Principles and

Applications,Acoustical Society of America, 1988.

Diffraction Applethttp://www.acoustics.salford.ac.uk/feschools/waves/diffract.htm#object

Animated Phasor Diagrams:http://www.kwantlen.bc.ca/science/physics/faculty/mcoombes/P2421_Notes/Phasors/Phasors.html

Superposition Java Applethttp://webphysics.davidson.edu/Applets/superposition/GroupVelocity.html

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http://www.acoustics.salford.ac.uk/feschools/waves/diffract.htmhttp://www.kwantlen.bc.ca/science/physics/faculty/mcoombes/P2421_Notes/Phasors/Phasors.htmlhttp://www.kwantlen.bc.ca/science/physics/faculty/mcoombes/P2421_Notes/Phasors/Phasors.htmlhttp://webphysics.davidson.edu/Applets/superposition/GroupVelocity.htmlhttp://webphysics.davidson.edu/Applets/superposition/GroupVelocity.htmlhttp://www.kwantlen.bc.ca/science/physics/faculty/mcoombes/P2421_Notes/Phasors/Phasors.htmlhttp://www.kwantlen.bc.ca/science/physics/faculty/mcoombes/P2421_Notes/Phasors/Phasors.htmlhttp://www.kwantlen.bc.ca/science/physics/faculty/mcoombes/P2421_Notes/Phasors/Phasors.htmlhttp://www.acoustics.salford.ac.uk/feschools/waves/diffract.htm


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Lecture Objectives

Review the Course Syllabus Explain why Acoustics is an important design

consideration in buildings

Explain basic physics of a simple wave

Understand the relations between frequency,

wavelength and speed of sound

Understand the idea of Root-Mean-Square

for measuring amplitude

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Syllabus Highlights

Instructor: Prof. Ralph Muehleisen Office: 228a AM

Office Hours: M 5:30-6:15, or by appt.

Email: [email protected], tel:630-252-2547

Text: Long: Architectural Acoustics (used, pdf, kindle versions

available, but you need this text very soon so buy it now) Marshall Long, Academic Press, 2005

IESNA: Lighting Fundamentals 2009

Blackboard: http://blackboard.iit.edu This course uses blackboard for posting lectures, homework,

solutions, and additional information.

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mailto:[email protected]://blackboard.iit.edu/http://blackboard.iit.edu/mailto:[email protected]


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Course Topics

Basics of Acoustics and Hearing Introduction to Environmental Acoustics

Introduction to Building Acoustics

Prereq for CAE 403/507 and 409/509

Basics of Lighting and Vision

Introduction to Lighting Devices

Introduction to Interior Illumination Design

Prereq for CAE 467

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Grading

~40% Exams Midterm covers acoustics, Final covers

Lighting

~40% Homework

I expect you to work 2-3 hours per credit

per week outside of class. So, you should

be working 5-8 hours per week on

reading and homework

Grad students taking CAE 502 have extrahomework and more homework points

~15% Journals

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Journals

I want you to start paying attention tothe acoustics and the lighting in thebuildings you use and write up yourexperiences in a Blackboard journal

As you learn in the course, your journalentries should become more completeand detailed.

You will be graded on both the quality

and the number of entries you make See info sheet about the journals that willbe posted to Blackboard and emailed toyou.

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Exams

There are two exams

The midterm covers only acoustics

The final covers only lighting.

Together they are about 40% of your grade.

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Questions?

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Acoustics, Sound and Noise

Acoustics is the science and engineering of sound Great!, but what is sound?

Sound is a vibration transmitted through an elastic

(restoring) medium. Well be concentrating on audible sound vibrations

in air that reach the ear.

Noise is an unpleasant or undesired sound

Noise is definitely in the ear of the beholder, but

some sounds are noise to just about everyone

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A classic philosophical question

If a tree falls in the woods and no one isaround to hear it - does it make a

noise?

Using our previous definitions, itdefinitely does make a sound

(vibrations are generated), but it does

not make a noise (no one hears it so

it cannot be considered unpleasant).

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Another philosophical question

What is the sound of one handclapping?

As so brilliantly demonstrated by Bart

Simpson in Season 2, Episode 6,Dead Putting Society, one hand can

clap.

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Wh h ld b i


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Why should we care about acoustics

in general building design?

For performance spaces like concert halls the

need for good acoustics is clear, by why do we

care about acoustics for general buildings?

Poor acoustics harms occupant performance

In Ordinary Commercial Buildings

In Green Buildings

In Schools

In Hospitals

In Multifamily Dwellings

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Complaints in Commercial Buildings

Poor building acoustics is the#1 complaint by building

occupants*

Effects of poor acoustics:

reduced worker satisfaction reduced worker productivity

higher worker stress levels

Bottom line:

Poor acoustics costs

money

Contributions to Workplace

Distractions

Lighting

9%

Noise

71%

Air

Quality

21%

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*Data from American Society of Interior Designers (1995)


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Problems in Commercial Buildings

Poor sound isolation between offices andcubicles

Occupant generated sounds are a background

noise to co-workers

Poor speech intelligibility

Background noise makes communication difficult

Especially a problem in locations where

communication is paramount

Conference Rooms

Executive Offices

Manager Offices

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Green Building Acoustics

Acoustics is becoming even more of aproblem with the design methods being used

for Green Buildings

Hard exposed concrete surfaces used in radiant

heating and cooling leads to high reverberation Large amount of windows/glass walls for

daylighting leads to high reverberation and noise

transmission from outside

Natural ventilation leads to high noisetransmission from outside to inside and from room

to room

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Occupant Surveys in Green Buildings

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-0.5 0 0.5 1 1.5 2

Overall Building

Overall Workspace

Office Layout

Cleaning/Maintenance

Lighting

Air Quality

Thermal Comfort

Acoustics

Berkeley Post Occupancy Survey Results

LEED/Green New Non-Green All Buildings


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Acoustics Survey Results are Poor

Acoustics is the only category where the

performance significantly decreased compared tonon-green buildings and it is the category with the

lowest ratings.

In shortgreen buildings have worse acoustics2/16/2014 CAE 334/502 - Week 124

-0.4 -0.2 0 0.2 0.4

Thermal Comfort

Acoustics

Berkeley Post Occupancy Survey Results

LEED/Green New Non-Green All Buildings


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Acoustics Complaints

One Obvious problem:

Poor speech privacyand sound isolation

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Acoustics in Schools

#1 UnsatisfactoryEnvironmental Condition inschools (1995 GAO) Effects of bad acoustics

Poor Speech Intelligibility

Decreased Attention Span Poor Speech Privacy

Bottom Line:

Teaching effectiveness is

compromised and couldbe improved

0

5

10

15

20

25

30

Acousti

cs

Ventilation

Security

IA

Q

Heating

Lighting

Percent of Schools withUnsatisfactory Conditions

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Data from US Government Accounting Office, 1995


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Problems in Schools

High background noise levels from HVAC

and external sources

Students have trouble hearing

Speech intelligibility drops

Concentration lapses

Teachers try to speak louder to overcome

background noise

Vocal strain is the #1 reason for teachers missing work

Noise from tables, chairs, feet on floors addsto background noise

This self generated noise is affected little by

classroom discipline and student activities

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Noise Problems in Hospitals

Noise is one of the most cited complaints in hospitals by

patients Noise adversely affects healing

Noise contributes to staff stress and burnout

Noise contributes to medical errors

Hospital Noise has been steadily increasing for decades

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Data from Noise Levels in JohnsHopkins Hospital


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Noise Problems in Multifamily Units

Noise is a commonproblem in multifamilydwelling units Apartments and Condos

Hotels and Motels

Excess Neighbor Noisecauses

Annoyance

Stress Sleep Problems

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Data from Indoor Environment Quality Related on Residential Satisfaction in Old Multi-Family Housing

Other reasons why acoustics is


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Other reasons why acoustics is

important in building design

Acoustics adds to the building designimpression poor acoustics is always remembered (badly)

good acoustics leaves a good impression

acoustics can be used to set a mood Acoustics is a big part of life-safety design

audible alarm and egress route indicators

Many spaces are acoustic venues where

acoustics are of primary importance Lecture Halls, Music Halls, Theater, Churches,

Recording Studios, Home Theaters

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Summary

Noise is a problem in a wide variety of

buildings including

Commercial Buildings

Schools

Hospitals Multifamily Dwellings

Major effects of excess noise include

Annoyance

High stress

Sleeping problems

Reduced productivity

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Questions?

If not, lets get started learning acoustics

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What is sound?

Sound is a vibration transmitted through an

elastic (restoring) medium.

Vibrations are compressions and expansions

Sound travels (propagates) as a wave. Since

it travels as a wave it follows the basicphysics of waves which you learned in

Physics 221 or 224

A simple wave is described by an amplitude (A), a

frequency (f), a wavelength (), and a speed ofpropagation (c)

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Notation Information

Acoustics a science of perturbationssmallfluctuations of density and pressure about astatic equilibrium.

We usually denote the static equilibrium (or mean

value) with a subscript 0(naught). Here aresome common static equilibrium values at sealevel that you should probably memorize Atmospheric pressure:P0=p0=100 kPa=10

5Pa

Air Density:0= 1.2 kg/m3

The textbook sometimes uses a naught todenote the static value, but sometimes does not.If you are confused, please ask.

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The Simplest Wave

The simplest wave in time is one that is

created by a source that is vibrating

sinusoidally at a single frequency. We call

this a pure tone.

The simplest wave in space is one that variesin only one dimension. We call this a plane

wave.

So, the simplest wave of all is a pure tone

plane wave. We can generate this by putting

a sinusoidally moving piston in a tube.

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A Pure Tone Sound Wave

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Dots are air molecules

which compress and

expand. Notice the

traveling density

oscillations

Oscillating density

variations mean

oscillating pressurevariations too

Notice that the

particles only move

back and forth

there is no nettransfer of mass!

Pressure peaks travel

at the speed of sound, c


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Snapshot of a Wave in Time

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2A

The pressureoscillates above and

below atmospheric

pressure with a peak

to peak amplitude of

2A

The pressure

variation repeatsover a distance we

call the wavelength

and denote by


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Snapshot of a Wave in Space

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Lets look at the time

variation at a single

point in space


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Snapshot of Wave in Space

If we look at the pressure at a fixed point in space we seea sinusoidal waveform just like was when we stoped time

The time between peaks is called the period, T

The period is related to the frequency as f= 1/T

The amplitude isA

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Pressure/V

elocity

Max

Min

P0

T

p

t

A


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Pure Tone Sound Waves

So we see that a pure tone has a sinusoidal waveform

in both time and space described by four numbers:

Amplitude,A, Frequency,f, Wavelength, and speed

of propagation, c

The waveform can be described with the equation

The constant is called the phase. Its purpose is to allow a

non-zero vale ofpwhen t=0 andx=0.

Note: We often use the angular frequency =2f in place offto

be able to remove the 2from all the equations

( , ) sin 2 /p x t A f x c t

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( , ) sin /p x t A x c t


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Frequency =f [Hz]

f = frequency, = 2f = angular frequency

Frequency is a measure of time variation

Frequency is the rate at which a pure tone wave repeats. It is also

how many times per second the molecule will move back and forth

and return to its original position

fis determined by motion of object generating the soundit hasnothing to do with the medium of travel.

Units offare cycles/sec or hertz [Hz] and is rad/sec

Audible sounds have 20


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Musical Scales and Frequency

See also Fig 2.2 of LAA

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Frequency Ranges

In acoustics we often talk about low, mid, and

high frequencies.

low frequencies: f< 250 Hz

mid frequencies: 250 Hz 2500 Hz

Notice that these frequency ranges are not

equal in span, the low frequency range is

much narrower than the high frequency range We do this because we perceive differences in

frequency in a non-linear (logarithmic) fashion

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Period = T[s]

Period is the time it takes a pure tone waveform to repeatin time so it is the reciprocal of frequency:

T=1/f and= 2f=2/T The units of period are seconds [s]

The period of audible sounds ranges from about 500s to

50ms to (f=20kHz tof=20Hz). While both Tandfdescribe the same physics, 1/Tand

2/T appears a lot in acoustic equations and we use sowe usefand instead

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Pressure/Velocity

Max

Min

P0

T

p

t

A


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Wavelength = ,[ft or m]

Wavelength, , is a measure of spatial variation A pure tone wave repeats, in space, after a distance

depends upon the medium of travel andf The of a wave will change as the sound moves from one

medium to another (i.e. air to water, air to solid, etc.)

Units are feet or meters [ft or m] Audible sounds have in< < 54 ft (1.7 cm17 m)!

The of a wave affects how it interacts with

objects in a room. When >> object size, waves will mostly bend around

the object (diffract) with little reflection

When


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Amplitude,A, [Pa]

The amplitude of a sound wave,A, is thepeak variation in pressure from theundisturbed mediumAis the maximum difference between the

perturbed and the static air pressures

Because pure tones are symmetric you canmeasure either the +peak, thepeak or the peak-to-peak value.

Units of pressure amplitude are pascals [Pa] No one in acoustics uses PSI

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RMS Pressure [Pa]

When sound waves are not pure tones,

characterizing amplitude gets more complicated. Instead of peak pressures, we usually use the Root-

Mean-Square or RMS amplitude since it is related tototal energy content of the sound

For a pure tone we can show that,

For multiple tones or other waveforms you must do the fullintegral shown above to findprms

Units are still Pa

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T

RMS dttpT

p0

2 )(1

0.7072RMS

Ap A

f


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Range of Pressure Amplitudes

The minimum audible pressure for a 1 kHz tone

isprms2x10-5Pa (20 Pa)

Major hearing damage occurs instantaneously

forprms 200Pa

Typical conversations haveprms1100 mPa

Recall that at sea level atmospheric pressure,P0100kPa, so we can hear atmospheric pressurefluctuations of under 0.0000001% !!!!!!!!!!

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cae 334/502 lecture 1a from spring 2014

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