Understanding Sound Measurements: Analyzing Annoying Noises and Protecting Hearing, Manuais, Projetos, Pesquisas de Controle de Ruído

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Introduction^

This booklet gives answers to some of the basic ques-tions^ asked^ by^ the^ newcomer

to^ a^ noise^ measuring programme.^ It^ gives^ a^ brief

explanation^ to^ questions like: What is sound? Why do we measure sound? What units do we use? How do we hear? What instruments do we use for measurement? What is a weighting network? What is frequency analysis? What is noise dose? How does sound propagate? Where should we make our measurements? How does the environment influence measurements? How should the microphone be positionedin the sound field ?How do we make a measurement report ?What do we do when levels are too high? Revision September 1984^

Why Measure Sound ?Measurements^ provide^ definite

quantities^ which^ de- scribe^ and^ rate^ sounds.^ These

measurements^ can^ pro- vide^ benefits^ such^ as^ improved

building^ acoustics^ and loudspeakers,^ thus^ increasing

our^ enjoyment^ of^ music, both in the concert hall and at home.Sound^ measurements^ also

permit^ precise,^ scientific analysis^ of^ annoying^ sounds.

However,^ we^ must^ remem- ber^ that^ due^ to^ the^ physiological

and^ psychological^ dif- ferences^ between^ individuals,

the^ degree^ of^ annoyance cannot^ be^ scientifically^ measured

for^ a^ given^ person. But^ the^ measurements^ do^ give^ us^ an^ objective^ means

of comparing annoying sounds under different conditions.Sound^ measurements^ also

give^ a^ clear^ indication^

of when^ a^ sound^ may^ cause^

damage^ to^ hearing^ and^ permit corrective^ measures^ to^ be^

taken.^ The^ degree^ of^ hearing damage^ can^ be^ determined

by^ audiometry^ which^ mea- sures^ a^ person's^ hearing^

sensitivity.^ Thus,^ sound^ mea- surements^ are^ a^ vital^ part

of^ hearing^ conservation^ pro- grammes.Finally,^ measurement^ and^ analysis

of^ sound^ is^ a^ power- ful^ diagnostic^ tool^ in^ noise

reduction^ programmes^ — from^ airports,^ to^ factories,

highways,^ homes^ and^

re- cording^ studios.^ It^ is^ a^ tool

which^ can^ help^ to^ improve the quality of our lives.

Just What is Sound ?Sound may be defined as^ any pressure variation

(in air, water or other medium)^ that the human ear can detect. The^ most^ familiar^ instrument

for^ measuring^ pressure variations^ in^ air^ is^ the^ barometer.

However,^ the^ pres- sure^ variations^ which^ occur

with^ changing^ weather^ con- ditions are much too slow for the human ear to detect—^ and^ hence^ do^ not^ meet

our^ definition^ of^ sound.^ But, if variations in atmospheric pressure occur more rapidly— at least 20 times a second — they can be heard andhence^ are^ called^ sound.^ (A^ barometer^ cannot^ respond quickly^ enough^ and^ therefore

cannot^ be^ used^ to^ mea- sure sound).The^ number^ of^ pressure^ variations

per^ second^ is^ called the^ frequency^ of^ the^ sound,

and^ is^ measured^ in^ Hertz (Hz).^ The^ frequency^ of^ a^

sound^ produces^ it's^ distinctive tone.^ Thus,^ the^ rumble^ of^

distant^ thunder^ has^ a^ low^ fre- quency,^ while^ a^ whistle^ has

a^ high^ frequency.^ The^ nor- mal^ range^ of^ hearing^ for^

a^ healthy^ young^ person^ ex- tends^ from^ approximately^20

Hz^ up^ to^20 000 Hz^ (or 20 kHz)^ while^ the^ range^ from

the^ lowest^ to^ highest^ note of a piano is 27,5 Hz to 4186 Hz.These^ pressure^ variations^ travel

through^ any^ elastic^ me- dium^ (such^ as^ air)^ from^ the

source^ of^ the^ sound^ to^ the listener's^ ears.^ You^ probably

already^ have^ some^ idea^ of the^ speed^ of^ sound^ from^

the^ familiar^ rule^ for^ determin- ing^ how^ far^ away^ a^ thunder

storm^ is:^ count^3 seconds per^ kilometer^ or^5 seconds

per^ mile^ from^ the^ time^ you see^ the^ lightning^ until^ you hear^ the^ thunder.^ This^ time interval^ corresponds^ to^ a^

speed^ of^ sound^ in^ air^ of 1238 km/hour^ or^770 miles

per^ hour.^ For^ acoustic^ and sound^ measurement^ purposes,

this^ speed^ is^ expressed as 344 meters per second at room temperature. 4

The dBThe^ second^ main^ quantity^

used^ to^ describe^ a^ sound^

is the^ size^ or^ amplitude^ of^

the^ pressure^ fluctuations.^ The weakest^ sound^ a^ healthy^ human

ear^ can^ detect^ has^ an amplitude^ of^20 millionths^ of

a^ Pascal^ (20^ μPa)^ —^ some 5000000000 times^ less^ than

normal^ atmospheric^ pres- sure.^ A^ pressure^ change^ of

20 μPa^ is^ so^ small^ that^

it causes^ the^ eardrum^ to^ deflect

a^ distance^ less^ than^ the diameter^ of^ a^ single^ hydrogen

molecule.^ Amazingly,^ the ear^ can^ tolerate^ sound^ pressures

more^ than^ a^ million times^ higher.^ Thus,^ if^ we^

measured^ sound^ in^ Pa,^ we would^ end^ up^ with^ some

quite^ large,^ unmanageable numbers.^ To^ avoid^ this,^ another

scale^ is^ used^ —^ the decibel^ or^ dB scale. The decibel is not an absolute unit of measurement. It isa^ ratio^ between^ a^ measured

quantity^ and^ an^ agreed^ ref- erence^ level.^ The^ dB^ scale

is^ logarithmic^ and^ uses^ the hearing^ threshold^ of^20 μPa

as^ the^ reference^ level.^ This is^ defined^ as^^0 dB.^ When^

we^ multiply^ the^ sound^ pres- sure^ in^ Pa^ by^ 10,^ we^ add

^20 dB^ to^ the^ dB^ level.^ So 200 μPa corresponds to 20 dB (re 20

μPa),^^2000 μ Pa^ to 40 dB^ and^ so^ on.^ Thus,^

the^ dB^ scale^ compresses^

a range^ of^ a^ million^ into^ a^

range^ of^ only^120 dB.^ The sound^ pressure^ levels^ (SPL)

in^ dB^ and^ Pa^ of^ various familiar sounds are shown in the figure.One^ useful^ aspect^ of^ the^ decibel

scale^ is^ that^ it^ gives^ a much^ better^ approximation^

to^ the^ human^ perception^ of relative^ loudness^ than^ the^ Pascal^ scale.^ This^ is^ because the^ ear^ reacts^ to^ a^ logarithmic

change^ in^ level,^ which corresponds to the decibel scale where 1 dB is the samerelative change everywhere on the scale. 6

What do we Hear ?We^ have^ already^ defined^

sound^ as^ any^ pressure^ varia- tion^ which^ can^ be^ heard^ by

a^ human^ ear.^ This^ means a range^ of^ frequencies^ from^20

Hz^ to^20 kHz^ for^ a^ young, healthy^ human^ ear.^ In^ terms

of^ sound^ pressure^ level, audible^ sounds^ range^ from^

the^ threshold^ of^ hearing^

at 0 dB to the^ threshold of pain which can be over 130 dB. Although^ an^ increase^ of^6

dB^ represents^ a^ doubling^ of the^ sound^ pressure,^ an^ increase

of^ about^10 dB^ is^ re- quired^ before^ the^ sound^

subjectively^ appears^ to^ be twice^ as^ loud.^ (The^ smallest

change^ we^ can^ hear^ is about 3 dB).The^ subjective^ or^ perceived

loudness^ of^ a^ sound^ is^ de- termined^ by^ several^ complex

factors.^ One^ such^ factor^ is that^ the^ human^ ear^ is^ not

equally^ sensitive^ at^ all^ fre- quencies.^ It^ is^ most^ sensitive

to^ sounds^ between^2 kHz and^5 kHz,^ and^ less^ sensitive

at^ higher^ and^ lower^ fre- quencies.

7

How do we Hear ?The^ human^ ear^ consists^ of

three^ main^ parts;^ the^ outer ear,^ middle^ ear^ and^ inner^

ear.^ The^ outer^ ear,^ consisting of^ the^ pinna^ and^ auditory

canal ,^ collects^ the^ airborne sound^ waves^ which^ then^ vibrate

the^ eardrum ,^ which^ is the^ interface^ with^ the^ middle

ear.^ The^ middle^ ear^ acts as^ an^ impedance^ matching

device^ and^ has^ three^ small bones^ operating^ as^ a^ set^

of^ levers.^ These^ bones^ trans- fer^ the^ vibration^ to^ the^ inner

ear^ which^ consists^ of^ two separate^ systems,^ the^ semi-circular

canals^ for^ control- ling^ balance^ and^ the^ cochlea

.^ The^ cochlea^ is^ a^ fluid- filled,^ snail-shaped^ tube^ which

is^ divided^ longitudinally into two parts by the^ basilar membrane

In^ response^ to^ an^ acoustic

stimulus^ the^ fluid^ in^ the^

co- chlea^ is^ disturbed^ and^ this

distorts^ the^ basilar^ mem- brane^ on^ whose^ upper^ surface

are^ thousands^ of^ very sensitive^ hair^ cells.^ The^ hair

cells^ register^ this^ distor- tion^ and^ transform^ it^ into

nerve^ impulses^ which^ are then transmitted to the brain.Prolonged^ exposure^ to^ loud

sounds^ causes^ damage^

to the^ hair^ cells^ with^ the^ result

that^ hearing^ ability^ be- comes^ progressively^ impaired.

At^ first,^ damage^ to^ a few^ hair^ cells^ is^ not^ noticeable,

but^ as^ more^ of^ the^ hair cells^ become^ damaged,^ the

brain^ can^ no^ longer^ com- pensate^ for^ the^ loss^ of^

information.^ Words^ run^ into each^ other,^ speech^ and^

background^ noise^ cannot^

be distinguished^ and^ music^

becomes^ muffled.^ Consider- able^ and^ irreparable^ damage

will^ have^ occurred^ by^ the time^ the^ listener^ becomes^

aware^ of^ the^ loss.^ Loss^

of hearing^ caused^ by^ noise^

exposure^ is^ normally^ greatest at^ those^ frequencies^ (around

4 kHz)^ where^ the^ ear^ is most sensitive.

The Basic Sound Level MeterA^ sound^ level^ meter^ is^ an

instrument^ designed^ to^ re- spond^ to^ sound^ in^ approximately

the^ same^ way^ as^ the human^ ear^ and^ to^ give^

objective,^ reproducible^ mea- surements^ of^ sound^ pressure

level.^ There^ are^ many^ dif- ferent^ sound^ measuring^

systems^ available.^ Although different^ in^ detail,^ each^ system

consists^ of^ a^ micro- phone, an processing section and a read-out unit.The^ microphone^ converts^ the

sound^ signal^ to^ an^ equiv- alent^ electrical^ signal.^ The^

most^ suitable^ type^ of^ micro- phone^ for^ sound^ level^ meters

is^ the^ condenser^ micro- phone,^ which^ combines^ precision

with^ stability^ and^ re-

liability.^ The^ electrical^ signal

produced^ by^ the^ micro- phone^ is^ quite^ small^ and

so^ it^ is^ amplified^ by^ a preamplifier before being processed.Several^ different^ types^ of

processing^ may^ be^ per- formed^ on^ the^ signal.^ The^

signal^ may^ pass^ through^ a weighting^ network.^ It^ is^

relatively^ simple^ to^ build^ an electronic^ circuit^ whose^ sensitivity

varies^ with^ frequen- cy^ in^ the^ same^ way^ as^ the

human^ ear,^ thus^ simulating the^ equal^ loudness^ contours.

This^ has^ resulted^ in^ three different^ internationally^

standardized^ characteristics termed the^ "A", "B"^ and^ "C" weightings. 10

When^ more^ detailed^ information

about^ a^ complex^ sound is^ required,^ the^ frequency^

range^ from^20 Hz^ to^20 kHz can^ be^ divided^ up^ into^ sections

or^ bands.^ This^ is^ done with^ electronic^ filters^ which

reject^ all^ sound^ with^ fre- quencies^ outside^ the^ selected

band.^ These^ bands^ usual- ly^ have^ a^ bandwidth^ of^ either

one^ octave^ or^ one^ third octave. An^ octave^ is^ a^ frequency^

band^ where^ the^ highest^ fre- quency^ is^ twice^ the^ lowest

frequency.^ For^ example,^

an octave^ filter^ with^ a^ centre

frequency^ of^1 kHz^ admits frequencies^ between^707 and

1414 Hz,^ but^ rejects^ all others.^ (The^ name^ octave^

stems^ from^ the^ fact^ that^

an octave^ covers^ eight^ notes^

of^ the^ diatonic^ musical^ scale). A^ third^ octave^ covers^ a^

range^ where^ the^ highest^

fre- quency is 1,26 times the lowest frequency.The^ process^ of^ thus^ dividing

a^ complex^ sound^ is^ termed frequency^ analysis^ and^ the

results^ are^ presented^ on^

a chart called a^ spectrogram. After^ the^ signal^ has^ been

weighted^ and/or^ divided^ into frequency^ bands^ the^ resultant

signal^ is^ amplified,^ and the^ Root^ Mean^ Square^ (RMS)

value^ determined^ in^ an RMS^ detector.^ The^ RMS^ is

a^ special^ kind^ of^ mathemati- cal^ average^ value.^ It^ is^ of^

importance^ in^ sound^ measure- ments^ because^ the^ RMS^ value

is^ directly^ related^ to^ the amount of energy in the sound being measured. 12

The^ last^ stage^ of^ a^ sound

level^ meter^ is^ the^ read-out unit^ which^ displays^ the^ sound

level^ in^ dB,^ or^ some^ oth- er^ derived^ unit^ such^ as^ dB(A)

(which^ means^ that^ the measured^ sound^ level^ has

been^ A-weighted).^ The^ sig- nal^ may^ also^ be^ available

at^ output^ sockets,^ in^ either AC^ or^ DC^ form,^ for^ connection

to^ external^ instruments such^ as^ level^ or^ tape^ recorders

to^ provide^ a^ record

and/or for further processing.CalibrationSound^ level^ meters^ should

be^ calibrated^ in^ order^

to provide^ precise^ and^ accurate

results.^ This^ is^ best^ done by^ placing^ a^ portable^ acoustic

calibrator,^ such^ as^ a sound^ level^ calibrator^ or^

a^ pistonphone,^ directly^ over the^ microphone.^ These^ calibrators

provide^ a^ precisely defined^ sound^ pressure^ level

to^ which^ the^ sound^ level meter^ can^ be^ adjusted.^ It

is^ good^ measurement^ prac- tice^ to^ calibrate^ sound^ level

meters^ immediately^ before and^ after^ each^ measurement

session.^ If^ recordings^ are to^ be^ made^ of^ noise^ measurements,

then^ the^ calibra- tion^ signal^ should^ also^ be recorded^ to^ provide^ a^ refer- ence level on playback.

The Impulse Sound Level MeterIf^ the^ sound^ to^ be^ measured

consists^ of^ isolated^ im- pulses^ or^ contains^ a^ high

proportion^ of^ impact^ noise, then^ the^ normal^ "F"^ and^ "S"

time^ responses^ of^ the^ sim- ple^ sound^ level^ meter^ are^

not^ sufficiently^ short^ to^ give

a measurement^ which^ is^ representative

of^ the^ subjective human response.For^ such^ measurements,^ sound

level^ meters^ having^ a standardized^ "I"^ (Impulse)

characteristic^ are^ needed. The^ "I"^ characteristic^ has^ a

time^ constant^ of^35 millisec- onds,^ which^ is^ short^ enough

to^ enable^ detection^ and display^ of^ transient^ noise,

in^ a^ way^ which^ takes^ into account the human perception of impulsive sounds.Although^ the^ perceived^

loudness^ of^ short^ duration sound^ is^ lower^ than^ that^

of^ steady^ continuous^ sound, the^ risk^ of^ damage^ to^ hearing

is^ not^ necessarily^ re- duced.^ For^ this^ reason,^ some

sound^ level^ meters^ include a^ circuit^ for^ measuring^ the

peak^ value^ of^ the^ sound, independent of it's duration.A^ Hold^ Circuit^ is^ also^ incorporated

to^ store^ either^ the peak^ value^ or^ the^ maximum

RMS^ value.^ Some^ stan- dards^ require^ the^ peak^ value

to^ be^ measured^ while^ oth- ers^ ask^ for^ a^ measurement

using^ the^ "I"^ time^ constant. In^ either^ case^ the^ Hold^ circuit

makes^ reading^ the^ mea- surement easy.

Energy ParametersAs^ sound^ is^ a^ form^ of^ energy

the^ hearing^ damage^ po- tential^ of^ a^ given^ sound^ environment

depends^ not^ only on^ it's^ level,^ but^ also^ it's^

duration.^ For^ example,^ expo- sure^ to^ a^ loud^ sound^ for^4

hours^ is^ much^ more^ harmful than^ a^ one^ hour^ exposure

to^ the^ same^ sound.^ So^

to assess^ the^ hearing^ damage

potential^ of^ a^ sound^ envi- ronment,^ both^ the^ sound^ level

and^ the^ duration^ of^ expo- sure^ must^ be^ measured^ and

combined^ to^ provide^ a^ de- termination of the energy received.For^ constant^ sound^ levels,^

this^ is^ easy,^ but^ if^ the^ sound level^ varies,^ the^ level^ must

be^ sampled^ repeatedly^ over a^ well^ defined^ sampling^ period.

Based^ on^ these^ sam- ples,^ it^ is^ then^ possible^

to^ calculate^ a^ single^ value known^ as^ the^ Equivalent^

Continuous^ Sound^ Level^

or L^ which^ has^ the^ same^ eq^

energy^ content^ and^ conse- quently^ the^ same^ hearing

damage^ potential^ as^ the varying^ sound^ level.^ For^ an^ A-weighted^ Lthe^ symboleq^ L^ is^ used.^ In^ addition Aeq^ to^ determining^ the^ hearing damage^ potential^ of^ a^ sound,

Lmeasurements^ areeq^ also^ used^ for^ many^ other^

types^ of^ noise^ measurements, for example community noise-annoyance assessments.If^ the^ sound^ level^ varies^ in

a^ stepwise^ manner,^ an^ L

eq can^ be^ calculated^ using^

measurements^ from^ a^ sound level meter and a stopwatch. 16

Noise DoseNoise^ exposure^ measurements

on^ individuals^ who^ move between^ many^ different^ noise

environments^ during^ the working^ day^ can^ be^ obtained

using^ Noise^ Dose^ Meters. These^ instruments^ are^ portable

and^ can^ be^ carried^ in^ a person's^ pocket.^ The^ microphone

can^ be^ separated from^ the^ dose^ meter^ body

and^ should^ preferably^ be mounted^ close^ to^ the^ individuals

more^ noise^ exposed ear.Noise^ dose^ meters^ display^

the^ percentage^ of^ the^ daily allowable^ noise^ dose.^ Two^

different^ ways^ of^ calculating the^ noise^ dose^ are^ used.^

The^ difference^ between^ the two^ methods^ is^ due^ to^ the

allowance^ incorporated^ for the^ recovery^ of^ hearing^ during

quiet^ periods.^ Currently, both^ methods^ essentially^ use

a^ basis^ of^90 dB(A)^ for^ an 8 hour day.The^ International^ Standards^

Organisation^ (ISO)^1999 de- fines^ one^ method^ which^ uses

only^ the^ energy^ criteria and^ makes^ no^ allowance^

for^ the^ recovery^ of^ hearing. Thus,^ an^ increase^ of^ 3dB^

in^ the^ sound^ pressure^ level halves^ the^ permissible^ exposure

period.^ For^ example^ an increase^ in^ sound^ level^ from

90 dB(A)^ to^93 dB(A)^ must be^ accompanied^ by^ a^ halving

of^ the^ permissible^ expo- sure duration from 8 hours to 4 hours.In^ the^ United^ States^ the^ Occupational

Safety^ and^ Health Administration^ (OSHA)^ defines

another^ relationship which^ permits^ a^ 5dB^ increase

in^ sound^ level^ for^ each halving^ of^ the^ allowable^ exposure

period.^ Thus,^ an^ in- crease^ in^ sound^ level^ from^

90 dB(A)^ to^95 dB(A)^ is^ ac- companied^ by^ a^ halving^ of^

the^ allowable^ exposure^ dura- tion from 8 to 4 hours. 18

Anechoic Chambers(Sound Absorbing Rooms)In^ order^ to^ make^ measurements

in^ a^ free-field,^ totally without^ reflecting^ objects,^

the^ measurements^ must^

be made^ outdoors^ at^ the^ top of^ a^ flagpole^ (or^ equivalent) or^ in^ an^ anechoic^ chamber.

In^ an^ anechoic^ chamber the^ ceiling,^ floor^ and^ all^

the^ walls^ are^ covered^ by^

a highly^ absorptive^ material^

which^ eliminates^ reflections. Thus,^ the^ sound^ pressure level^ in^ any^ given^ direction from^ the^ noise^ source^ may

be^ measured^ without^ the

presence of interfering reflections.Reverberation Chambers(Sound Reflecting Rooms)The^ opposite^ of^ an^ anechoic

chamber^ is^ the^ reverbera- tion^ chamber^ where^ all^ surfaces

are^ made^ as^ hard^ and reflective^ as^ possible^ and

where^ no^ parallel^ surfaces exist.^ This^ creates^ a^ so-called

diffuse^ field^ because^ the sound^ energy^ is^ uniformly

distributed^ throughout^ the room.^ In^ this^ type^ of^ room,

it^ is^ possible^ to^ measure the^ total^ acoustic^ power^ output

from^ the^ noise^ source, but^ the^ sound^ pressure^ level

at^ any^ point^ will^ be^ an average^ value^ due^ to^ the

reflections.^ As^ such^ rooms are^ cheaper^ to^ construct^

than^ anechoic^ chambers,^ they find^ widespread^ use^ for machinery^ noise^ investiga- tions.