Chapter 2: Noise and Vibration By: Dr. Sara Yasina Yusuf | School of Environmental Engineering EAT...

Chapter 2: Noise and VibrationBy:

Dr. Sara Yasina Yusuf

| School of Environmental Engineering

EAT 342: Noise Pollution Control

Course Objectives & Learning Objectives

This chapter reflects CO2:

– Ability of defining the properties of sound, quantifying the noise levels and decibel as well as to characterize the noise.

Learning Objectives for the chapter:

– LO-1: Able to SKETCH pure tone wave and DEFINE the properties of sound waves (i.e. Frequency, amplitude, wavelength and period). CALCULATE the speed of sound, sound pressure, frequency and wavelength.

– LO-2: DEFINE sound scale in decibel.

– LO-3: DIFFERENTIATE and CALCULATE the Sound Power Level, Sound Intensity and Sound Pressure Level.

– LO-4: CALCULATE the Sound Pressure/Power Level, summation of sound pressure using formula, table and graph, substraction and averaging of Sound Pressure/Power Level

– LO-5: DISCUSS and ANALYZE noise characteristics, such as weighting networks, octave bands and rating systems

– LO-6: DISCUSS the techniques in measuring community/environmental noises. COLLECT and ASSESS the Leq as well as Ldn measured through the PBL assignment.

– LO-7: DISCUSS various types of community noise sources and its criteria. COMPARE the measured data obtained via PBL and CRITIC on the levels to protect human health and welfare

Content

1. The basic physics of sound – Sound and types of sound– Speed of sound– Sound pressure– Properties of Sound Waves

– Frequency– Wavelength

2. Characteristics of noise and Desibel scale– Frequency and Loudness– Sound level and decibel scale– Weighting networks– Octave bands– Rating systems

3. Noise measurement

4. Community Noise Sources and Criteria– Transportation noise– Other internal combustion

engines– Construction noise– Zoning and siting

considerations– Levels to protect Health and

Welfare

1. The basic physics of sound – Sound & Types of Sound

What is sound wave?

A sound wave is an air pressure disturbance that results from vibration that propagates through an elastic medium (air, water, etc.) at a speed characteristic of that medium.

Patterns of noise:

− Steady-state or continuous

− Intermittent

− Impulse or impact


Continuous noise is an uninterrupted sound level that varies less than 5 dB during the period of observation

Intermittent noise is a continuous noise that persists for more than 1 second that is interrupted for more than 1 second

Impulse noise is a change of sound pressure of 40 dB or more within 0.5 second with a duration of less than 1 second high pitch or intensity, lifetime of less than 1 sec.

1. The basic physics of sound - Sound & Types of Sound

Behaviour of sound waves

ReflectionWaves bounce off a surface

Echoes & reverberation

Refraction Waves bend when they pass through a boundary

Diffraction Waves spread out when they pass through a small gap

Interference2 waves superpose to form a resultant wave of greater or lower amplitude


Reflection

Interference


Example 2.0

A fishing boat received the echo 50 ms after sending it.

The speed of sound in water is 1500 m/s

Determine the depth of the water.

1. The basic physics of sound – Speed of Sound

In a free field, sound propagates with the velocity c defined by

For the velocity of sound in air sufficiently accurate at normal temperatures, 0–30oC

where TK and TR are the temperature in Kelvin and Rankine, respectively

where TC is the temperature in centigrade

0F=R - 459

Eqn. 2.1

Eqn. 2.2

Eqn. 2.3

1. The basic physics of sound – Speed of Sound

Example 2.1 Determine the speed of sound at 20oC (68oF) in both metric (m/s) and

English (ft/s) units.

0F=R - 459

1. The basic physics of sound – Sound Pressure

For a pure tone, the sound pressure p can be described as

where a is the amplitude in Pascals, ω is the angular frequency in radians per second, t is the time in seconds, and f is the frequency in hertz.

Eqn. 2.4

Eqn. 2.5

A pure tone is a tone with a sinusoidal waveform, i.e. a sine or cosine wave

1. The basic physics of sound – Sound Pressure

Figure 2.1, and 2.2 show the pure tone oscillation, although pure tones do not often exist in nature.

The field of acoustics and noise control has nearly uniformly adopted the metric system throughout and, as such, the unit used for measuring sound pressure is the Pascal(Pa).

1. The basic physics of sound – Properties of Sound Waves

Figure 2.1 : Pure tone


Figure 2.2: Pure tone from a tuning fork


The frequency of a sound indicates the number of cycles performed in 1 s:

where T is the period of one full cycle.

The unit for frequency is the hertz (Hz):

A high frequency pure tone is perceived to have a high pitch.

The audible frequency range to humans is 20–20,000 Hz Above 20,000 Hz is ultrasonic. Below 20 Hz is sometimes called

infrasonic.

Eqn. 2.6


Figure 2.3 : Frequency and amplitude


The wavelength λ is equal to the distance the oscillations have propagated in the time period T:

This shows that the wavelength is inversely proportional to the frequency.

In the audio frequency range, the low frequencies have wavelengths of several meters (or feet), whereas the wavelengths for the high frequencies are only a few centimeters (or fractions of an inch).

Eqn. 2.7


Figure 2.5 : Harmonic oscillation of pressure

2. Characteristics of Noise – Frequency and Loudness

There are two important characteristics of sound or noise - frequency and loudness.

The number of pressure variations per second is called the frequency of sound, and is measured in Hertz (Hz) which is defined as cycles per second.

The higher the frequency, the more high-pitched a sound is perceived.

Frequency Pitch

Amplitude Loudness

No oscillations?


Example 2.2

Determine the wavelength of a 125-Hz and an 8000-Hz tone at 20oC (68oF) in both metric and English units.


The curves in Figure 2.6 indicates the loudness – the subjective interpretation of the magnitude of sound – for pure tones.

The loudness of a sound depends on the wave's amplitude.

The louder the sound, the higher the amplitude. So, amplitude is also a way of measuring the energy has.

The higher the energy, the higher the amplitude resulting a louder sound.


Figure 2.6 : Normal equal loudness contours for pure tones.

20Hz


For example, as can be seen in Fig. 2.6, the threshold of hearing at 1000 Hz is about 4 dB (5 x 10-5 Pa).

A 20-Hz tone must have a sound pressure level about 70 dB higher than a tone at 1000 Hz in order for a person just to hear the tone.

The curves also indicated the loudness—the subjective interpretation of the magnitude of sound—for pure tones.

The units describing loudness are called phons. By definition, the phon is equal to the sound pressure level (in dB) reference to 20 μPa of an equally loud 1000-Hz tone.

Pain will occur when the loudness exceeds 120 phons.

2. Characteristics of Noise – Sound Level and Decibel Scale

As explained previously, sound is measured in unit of pressure and normally referred in Pascal and Newton per meter square or “psi”.

The lowest audible sound pressure is 0.00002 Pa (or 20x10-6 Pa equivalent 20 μPa).

This value is very low as compared to sound emitted from a taking off jet i.e., 200 Pa. Note that the range of difference between these is so great.

Therefore, a scale based on the logarithm of the ratios of the measured quantities is used.

Measurements on this scale are called levels.


rms Sound Pressure

Physically, the rms value is indicative of the energy density of the disturbance.

Mathematically, the rms value is obtained by squaring the sound pressures at any instant of time and then integrating over the sample time and averaging the results.

The rms value is then the square root of this time average:

Where the overbar refers to the time-weighted average and T is the time period of the measurement

2/1

02

2/12 1

Trms dttp

Tpp Eqn. 2.8


Sound Power

Travelling waves of sound pressure transmit energy in the direction of propagation of waves magnitude of the displacement times component of force in the direction of the displacement

The rate at which this work is done

Sound Power Level is defined as :

Where,

Lw = Sound Power level in dB

W = Sound Power in watt

Wo = Reference sound power

= 10-12 watt

The standard reference power 10-12 watt is the threshold power of our hearing.

Eqn. 2.9

= WORK

= (SOUND) POWER, Watt


Please note that, direct reading of sound power is not possible. Sound Level meter functions by measuring sound pressure or the difference in pressure due to vibration of air molecule, compared to 1 atm.

Equation 2.2 describes the power emitted from a noise source.

For example, when we speak, our voice vibrates the air molecule causing the pressure to increase and a sound power is generated.

What is measured by the instrument is the pressure.

Development of sound measurement instruments are currently based on the measurement of pressure and not the power.


Example 2.3

Given Lw = 90 dB. What is the sound power in watt?

Wo = Reference sound power

= 10-12 watt

Eqn. 2.10


Sound Intensity

Sound intensity (I) is defined as the time-weighted average sound

power per unit area normal to the direction of propagation of the

sound wave

I = Sound Intensity (watt/m2)

W = Sound Power (watt)

A = area (sphere) normal to source (m2)

= 4πr2, r is distance from source

The higher the area, the lower will be the sound heard at a distance

from origin.

Therefore, sound intensity is reduced proportionately with increase

in coverage area.

A

WI Eqn. 2.11


Intensity is related to sound pressure in the following manner:

Where,

I = Intensity (watt/m2)

prms = root mean square pressure, Pa

ρ = density of medium (kg/m3)

c = speed of sound in medium (m/s)

c

pI rms

2

Eqn. 2.12


Both the density of air and speed of sound are a function of temperature. Given the temperature and the pressure, the density of air may be determined from Standard Table.

Sound Intensity Level can be written as:

Where,

LI = Sound Intensity Level (dB)

I = Sound Intensity (watt/m2)

dB 10

log1012I

LI Eqn. 2.13


Example 2.4

Given that a sound power in watt from a pile driver in a construction site is 1x10-3 watt. Determine the sound intensity at the perimeter which will be heard by the following two cases:

i) Heidi stands at a distance 15 meter from source

ii) A hawker stands at a distance 50 meter from source

Sound Intensity level?

dB 10

log1012I

LIA

WI

A = 4πr2, r is distance from source


Sound Pressure Level

In order to cope with the problem of an ‘astronomical’ range of numbers of the sound pressure level (i.e; normal healthy level of 0.00002 Pa vs. Saturn rocket at liftoff of >200 Pa), a scale based on the logarithm is introduced as “levels”

The unit for these types of measurement scales is the Bel, named after Alexander Graham Bell:

L’ = levels, Bel

Q = measured quantity

Q0 = reference quantity

L’ = log10 (Q/Qo) unit: Bel Eqn. 2.14


The sound pressure level then is a logarithmic ratio Lp defined as:

where

prms = the sound pressure of interest (in Pa) and

pref = reference sound pressure (in Pa) usually chosen as the limit of hearing of 20 μPa.

NOTE: (P log10 xn) = P (n) log10 x

The unit for the sound pressure level, SPL or Lp, is the decibel (dB)

Eqn. 2.15


The LP is measured against a standard reference pressure, pref = po = 2 x 10-5 N/m2 which is equivalent to zero decibels.

The relationship between sound pressure and sound pressure level (with 20 μPa as the reference sound pressure) is shown in Table 2.1.

A scale showing some common sound pressure level is shown in Figure 2.7.


Table 2.1


Figure 2.7: Relative scales of sound pressure levels


Example 2.5

Determine the sound pressure level for sound pressures of p = 1 Pa and p = 1 atm (1.013 × 105 Pa) (reference to 20 μPa)


Combining Sound Pressure Levels

Since we’re dealing with the logarithmic heritage in SPL, adding the decibels is the same as multiplying them.

For example, adding 0dB (20µPa) noise with 0dB to it, you’ll get a 6.02dB noise.

Two approaches: 1) addition through formula, 2) addition through graphical solution


For skeptics, this can be demonstrated by converting the dB to SPL, adding them and converting back to dB.

Thus, the addition of these Sound Pressure Level is denoted by:

Lp = 20 log10 (P/Po) dB

Lpt = 10 log10 [ Σ (10)Lpi/10 ] dBEqn. 2.17

Eqn. 2.16


The addition of these Sound Power Level is denoted by:

Lw = 10 log10 (w/wo) dB

Lwt = 10 log10 [ Σ 10(Lwi/10) ] dB Eqn. 2.19

Eqn. 2.18


A graphical solution for this type of problem is provided as in Figure 2.8.

For noise pollution work, results should be reported to the nearest whole number.

For equal decibel values, a shortcut method can be applied:


i

PiP

L Lnn 1010/

10 log10 10log10 SPL total

n 10 log10 (n) n 10 log10 (n)

1 0.00 6 7.78

2 3.01 7 8.45

3 4.77 8 9.03

4 6.02 9 9.54

5 6.99 10 10.00


Figure 2.8: Graph for solving decibel addition problems


Example 2.6

Three SPL’s 68 db, 79 dB and 75 dB, what is SPL of combination?

68 dB

75 dB

= 775.8 dB

80.7dB

79 dB

= 3.2


Alternatively, this can be solved by converting the readings to SPL, adding them and convert back to SPL:

dB7.80

3117,365,17 log 10

101010log10 10/7910/75)10/68(

WL


Calculate the final sound power level that would be heard for noise levels 92, 98, 100, 95 and 85 dB using:

I. Formula

II. Graph


Averaging Sound

Average sound can be calculated as the same as the calculation of summation of sound.

As sound is referred in form of log, so the average requires calculation in form of pressure and power. The pressure is then calculated, averaged and finally antilog.


Average Sound Pressure Level is given as:

Where,

= Average Sound Pressure Level at reference pressure 20 μPa,

dB (A)

N = Number of sample

Lj = Sound Pressure level measured at reference pressure 20 μPa,

dB(A)

j = 1,2,3…n

N

j

Lp

j

NL

1

20/10

1log20

pL

Eqn. 2.20 (a)


The latter equation is only applicable to sound levels in dBA It may also be used to compute average sound power levels if the

factors of 20 are replaced with 10 s

Where,

= Average Sound Pressure Level at reference pressure 20 μPa,

dB (A)

N = Number of sample

Lj = Sound power level measured at reference power level 10-12 W,

dB(A)

j = 1,2,3…n

N

j

Lw

j

NL

1

10/10

1log10 Eqn. 2.20 (b)

wL


Example 2.7

Average the Sound Pressure Level for following field monitoring data, 51, 38, 78 and 68 dB(A).

N

j

Lp

j

NL

1

20/10

1log20

Eqn. 2.20 (a)

2. Characteristics of Noise – Weighting Networks

Weighting networks are used to account for the frequency of a sound.

They are electronic filtering circuits built into the sound level meter to attenuate certain frequencies – with a prejudice something like that of the human ear.

Normally, there are 3 weighting characteristics: A, B and C

The very low frequencies are filtered quite severely by the A network, in a manner similar to the response of the ear, but only moderately by the B network and hardly at all by the C network.

Therefore, if the measured sound level on the C network is much higher than that on the A network, much of the sound energy is concentrated in the low frequency region.


“A” Scale– Filters out low frequencies– Response curve is similar to sensitivity of human ear

“C” Scale– Filters out very little (only the extreme low frequencies)– If a measurement is higher on the C scale than the A scale, the noise has

a low frequency component– Used to estimate the effectiveness of ear protectors

A specialized filter, the "D" weighting, has also been introduced for aircraft noise measurements.

Figure 2.9 shows the response characteristics of the three basic networks as prescribed by the American National Standards Institute (ANSI) spec. no. S1.4 – 1971.


Figure 2.9: Frequency Response Characteristics of Various Weighting Networks


When a weighting network is used, the sound level meter is electronically subtracts or adds the number of dB shown at each frequency shown in Table 2.2

Readings taken when a network is in use are said to be “sound levels” rather than “sound pressure levels”.

The readings taken are designated in decibels in one of the following forms: dB(A), dBa, dBA; dB(B), dBb, dBB; dB(C), dBc, dBC. Tabular notations may refer to LA , LB , LC


Table 2.2: Sound Level

Meter network weighting

values - CFA


Example 2.8

A new type 2 sound level meter is to be tested with two pure tone sources that emit 90 dB. The two sources are at 1,000 Hz and 100 Hz. Estimate the expected readings on the A, B and C weighting networks.


Example 2.9

The following sound levels were measured on the A, B, and C weighting networks:

Source 1: 94 dB(A), 95 dB(B) and 96 dB(C)

Source 2: 74 dB(A), 83 dB(B) and 90 dB(C)

Characterize the sources as “low frequency” or “mid/high frequency”.

Added Problem

The measured octave band sound pressure levels around a punch press is given in table below.

Determine the A-weighted sound level and the overall sound pressure level.

Octave band center frequency, Hz

31.5 63 125 250 500 1000 2000 4000 8000

LP (dB) 70 81 89 101 103 93 83 77 74

CFA, dB

LP + CFA (dBA)


2. Characteristics of Noise – Octave Bands

The human ear is sensitive to sound in the frequency range from approximately 16 Hz to 16 kHz.

Impractical to measure the sound pressure level at each frequency in this range

The measurements are made over an interval of frequency which is called the bandwidth and is specified by an upper and lower frequency limit

fi+1 and fi are called cut-off frequencies.

In acoustics the frequency bandwidths are usually specified in terms of octaves and one-third-octaves

Normally, considering an 8 to 11 octave bands


An octave is defined as an interval of frequency such that the upper

frequency limit is twice the lower limit, that is:

For the 1/3-octave bands, it is defined as:

The center frequency of the band is defined as the geometric mean of the

upper and lower frequencies for the interval:

Relationship of center frequency for an octave? For 1/3 octave?

2 or 2 1212 ffff

260.12 3112 ff

21210 fff

Generation law for octave and third octave bands


Octave band − oct. filter

1/3 Octave band − third oct. filter


Table 2.3: Octave bands

Added Problem

Find the geometric mean frequency for 1:1 octave and 1:3 octave bands for the following band no.


Octave Band 1/3 Octave Band

Lower Center Upper Lower Center Upper

Frequency Frequency Frequency Frequency Frequency Frequencyf1 (Hz) f0 (Hz) f2 (Hz) f1 (Hz) f0 (Hz) f2 (Hz)

22

44

88

177


Figure 2.10: (a) One-third octave band analysis of a small electric motor. (b)

Narrowband of analysis of a small electric motor


Noise level measured with 1:1 Octave Band Filters


Noise level measured with 1:3 Octave Band Filters

2. Characteristics of Noise – Rating Systems

Goals of Noise-Rating System

– An ideal noise-rating system is one that allows measurements by sound level meters or analyzers to be summarized succinctly and yet represent noise exposure in a meaningful way

– Our response to sound is strongly dependent on the frequency of the sound

– Significant factors in annoyance: type of noise & time of day that it occurred

– Ideal system: a) frequency, b) daytime or nighttime noise, c) capable of describing the cumulative noise exposure.


The LN Concept

LN is a statistical measure that indicates how frequently a particular sound level is exceeded.

Example: if L30 = 67 dB, means that 67 dB(A) was exceeded for 30% of the measuring time.

A plot of against N (where N = 1%, 2%, 3%,….) look like the cumulative distribution curve (Figure 2.11)

Allied to the cumulative distribution curve is the probability distribution curve (Figure 2.12) – showing how often the noise levels fall into certain class intervals.


Figure 2.11: Cumulative distribution curve


Figure 2.12: Probability distribution plot

Fre

que

ncy

of o

ccu

rre

nce

, % Calculation of L30 = 67 dB:

Where;

N = the sum of the percentages

L = lower limit of the left-most class interval added

Eqn. 2.21


The Leq Concept

The equivalent continuous equal energy level (Leq) can be applied to any fluctuating noise level.

It is expressed as:

Eqn. 2.22

Added Problem

Refer to the attachment distributed in the class.

Construct a cumulative distribution curve (Sound level vs. Percentage time greater than stated value)

Find:

a) Lmax

b) Lmin

c) L1

d) L50

e) L90

f) Leq



Example 2.10

Consider the case where a noise level of 90 dBA exists for 10 minutes and is followed by a reduced noise level of 70 dBA for 30 minutes. What is the equivalent continuous equal energy level for the 40-minute period? Assume a five-minutes sampling interval.


The Ldn Concept

Ldn is the Leq computed over a 24-hr period with a “penalty “ of 10 dBA for a designated night time period.

Day-night average – subscript “dn”

In airport noise applications, Ldn is referred to LDN

Night time period 10 pm to 7 am

Ldn equation is derived from the Leq equation with the time increment specified as 1 s (1 86, 400 seconds)


So, eqn. 2.22 becomes:

10 log [1/86400] ≈ 49.4, the day-night average sound level:

iL

iL

dn ttL ji10/1010/ 1010

86400

1log10 Eqn. 2.23

Eqn. 2.24 4.491010log1010/1010/

iL

iL

dn ttL ji


Example 2.11

The USEPA estimated that, in 1974, the following was a typical noise exposure pattern for a factory worker living in an urban area. Estimate the Ldn for the exposure shown.

Time (h) Sound Level (dBA)

0000 – 0500 52

0500 – 0700 78

0700 – 1130 90

1130 – 1200 70

1200 – 1530 90

1530 – 1800 52

1800 – 2200 60

2200 – 0000 52

4.491010log1010/1010/

iL

iL

dn ttL ji

Content

1. The basic physics of sound – Sound and types of sound– Speed of sound– Sound pressure– Properties of Sound Waves

– Frequency– Wavelength

2. Characteristics of noise and Desibel scale– Frequency and Loudness– Sound level and decibel scale– Weighting networks– Octave bands– Rating systems

3. Noise measurement

4. Community Noise Sources and Criteria– Transportation noise– Other internal combustion

engines– Construction noise– Zoning and siting

considerations– Levels to protect Health and

Welfare

3. Noise Measurement – Measuring noise

Noise measurement equipment depends on the task to be performed

For an initial survey – a sound level meter (SLM) is adequate for a rapid evaluation and identification of potential problem areas

To study and also determine the characteristics of a noise problem area – an SLM, frequency analyzer and recorder are needed


– Sound Level Meter

Used to measure the sound pressure level

Available to cover a range of 20 to 180 dB

Specifications refer to the American National Standards Institute (ANSI) – referred as “Specifications for Sound Level Maters (ANSI S1.4-1971)

Weighting networks of A, B, C are provided – total loudness level for a particular situation with consideration of the sound frequency, intensity and impact levels.

Weighting network A – most commonly used: discriminates against frequency below 500 Hz – encompasses the most sensitive hearing range.

Measuring environmental noise should be supplemented by the time/duration to determine the total quantity of sound affecting people


– Sound Level Meter (cont’d)

Types of SLM:

SLM provides setting for “F” (fast time response) and “S” (slow time response)

Calibration by calibrator – generates a known decibel standard for QA/QC

SLM Type Intended Use

0 laboratory reference standard

1 for laboratory use, and for field use where the acoustical environment has to be closely specified and controlled

2 suitable for general field applications

3 primarily for field noise survey applications


– Noise Dosimeter

Measure the amount of potentially injurious noise to which an individual is exposed over a period of time

Can be set to the desired level – total up the exposure time to noise above the set level

Does not identify the noise sources

To determine the noise exposure and culpability – dosimeter should be coupled with a frequency analyzer/ human observer to record noise source identities


– Noise Dosimeter (cont’d) – Interpretation of results

To calculate the noise exposure level of an employee working shifts of more or less than eight hours, it is necessary to normalise the employee’s exposure to an equivalent eight hour exposure (LAeq,8h).

where:

LAeq equals the equivalent continuous A-weighted sound pressure level occurring over the measured time; and

T represents the shift length in hours (not to be confused with the sampling time).

For shifts between 10 to 12 hours, add 1 dBA – extended shift

In addition, shifts of 10 hours or more require adjustments to LAeq,8h values, as indicated in Table 3.1.

8log10 108,

TLL AeqhAeq


Table 3.1

Correction factors for computing LAeq,8h from LAeq records

Example 3.1

A personal noise dosimeter is placed on an employee for a representative period of six hours. At the end of the six hours, the LAeq

reading is 93 dB(A). The employee works a 10 hour shift.


8log10 108,

TLL AeqhAeq

– Sound Analyzer

Frequency analyzer – measure complex sound and sound pressure according to frequency distribution.

Supplemented with SLM

Covers different frequency bands

Example: octave band analyzer, impact noise analyzer (peak level and duration of impact noise)


– Cathode-Ray Oscillograph

Observing the wave form of a noise and pattern

Magnetic tape recorder makes possible the collection of noise information in the field and subsequent analysis of the data in the office or laboratory


– Background Noise

Noise in the absence of the sound being measured that may contribute to and obscure the measured sound

Correction can be made through subtraction method or application of correction factors (CF) as in Table 3.2


ΔL, dB AΔ, dB

1.0 6.9

1.5 5.3

2.0 4.3

2.5 3.6

3.0 3.0

3.5 2.6

ΔL, dB AΔ, dB

6.5 1.1

7.0 1.0

7.5 0.9

8.0 0.7

9.0 0.6

10 0.5

12 0.3

14 0.2

16 0.1

18 0.1

20 0.0

Table 3.2: Background noise correction factor

10/background10/measured10 1010log10corrected LLL

4.0 2.2

4.5 1.9

5.0 1.7

5.5 1.4

6.0 1.3

ALL measuredcorrected

Example 3.2

The measured overall sound pressure level around a fan is 83 dB. The measured overall sound pressure level for the background (ambient) noise in the room where the fan is located is 77 dB. Determine the overall sound pressure level produced by the fan alone.




Added problem

The experimental data shown below were measured around an air vent. The readings are the octave band sound pressure levels with the air flow stopped (background noise) and with the air flowing (data). Determine the octave band sound pressure levels and overall sound pressure level for the vent noise alone




Octave band center frequency, Hz

63 125 250 500 1000 2000 4000 8000Background LP, dB

79.0 76.1 73.6 70.7 68.1 66.0 64.3 63.0

Data LP, dB 79.1 76.6 76.3 78.7 81.2 83.1 81.2 78.1

Estimation of Community Reaction

If noise spectrum data are not available, the LDN of the background noise, with suitable correctors may be used to estimate the anticipated community response to the environmental noise:

The correction made to measured Ldn accounted for the effect of annoyance due to several influencing factors (presented in Table 4.1) such as below:

• Nighttime

• Location

• Time of the year

• Previous noise exposure

Average community reaction to noise based on Ldn is given in Table 4.2.

4. Community Noise Sources and Criteria

dndndn CFLL measuredcorrected

Estimation of Community Reaction – Table 4.1. Correctors to be Added to the Measured Day-Night Level for Various Influencing Factors for Community Noise Reactionb


Influencing Factor Description of condition CFdn, dBANoise Spectrum Pure tones or impulsive noise present +5 No pure tone or impulsive sounds 0 Type of location Quiet suburban or rural community +10 Normal suburban community +5 Urban residential community 0 Noisy urban residential community -5 Very noisy urban community -10 Time of year Summer or year-round 0 Winter only or windows always closed -5

Previous noise exposure No prior experience with the intruding noise +5

Some prior experience with the noise or where the community is aware that good-faith efforts are being made to control noise

0

Considerable experience with the noise and the group associated with the source of noise has good community relations

-5

Aware that the noise source is necessary, of limited duration, and/or an emergency situation

-10

bOnly one correction factor should be used from each category


Estimation of Community Reaction – Table 4.2. Average Community Reaction to Noise Based on the Day-Night Level, (Ldn)

Corrected day-night level Ldn(corrected) Expected community response

<62 dBA (dn) No reaction

62 - 67 dBA (dn) Complaints

67 - 72 dBA (dn) Threats of community action

>72 dBA (dn) Vigorous community action

Example 4.1

The noise levels in a suburban area are given in Table 4.3. The area has had some prior experience with intrusive noises. There are no pure tone components of the noise, and it is not impulsive. The noise source will be present year-round.

Determine the anticipated community response to the noise source.


Duration A-weighted level

Daytime4 hours 60 dBA6 hours 55 dBA5 hours 50 dBA

Nighttime2 hours 45 dBA7 hours 40 dBA

A. Aircraft Noise– The noise spectra of a wide fan jet (i.e. Boeing 747) reveal that

sound pressure levels are higher on takeoff compared to landing

– Smaller aircraft have lower sound pressure levels (except for turbojets)

B. Highway Vehicle Noise– Predominant source of most automobiles during normal operation

below about 55 km/h is the exhaust noise

– At speed 80 km/h, tire noise is dominant source. For truck, at speed more than 80 km/h tire noise is dominant – the noisiest is “cross-bar” tread

– Diesel trucks are 8 to 10 dB noisier than gasoline-powered

– For motorcycle, dominant source of noise is exhaust – highly dependent on the speed

– The U.S. Federal Highway Administration (FHA) has developed standards shown in Table 4.3

4. Community Noise Sources and Criteria– Transportation noise

4. Community Noise Sources and Criteria– Transportation noise

Table 4.3. FHA noise standards for new construction

a Either Leq or L10 may be used, but not both. The levels are to be based on a 1-hour sample

Land Use Category

Exterior design noise level dBAa

Description of land use categoryLeq L10

A 57 60

Tracts of land in which serenity and quiet are of extraordinary significance and serve an important public need, and where the preservation of those qualities is essential if the area is to continue to serve its intended purpose. For example, such areas could include amphitheaters, particular parks or portions of parks, or open spaces, which are dedicated or recognized by appropriate local officials for activities requiring special qualities of serenity and quiet

B 67 70Residences, motels, hotels, public meeting rooms, schools, churches, libraries, hospitals, picnic areas, recreation areas, playgrounds, active sports areas, and parks

C 72 75Developed lands, properties, or activities, not included in categories A and B above

D Unlimited Unlimited Undeveloped lands

E52

(interior)55

(interior)Public meeting rooms, school, churches, libraries, hospitals and other such public buildings

These devices as listed in Table 4.4 are not significant to average residential noise levels in urban area

However the relative annoyance of most of the equipment tends to be high – U.S. EPA, 1971

The 8-hour exposure level is in reference to the equipment operator

4. Community Noise Sources and Criteria– Other internal combustion engines

Table 4.4. Summary of noise characteristics of internal combustion engines

19 common types of construction equipment – range of sound levels as in Table 4.5.

Annoyance resulting from construction noise:

– Single house construction in suburban communities will generate sporadic complaints if the boundary line 8-hour Leq exceeds 70 dBA

– Major excavation and construction in a normal suburban community will generate threats of legal action if the boundary line 8-hour Leq exceeds 85 dBA

4. Community Noise Sources and Criteria– Construction noise

Table 4.5. Range of sound levels from various type of construction equipment (based on limited available data samples. (Source: U. S. EPA, 1972)

The U.S. Dept. of Housing and Urban Development (HUD) was charged with developing guides for zoning modifications or for siting of dwellings

Annoyance for a specific noise exposure depended on both the average level of the noise and on the variability of the source of noise (Griffiths and Langdon, 1968)

Table 4.6 list out criteria for new residential construction by HUD

4. Community Noise Sources and Criteria– Zoning and siting considerations

General external exposures AssessmentExceeds 89 dBA 60 minutes per 24 hours

UnacceptableExceeds 75 dBA 8 hours per 24 hoursExceeds 65 dBA 8 hours per 24 hours Discretionary:

normally unacceptableLoud repititive sounds on site

Does not exceed 65 dBA more than 8 hours per 24 hours

Discretionary: normally acceptable

Does not exceed 45 dBA more than 30 minutes per 24 hours Acceptable

Noise criteria levels that is necessary to protect the health and welfare of U.S. citizens listed in Table 4.7

4. Community Noise Sources and Criteria– Levels to protect Health and Welfare

Chapter 2: Noise and Vibration By: Dr. Sara Yasina Yusuf | School of Environmental Engineering EAT...

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