Engineering Acoustics Lecture 4

7/28/2019 Engineering Acoustics Lecture 4

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Chapter 2 . . .



Personal Daily Noise Exposure Level

The equivalent continuous A weighted sound

level over an 8 hour period is called personal daily

noise exposure level, according to the Noise at Work

Regulations, 1989.

This is used for assessing noise exposure in thework place.



Personal Daily Noise Exposure Level . . .

In the regulations recommended level foroccupational noise is a personal daily noise

exposure level of 85 dB (A).

i.e. 1st action level is 85 dB (A)

This is taken as the industrial first action level for

occupational noise.

The second action level is 90 dB (A).




Above the second action level (90 dB (A)) it ispossibly hazardous and ear protection (muff or

plugs) must be provided to the workers or change

their work shifts.

Earmuffs Ear defenders on a

safety helmet




A 12 hour LAeq,T

value of 75 dB (A) is the common limit

for construction site noise.

Above this level site operations can be stopped by

legal action.



Example

What is the maximum time for which an employee

may spend in a particular work shop where the

noise level is 106 dB (A) without using ear

protection if his noise dose is not to exceed an

equivalent continuous noise level of 90 dB (A) over

the period of 8 hour work shift?

Assume that for the rest of the shift the employee is

subjected to a constant sound level of 85 dB (A).



Answer

For L=106 dB (A)

=> I1

= 1010.6 x I0

Let t be the required time.



Answer . . .



Example

The noise of a construction site is caused by the following.

Source dB (A) Duration (hrs)

Compressor 89 8

Excavator 85 2

Truck 78 6

Pump 76 7

Calculate the equivalent continuous sound level

over a 12 hour working day of a worker exposed to

the above noise levels.



Noise exposure from single discrete events

In many situations the total noise exposure

over a period of time is made up from a number

of different individual events such as passing of an air craft over head or a train near by or a

short bursts of machinery noise.




The measurements of noise from different eventswill be made over different durations.

For comparison of different types of events itwould be convenient if the equivalent continuous

sound level is averaged over the same duration.

For convenience that duration is taken as 1second.




To access such transient sound levels an index

called sound exposure level LAE is defined as

follows,

Where to

– reference duration

t2 – t1 –stated time interval (actual duration of

sound)

t2 – t1 = T




; t0 = 1s

LAE = LAeq,T + 10logT



Example

Sound levels of the following types of trains which

pass a railway station are as follows.

Type of train LAeq dB (A) Duration (s)

Express 85 12Slow 78 18

Freight 76 22

During the 24 hour period of the day, if 8 fast trains,20 slow trains and 3 freight trains passing the station.

Calculate the Equivalent continuous A weighted

sound level over a 24 hour period.



Answer

Equivalent continuous sound level,

LAeq = 58.09 dB (A)



Noise Pollution

Noise

The unwanted sound is called noise.

The effects of noise on human life are;

1) Annoyance to daily life

2) Physiological effects

3) Social effects



Noise Pollution . . .

1) Annoyance to daily life

Noise disturbs

- concentration

- speech

- rest

- sleep

And also noise causes stress.




2) Physiological Effects

The environmental noise such as aircraft noise

affects the growth of children.

Prolonged exposure to excessive loud noise will

result in permanently elevated blood pressure.

The most distinct physiological effect is the loss of hearing.




Noise Induced Hearing Defects

a) Temporary Threshold Shift

b) Permanent Threshold shift

a) Temporary Threshold shift

When a person of normal hearing is exposed

to intense noise (above 70 –

75 dB) for a length of time his or her threshold of hearing is temporarily

raised.




As a result the person suffers a temporary loss

of hearing sensitivity called Temporary Threshold

Shift (TTS).

This loss of hearing sensitivity is regained after

a period (16 hours or more) in a quite

environment.

That is the recovery from temporary thresholdshift takes 16 hours or longer.




b) Permanent Threshold Shift

People working in extremely high noise

environments for long hours have a risk of

permanent damage to their hearing if therecovery is incomplete. This is referred to as

Permanent Threshold Shift.




In the case of someone exposed to intense

occupational noise each at work for many

years (15 - 20) the threshold shift becomes

permanent if temporary threshold shift has

not recovered over night before the next

exposure.

This permanent damage to hearing may often

be indicated by signs of dullness of hearing

together with tinnitus often exposure to noise.




3) Social Effects

The demand for land use is reduced near roads

with heavy traffic, busy airports or noise factories.

Transport noise also affects the growth of cattle

and poultry. It is said that is decreases milk and

egg production.



Noise Criteria In Sri Lanka

L



Reference book:

Acoustics and noise control

2nd edition

B J Smith, R J Peters and S Owen



Practical schedule

3 Practical

2 - Outdoors

1 – Industrial visit

Assignments:

Three (3) in-class assignments, each carry 10 marks.

3 – for performance

7 –

for assignment

Engineering Acoustics Lecture 4

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