1. NOISE AND ITS MEASUREMENT
DEALING WITH TIME VARYING SOUND
Percentile Levels, LN
Equivalent Continuous Sound Level, Leq
Sound Exposure Level, SEL
Assessing Noise over given Time Periods

2. TIME VARYING SOUND Sound Typical Road Traffic Noise Pressure Level, Lp
(dB)
Time
Fluctuating sound pressure level has no single value, although it may
be possible to estimate the peaks and troughs by watching the display
on a sound level meter.
This was a problem before microprocessor were developed as SLM only displayed the instantaneous fluctuating level

3. Percentile Levels, LN Sound L1 Pressure Level, Lp (dB) L10 L90 L 99
Time
One way of finding out more information about the character of the fluctuations
is to measure a percentile level, that is, find the sound pressure level which
the sound level exceeds for a given percentage of the time.
These were originally constructed manually from instantaneous readings of the Lp at 4 second intervals using the “tap on shoulder” method.
Used mainly in older standards and assessment methods like BS4142, CRTN

4. Problems with Percentiles
L1 for first hour
Sound
Pressure
Level, Lp (dB)
L1 for two- hour period
L1 for second hour
Second one-hour period
First one-hour period
Time
The percentile for the two-hour period CAN NOT be calculated from the
percentiles for the individual one-hour periods.
Averages can not be constructed from a set of measurements.

5. Equivalent Continuous Level, Leq,T
Sound
Pressure
Level,Lp
(Energy Units)
Typical Noise
Leq,T T
Time
This should be thought of as the equivalent continuous energy level as it is the steady
level that has the same energy content as the fluctuating sound pressure level.
The area of the rectangle representing the equivalent Continuous Level is the same
as the hatched area under the curve
Easy to manipulate -- average, add etc.
Now the preferred unit – Building Acoustics, Environmental Noise, Noise nuisance

6. Equivalent Continuous Level, Leq,T
Mathematically this is found from numerical integration of the curve which is converted into a steady level with the same amount of energy.
Sound
Pressure
Level, Lp
(Energy Units)
Typical Noise
Leq T
Each strip has a width of δt (a small time step)
Each strip is Lp in length – in energy term that’s 10Lp/10
The sum of all these strips between 0 –T has equivalent energy as the steady level rectangle giving the Leq

7. Rearranging and converting back to a dB level we get,
Sound level meters perform this automatically when set to measure the Leq. Normally it would be based on the “A” weighted sound pressure level LA and hence we would have the LAeq,T - The measurement time period must always be stated.

8. Finding the overall Leq,T from individual levels and time-periods
L2, t2
Leq, T
Sound
Pressure
Level, Lp
(dB)
L1, t1
Energy
Units
L3, t3
Time period, T (where T = t1+t2+t3)

9. CONTROL OF NOISE AT WORK REGULATIONS For the Control of Noise at Work Regulations we use the Daily Personal Noise Exposure Level, LEP,d as a measure of the daily dose. Essentially this is all the energy received by the ear during the working day and converted to an equivalent 8 hour level

10. Example Calculate the LEP,d if the noise levels are: 2 hours @ 56 dBA
1 hour @ 62 dBA
Note the extra brackets I have included, this is to make sure that the 8 divides all of the top line, not just the last term!

11. Assessment time longer than the noise-on time
1 time unit
Noise-on (measured level)
16 energy flow units
Assessed level over a longer time period
Energy flow units
8 time units
2 energy flow units
Time

12. Example
Find the LAeq, 1 hour for a noise source that operates for 10mins per hour at a steady level of 63 dBA
Here L is 63 dBA, t is 10 mins and T is 60 mins
So,

13. Sound Exposure Level, SEL or LAE
Equivalent one second Leq
Sound
Pressure
Level, Lp
(dB(A))
SEL in dB(A)
Train bypass
Time
Useful for capturing events such as train bypasses. A Sound level meter converts the sound energy of the train bypass into an equivalent one second level.
The SEL can be multiplied by the number of bypasses in a given period
to give the L Aeq,T

14. Example
A blacksmith uses a hammer to forge metal bars into ornamental swords. If an individual blow give a LAE of 99 dB and he works at a rate of one hammer blow every 5 seconds, how many hours can he work before exceeding the lower exposure action value of 80 dBA for 8 hours? (Limit set in CNWR)
To find the number of blows we need to use energy boxes - we divide the box representing the allowable dose 8 x 108 by the box representing a single blow 1 x 109.9
No of blows allowed x 5 seconds between blows gives the total time.

15. 8 x 3600 x 108 Number of blows = 1 x 109.9 Number of blows = 362.6
One blow every 5 seconds, therefore x 5 seconds or about ½ an hour
[Note that we need to be consistent with time units --- work in hours or minutes or seconds]
1 x 109.9

16. References. http://www.bksv.com/
You need to register then look under library for books on environmental noise &building acoustics
Advice on noise control in the entertainment industry