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

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

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

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.

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

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

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.

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)

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

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!

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

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,

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

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.

8 x 3600 x 108 Number of blows = 1 x 109.9 Number of blows = 362.6 One blow every 5 seconds, therefore 362.2 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

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