HoareLeaPresentation -NoiseIssuesBath

8/3/2019 HoareLeaPresentation -NoiseIssuesBath

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Importance of technicalresearch in long-distance

sound propagation

Andrew Peplow

Andrew Bullmore

Contact emails: [email protected] [email protected]

A C O U S T I C S


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warns of danger

provides information on surroundings

allows us to communicate and learn

enjoyment of recreational sound (music)

Why do we want to hear sound ?

Image courtesy Bruel & Kjaer


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Why do we not want to hear noise ?

lowers quality of life causes annoyance

interferes with work and ability to learn

damages health



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When does sound become noise ?

depends on individual

depends on activity of individual

depends on attitude of individual

depends on hearing acuity of individual

depends on level of noise

depends on character of noise



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The human auditory field

Threshold of hearing

Please click on small pictures to hear audio samples or anywhere on main figure to

hear pure tones at 200Hz, 1000Hz, 2000Hz and 10,000Hz


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LAeq,T Equivalent continuous noise level. The continuous noise level over the assessment time

period, T, that would result in the same total sound energy at the assessment location as

produced by the actual time varying sound. The LAeq,T tends towards the peaks in the time

varying noise.

LA90,T

Background noise level. The noise level exceeded for 90% of the time over the assessment

time period, T. The LA90,T tends towards the troughs in the time varying noise and thus

provides a measure of the typical lower level of noise that will always be present to mask out

any specific source of noise introduced into the noise environment

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Time (125ms per sample, total time = 1 minute)

S

oundpressurelevel,dB(A)

LAeq,T = 44 dB

LA90,T = 34 dB

Sample plot showing the time varying sound pressure level measured over a minute long period


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Time (5 seconds per gridline)

Soundpressurelevel,d

B(A)

LAeq,T = 57.2dB

LA90,T = 56.8dB

LAeq,T versus LA90,T

For a steady noise environment, such as that at some distance from a busy motorway, the LAeq,T will

be similar to the LA90,T, although it will always be higher

Please click anywhere on chart to hear the noise, or on picture to see and hear noise


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Soundpressurelevel,dB(A)

LAeq,T = 57.2dB

LA90,T = 51.0dB

LAeq,T versus LA90,T

For a variable noise environment, such as that close by a road with distinct passing vehicles, the

LAeq,T will be significantly higher than the LA90,T

Please click anywhere on chart to hear the noise, or on picture to see and hear noise


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The subjective perception of specific sounds

The two previous traffic noise examples (shown together below for direct comparison) have the same

LAeq,T noise levels but quite different temporal characteristics, as evidenced by the differences in their

respective LA90,T levels. Thus the subjective perception of the same specific noise introduced into each

of the two different environments may be quite different.

Please click anywhere on chart to hear the combined noise from distant and close traffic

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Soundpressurelevel,dB(A

)

Distant

traffic

Close by

traffic


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Soun

dpressurelevel,dB(A)

LAeq,T = 54.9dB

LA90,T = 51.2dB


As an example, the sound of kart racing activity is now introduced into the two different environments.

The following figure shows that the LAeq,T of the kart noise is 54.9 dB. This is around 2 dB(A) lower than

the LAeq,T of both the more constant in level distant traffic noise case and the more variable in levelclose by traffic noise case

Please click anywhere on chart to hear the kart noise


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The preceding examples have considered the effects on subjective audibility of different temporal

characteristics of the existing (residual) sound field. The following example shows the effect of

introducing the same kart noise into environments with the same (steady) noise environment, but with

different levels of steady noise

Please click anywhere on the relevant traffic noise label to the right of the chart to hear the kart

noise together with the constant traffic noise at the stated level

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So

undpressurelevel,dB(A)

Traffic noise

Kart noise

Traffic noise

+ 5dB

Traffic noise

- 10dB

Traffic noise

+ 5dB

Traffic noise

Traffic noise 10 dB


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0

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50

60

70

20Hz

25Hz

31.5

Hz

40Hz

50Hz

63Hz

80Hz

100Hz

125Hz

160Hz

200Hz

250Hz

315Hz

400Hz

500Hz

630Hz

800Hz

1kH

z

1.25

kHz

1.6kH

z

2kH

z

2.5kH

z

3.15

kHz

4kH

z

5kH

z

6.3kH

z

8kH

z

10kHz

12.5

kHz

16kHz

Third octave band centre frequencies, Hz

Thirdoctavebandsoundpressurelevels,d

BKart NoiseAmbient Low Frequency Bird Song

Third octave band frequency analysis

Please click on coloured labels at top of chart to hear recorded noise in that frequency range

Ambient Low Frequency Kart Noise Bird Song


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Soundpressur

elevel,dB(A)

LAeq,T = 44 dB

LA90,T = 34 dB

Please click on chart to hear recorded noise in the upper frequency range

Time history of upper frequency noise (predominantly bird song) only


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Soundpressurelevel,dB(A)

LAeq,T = 52 dB

LA90,T = 47 dB

Please click on chart to hear recorded noise in the mid frequency range

Time history of mid frequency noise (predominantly karts) only


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Soundpressur

elevel,dB(A)

LAeq,T = 52 dB

LAeq,T = 44 dB

Please click on chart to hear recorded noise in the combined frequency ranges

Comparison of mid (top trace) and upper (lower trace) frequency noise


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Background noise and wind farms

background noise levels often fall below 30dB(A)

noise levels also vary with wind speed

Measured background noise levels - quiet daytime

y = 0.2603x2

- 0.0533x + 21.225

R2

= 0.7092

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10m height wind speed, m/s

Soundpressurelevel,LA90


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Ground effect as a source of uncertainty

Hard paving, concrete, etc. G = 0.0 39 dB(A)

Mixed - hard and porous ground G = 0.5 37 dB(A)

Porous - ground suitable for vegetation G = 1.0 35 dB(A)

35 dB(A) to 39 dB(A)(ground effect only)


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Background Sound Variability

Distance from source

Noiselevel,d

B


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Industry Sound Variability


Noiselevel,dB


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Noiselevel,d

B

Uncertainty and Potential Risk

critical region = risk

RAY TRACING & PARABOLIC EQUATION METHODS


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RAY TRACING & PARABOLIC EQUATION METHODS

TWO MOST POPULAR METHODS


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343 343. 5 344 344. 50

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Sound speed (m/s)

Height(m)

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Distance (m)

S R

Propagation effects. Wind or Temp term included

in sound speed profileSound shadow region results under

temperature lapse and/or upwind conditions

Result is large decreases over neutral of

typically -10dB(A) to -15dB(A) coupled with

highly variable noise level

Sound enhancement results due to multiple paths

under temperature inversion and/or downwind

conditions

Result is small increases over neutral

of typically +1dB(A) to +3dB(A) and much morestable noise level

Barrier effects of topographical screening can be

greatly reduced compared with the neutral case

Sound enhancement at receiver due

to multiple source-receiver paths

Sound energy enters shadow

region via turbulent scattering

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Sound speed (m/s)

Height(m)

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Distance (m)

Limiting ray

Shadow


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Benefits of using Parabolic Equation models

can aid understanding of complex effects

(e.g. linear/logarithmic sound speed gradients)

can provide guidance as to potential range of noise levels for a

given range of input parameters

can serve as benchmarks for testing the output of engineering

type models


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ATTENUATION DECREASE IN NOISE LEVEL


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ATTENUATION DECREASE IN NOISE LEVEL


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Low-level source. Difference Ray and PE

Ray tracing does not include surface wave ???


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Raspet, JASA, 1991


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Benefits of using Asymptotics

can provide understanding of mechanisms of complex physicaleffects (e.g. linear/logarithmic sound speed gradients,

impedance)

can provide guidance as to potential decay rate of noise levels

against homogeneous, no wind, conditions.

p ~ Z / (kr) squared in homogeneous conditions, Real(Z) > 0.

can serve as a benchmark for testing the output of engineering

type long-range models


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Offer two MSc projects:

PARABOLIC EQUATION METHODS

ASYMPTOTICS

A C O U S T I C S

Documents

HoareLeaPresentation -NoiseIssuesBath