Measuring Background Noise

Doug · science forum beginner Joined: 02 Apr 2005 Posts: 15

I have just got myself involved in some noise measurement work and have
some received basic training. (I plan to do more training) However, in
the meantime I need some advice on measuring background noise.

The situation:
A local factory/ warehouse has a planning condition that the LAeq (15
minutes) between 0800 and 2200 must must not exceed the background
noise (defined as L95) by more than 5dB(A) and from 2200 to 0800 must
not exceed by more than 3dB(A).

To establish the backgrond noise I am assuming that I need to get an
L95 reading for at least 15 minutes during the day and another reading
during the night, at times when the factory is not operational.

My question is - do I just take one 15 minute reading for day time
background and one for night time or do I need to take readings at
different times - as the backgound may vary e.g. noise at 8.00 in the
morning may not be the same at say 20.00 in the evening. If I do take
reading at more than one time - what do I do with the results. Average
them?

Advice would be appreciated.

Doug

Noral Stewart · science forum addict Joined: 23 May 2005 Posts: 77

First, I do not like the idea of mixing measurement methods and basing the
limit on an leq exceeding an L95 by some amount. The existing leq may
already be 10 or 20 dB higher than the L95. However, it is the regulators
and not you who have set that method and you have to live with it. I think
you have already answered your question somewhat by recognizing that the L95
with vary with different 15 minute periods. You really need to have some
idea how the local authorities will interpret the situation. However, they
could easily say that you must meet the limit for the quietest 15 minutes of
the day on the quietest day of the year. Did you catch that, the sound will
vary from day to day and possibly seasonally as well. In our area, the L95
is often higher in the summer and early fall due to insects, and then lower
in the winter and early spring. If you do not have nearby sound sources at
night, you will find the nighttime levels are often dominated by distant
sources, and levels due to these sources can vary strongly with atmospheric
conditions.

"Doug" <doug.corrie@pateltonra.com> wrote in message
news:1152708791.591240.306800@75g2000cwc.googlegroups.com...

Kev · science forum beginner Joined: 29 Aug 2005 Posts: 3

Doug,

Where are you located? Your state/country should have an environmental
body that sets the industrial noise policy, and this document will
guide you when carrying out noise assessments.

BACKGROUND NOISE
To get a genuine L95 reading it will generally be necessary to measure
noise levels (for example at 15 min intervals) over a period of a week
with an unattended noise logger. If you were being thorough with the
assessment, you would then match this data against meteorological data
to exclude periods of high winds and/or heavy rain.

How you then determine the background noise level from these
measurements will vary according the noise policy, however it will
involve something like sorting the L95(15min) values in ascending order
and selecting the 10th percentile value.

FACTORY NOISE
To determine the LAeq from the factory, you should make attended 15
minute measurements, so that you can exclude extraneous nosie sources
such as aeroplanes, lawn mowers and so on. The number of measurements
you make should be based on your assessment of the factory opertations
- i.e does the factory equipment operate consistently, or does it vary
throughout the day?

SEASONS
It is true that the background level will vary with the season due to
insects, and if possible you should avoid measuring noise during the
periods (the insects would be classed as extraneous noise). I don't
want to complicate things too much for you mate, but you should also be
aware that in rural areas, meteorological phenomena such as temperature
inversions (which occur in some areas during winter) will also affect
noise levels.

Kev

Brian Marston · science forum beginner Joined: 13 May 2005 Posts: 25

Hello in Ireland <www.pateltonra.com>

Define operating hours. It looks like the bare minimum will be 24 hours
of measurement, if you're in a fully urban area. From what you say, and
without knowing the area, I'd recommend nothing less than 24 hours. It
can take years of measurement before you get the feel for the daily /
weekly fluctuations.

Averaging is a debate that can go on for years - logarithmic average,
arithmetic average, absolute lowest LA95,15min or average of lowest
LA95,15min values over a number of days, statistical trimming of each
periods LA95,15min values or a combination.

Talk to the authority who set the conditions, and get a copy of the
policy those conditions are based on. Talk through what measurements
they expect and how and where they expect them to be measured. If you
get it wrong, you are playing with the the livelihood of that company so
it can't be taken lightly.

Doug wrote:

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