AP3267 Noise Pollution
Mini-Experiment- Sound Power Measurement
Group Member:
xxx
xxx
xxx
Date:
14th November, 2000
Experiment:
Sound Power Measurement of and electric drill at outdoors.
Objective:
„Y Practical using of the sound level meters for noise measurement.
„Y To determine the sound power and hence calculate the loudness of the source.
Procedure:
1. Calibrate the sound level meter.
a) Internal calibration
The K-factor for microphone was checked.
The measuring range FSD was set to "REF".
The Displayed Parameter was set to "SPL".
The Sens. Adj was adjusted until the display shows a value of 94.0 dB +
K-factor by using a small screwdriver.
b) Using sound level calibrator
The measuring range FSD was set to 110 dB.
The Displayed Parameter was set to "SPL".
The calibrator was fitted over the microphone and the Sens. Adj was
adjusted until the display shows a value of 93.8 dB by using a small
screwdriver again.
2. Measurement of Sound Pressure Levels
The setting was changed by holding hold down the button and use the „F and „G
buttons to scroll through the options.
Time W Time weighting. "SLOW" or "FAST" should be selected.
Freq. W. Frequency weighting. Most of your measurements will be made
using the linear or flat filter "LƒÂƒu ƒu ƒuƒÈ" or "LƒÂ ƒu ƒÈ", the "A"
weighting also needed.
Fr./Rdm. Free field or random type sound field. Free field response was
selected.
FSD Full-scale deflection. This sets the range on the instrument. A
suitable range was selected for the measurements. Sound goes
out of range appears ƒî or „P for over range and U_ _ _ for under
range. The scale was changed if both the above cases appear.
The measurement was started again.
Displayed Parameter "LEQ" was selected that's a long integration time
setting.
ƒÓ
Reset All Two buttons were pressed together start a new measurement
was started.
Ext. Filter The external octave filter was inserted or removed by switching
the filter switches in or out.
3. Experimental Procedure
a) The electric drill was placed outdoors on a hard road that's far away from
reflecting objects such as buildings and cars.
b) The x,y coordinate system was marked on the ground by using tape. The
position of each of the five measuring points was also marked. A
hemisphere of radius 1m was employed so the numbers in figure 1 were
used directly.
c) The sound level meter was placed on the tripod provided safely so that the
microphone was at the correct position and pointing towards the sound
source. The foam ball over was put on the microphone for measurements
outdoors in order to reduce wind noise.
d) Leq (with linear filter on sound level meter and external filter switched in)
was measured at each of the 5 points and for each octave filter. Leq without
the octave filter (still on linear filter) and LeqA (using A weighted filter)
were entered in the table provided as well.
e) Sound source from other background sound sources such as cars was kept
minimized to avoid errors.
f) The sound power of electric drill was determined by using the data.
Figure 1 5-Point Measurement Locations for a Hemisphere
Discussion:
The sound source we used is an electric drill provided by Ruel that is relatively small,
portable and able to produce a reasonable amount of steady noise. The sound power
and loudness of the noise produced by the electric drill can then be calculated from
the data recorded.
As the electric drill produces steady noise, the readings at each of the five measuring
points for a hemisphere should be more or less the same. However, we found that the
readings vary time to time at different positions. This may due to the following
reasons.
Firstly, the background sound can never be neglected even though the background
noise levels was at least 10dB lower than those from the electric drill. The location we
carried out the experiment is a public car park and another group carried out the
experiment at the same time next to us. We have abandoned those data that possibly
influenced, nevertheless, sounds produced by the car passing by and other sound
source still may affect the results.
The sound source is placed on the hard concrete ground, we assumed there is a little
absorption by the ground and the sound is propagated into a hemisphere. However,
the hemisphere is not propagate isotropically due to the wind naturally exists in the
environment. Besides, when we perform the experiment, our body and instruments
used may reflect or absorb some of the sound power. Thus, leads to inaccurate
readings recorded.
To improve the accuracy of the data recorded, we have to wait for the number display
settles down to a stable value before taking data. This can minimise the influence of
fluctuations of the sounds.
We should use more points instead of just five points to calculate the sound power of
the electric drill. It is much more reliable to recording the Leq at more different points
as to increase the accuracy but not just demonstrate the principle.
If possible, it would be better that all objects are far away from the sound source to
avoid any influence caused.
The sound level meter should be calibrated before starting the experiment and again
after you have finished your measurements. For the instrument drifts by more than
1dB then the overall measurements are not valid. It is remarkable that for quality
instruments, calibration should merely be a check. Paid attention to any significant
drift, as this may be a signal means that there is something wrong with the instrument.
The sound level meter is very expensive that is nearly over HK$30,000. We have to
handle it with extreme care, especially the delicate microphone. Never get the
instruments wet or get shocked.
Conclusion:
The sound power of the electric drill measured is 6.42 x 10 ¡V4 W and 1.46 x 10 ¡V4 W
(A-weighted).
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