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34
Spécial “ Noise at work 2007 ”
Acoustique
&
Techniques n° 49
Practical noise attenuation of hearing protectors according to noise directive 2003/10/EC
workplaces (Pekkarinen 1989). However, particularly
during short inspections, HPDs are not always used, and
these short periods without use of protectors can reduce
effective attenuation of HPDs significantly, as Figure 1
shows. If a person is exposed to high-level noise without
hearing protectors, even for five minutes, the efficient
attenuation of hearing protectors is seriously reduced.
The noise attenuation of HPDs in practice is also reduced
by communication, application difficulties — especially
with ear plugs, wearing eyeglasses or headgear —
protector maintenance problems, beards and hair. In the
laboratory, the attenuation of HPDs is determined by the
standard series of EN 352. Practical noise reduction can
be made in audiometric chambers by subjective hearing
threshold (REAT = real ear at threshold) or at real work
(MIRE = microphone in real ear) (Berger 1986, Pääkkönen
2000c). According to a standard EN 458, some properties
and evaluation models are defined for workplace use
(EN 1993). The noise attenuation of HPDs against low-
frequency noise and impulse noise can be difficult to
evaluate.
Materials and methods
The data were collected on workplace visits where
there was discussion on the real attenuation of hearing
protectors. In the MIRE method, a miniature microphone
(<5x5 mm) was fixed to the end of the earplug and
inserted into the ear canal. When an earmuff was used
the microphone was located at the entrance of the ear
canal. The microphone signal was transferred through
thin insulated wires (diameter less than 0,1 mm) to the
recording device. A second microphone was located
outside the HPD to measure the ambient noise level. The
recording system consisted of a measurement amplifier
and tape-recorder (Sony TCD D7-8) (7). In addition, a
logging noise dose meter CEL-460 was also used. The
logged noise profiles were analysed by a computer
program, and the tape-recorded signals were analysed by
a sound analyser (B & K 2260). The sound level analyser
was also used directly in field conditions.
The earplug attenuation was determined by the TR method
(TR = transmission reduction) ; in other words, the SPL
in the ear canal with an earplug was subtracted from the
SPL measured outside the earplug. The ear amplification
(TFOE – transmission function of the open ear) of the SPL
difference between the SPL in the ear canal without the
earplug and the SPL outside the ear was calculated. From
the measurement results it was also possible to determine
the insertion loss (IL) by summing TR and TFOE :
IL = TR + TFOE
(1)
Results
Table 1 shows examples of noise attenuationmeasurements
in an industrial workplace. The noise attenuation varied from
10-30 dB. In jet engine testing the average noise exposure
over the test period was 130 dB and the measured noise
level inside hearing protectors was 100-107 dB (attenuation
23-30 dB), which indicates an allowed daily exposure
duration of less than 15 minutes. In a cellulose factory
the average noise attenuation of
hearing protectors was 18 dB,
which is significantly less than is
usually claimed (SNR 25-30 dB).
We have also previously measured
that if there is oral communication
or if the test subject is eating, then
the noise attenuation against usual
industrial noise is reduced by about
5 dB.
There are work tasks in which
communication noise directed
inside a headset can be too loud or
workers are exposed to loud music
directly into their ear canal. The
quality of communication can be
vitally important, for example, when
transportation safety is evaluated.
Among jet engine testing crew the
attenuation of helmets was 23-
30 dB, but this attenuation was
reduced due to communication noise exposure. In addition,
the background noise inside the helmet was elevated by
20 dB (from 70 dB to 90 dB) when the communications
system was activated.
At a power plant the noise exposure was 95 dB, while
under the hearing protector it was 65-75 dB, depending
on the protector type. In the worst case, protective eye
glasses cancelled out the attenuation and in the best case
they did not have any effect on the attenuation (Figure 2).
Therefore, the selection of eyeglasses has a very important
influence on noise exposure. In particular, the insertion of
ear plugs is a critical factor for noise attenuation, and
noise attenuation against low-frequency noise is difficult
to achieve. This can be seen in Figures 1 and 2. In some
cases the use of eyeglasses can destroy the attenuation
of ear muffs. In our examples the noise attenuation was
reduced by 0-22 dB. The worst cases were those in which
the noise content was of low frequency and the eyeglasses
Fig. 1 : The deterioration of the attenuation of hearing protectors if they are not used all the
time The purpose of this presentation is to evaluate the practical issues of hearing
protection, especially regarding the problems for evaluation due to the noise directive