Keep the noise down

Growing awareness of the dangers of
exposure to excessive noise is driving the
requirement for better performing hearing
protectors at work. Andy Todd reports on
the testing undertaken at SATRA’s recently
installed state-of-the-art hearing protection
test facility, a service that is in demand but
not widely available in Europe

Hearing protection comes in
many different forms: standard
ear-muff or ear-plug designs,
known as passive devices, or more
complex models incorporating
electronic systems which react
differently in varied noise
environments, known collectively as
active devices.

The protection provided by any device
will be compromised if it is poorly fitted
or incorrectly worn – even the slightest
break in a seal around an ear-plug or the
cushion of an ear-muff will reduce the
attenuation or noise reduction provided
by the hearing protection. It is, therefore,
important that health and safety managers
know which type of protection is suitable
for particular tasks and that wearers
understand how to use their hearing
protection correctly.

The performance of models in their
passive state is dependent on several
factors. The depth of the cups which
enclose the ears, the force of the
headband, and acoustic absorption
characteristics of liners are just a few of
the variables contributing to the
effectiveness of ear-muffs, while depth, fit
and construction material generally
govern the performance of ear-plugs.

Selecting the correct protection
It is vital that the choice of hearing
protector is suited to the noise
environment in which it will be used,
providing adequate protection while
avoiding over-protection (this is when the
hearing protection provides too much
attenuation, making it difficult to hear
speech, alarms and warning signals).

Health and safety managers can ensure
they are selecting the right device by
referring to the SNR and HML values
provided by the manufacturer.

Simplified Noise level Reduction
(SNR), also referred to as Single Number
Rating, provides a single attenuation value
based on the subjective attenuation tests.

Theoretically, this value can be subtracted
from measured external noise levels to
estimate the noise level at the ear beneath
the hearing protection. It should be noted,
however, that this method does not
provide any information as to how much
protection is provided in different
frequency ranges – this is why the HML
rating system is also required.

High – Medium – Low (HML)
provides further detail, allowing the
attenuation provided to be assessed across
high, medium and low frequency ranges.

This is particularly useful if a staff
member is subjected to narrow-band
noise, which covers a small range of
frequencies such as continuous hissing
from compressed air, as it allows a more
accurate assessment of the noise level at
the ear.

Ensuring quality
At SATRA our acoustic test programme
consists of two parts: subjective
attenuation and insertion loss. Subjective
attenuation testing is undertaken using 16
human test subjects. During this test, we
measure the threshold of hearing (the
quietest sound a person can hear) with
and without the hearing protection under
test worn; from these two measurements
we can calculate the acoustic performance
of the device. This test procedure requires
extremely stringent test conditions, which
is why SATRA has recently invested in a
high performance hemi-anechoic
chamber. The new chamber is isolated
from external noise and provides the
environment required to test the
subjective attenuation provided by hearing
protection devices.

The second acoustic test is the insertion
loss assessment which is conducted in our
acoustic tunnel. This test is designed to
calculate the deviation in performance
between ten samples of the same device.

The physical test programme is
designed to replicate the day-to-day wear
and tear which the device will be
subjected to. During this test schedule we
assess resistance to damage when
dropped, ignitability, headband flexing
and cushion pressure to name a few, as
well as a sizing assessment to ensure that
the size of the device corresponds to the
size designated by the manufacturer.

Within SATRA’s chemical and analytical
testing laboratories, we are also able to
conduct
innocuousness
assessments, to
confirm that the
materials used in the
manufacturing
process are not
harmful to the wearer.

When purchasing a
hearing protection device, it is important
to check that it is marked ‘EN 352’ as this
shows it has been tested to and meets the
required European Standards. Without
this marking, the quality of the device
cannot be assured and may be putting
employees at risk.

Andy Todd is acoustic engineer at
SATRA