Noise-induced hearing loss (NIHL) is the most common,
permanent occupational injury in the world, according to
the World Health Organisation, yet it is preventable. Andy
Todd, acoustic engineer at SATRA, outlines why fully tested
and certified hearing protection products are vital

The dangers of exposure to excessive
noise are well documented yet noiseinduced
hearing loss remains a problem
all over the world. Measuring noise levels in
the workplace and finding ways to reduce
noise at source is the most effective way of
preventing occupational NIHL. In workplaces
where noise limits are exceeded and cannot be
reduced, hearing protection is required.

Effective protection 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. Wearers take it for granted that these
devices are safe to use and are protecting their
hearing, but how can they be sure? Hearing
protection test laboratories such as the new
facility at SATRA help to ensure products
meet the specifications of international and
European Standards, and provide wearers
and employers with peace of mind.

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.

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 require.

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.

Acoustic testing 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 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 10 samples of the same device.

Chemical and physical testing 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.

Chemical testing ensures that the
materials used in the manufacture of the
device and which will come into contact with
the skin are non-staining and not likely to
cause skin irritation, allergic reaction or any
other adverse effect on health.

Quality assurance When choosing a hearing protection device,
a level of 70 to 80dB under the protection is
recommended. This should be adequate to
prevent any damage to the wearer’s hearing
while avoiding over protection.

It is also vital to ensure that the device
bears the CE mark if being sold in Europe, or
the marking of other regional recognised
standards and certification schemes if being
sold in other parts of the world. This should
provide extra confidence that the product
has been properly tested before being placed
on the market. Without it, the quality of a
hearing protection product cannot be
assured and your employees may be at risk of
damaging their hearing.