What is Passive Noise Reduction (PNR)
PNR
is the attenuation of acoustic
noise by the creation of a physical barrier between the sound
source and the ear. This is achieved by the specific design of the
barrier, in the case of a typical military communications headset,
the earshell. The construction of the earshell, material mass,
volume and stiffness, coupled with the compliance of the seal
between the earcushion and the head, provides the mechanism to
achieve passive attenuation. Such passive techniques are most
effective at higher frequencies.
The integrity of the seal has a significant
impact on the attenuation of the headset and is normally aided by a
spring that exerts force on the earcups and ear cushions. The
spring mechanism can be over the head (headband) or behind the head
(neck/nape band). Force can also be applied without a spring
by means of strap arrangement, as is the case with some Combat
Vehicle Crew (
CVC
)
helmets.
The neckband is the established means for
mechanised infantry, enabling operatives to ‘don & doff’
headsets without having to remove ballistic helmets. Headsets
are worn both mounted and dismounted, and helmet design is changing
to better accept the headband, for example with the US and
Australian advanced/enhanced combat helmet (
ACH
/ ECH ).
However, as with many engineering solutions
there are sometimes compromises to be made. Too much spring
pressure results in wearer discomfort, too little means the headset
is not secure. Similarly, if the earcushion is too soft it
becomes ineffective, too hard and it becomes
uncomfortable. There is also a limit on the physical size of
the earshell, which needs to be compatible with ballistic
protective equipment, such as helmets. Human factors have
generally become much more relevant in recent years and it is no
longer just a question of technical
performance. Nevertheless, PNR is most
effective at higher frequencies with any ‘shortfall’ in attenuation
at low frequencies being complemented by
ANR
.