Noise Reduction

There are compelling reasons for achieving noise reduction in military environments because of the high sound levels that frequently prevail.  Noise can severely impair communication; at best it is fatiguing and can cause short term hearing impairment, at worst, it can lead to permanent hearing damage. Health & safety, soldier effectiveness and mission success can all be compromised by noise.  

Levels can range anywhere from the medium noise found in tactical wheeled vehicles, typically mid-90 dB(A), to high levels over 110 dB(A) in tracked vehicles, to extreme levels approaching 150 dB(A) experienced by ground crews working around fast jets.  As a ‘medium level’ of 95 dB(A) is actually high in civilian workplace terms, it is understandable why clear communication at higher military noise levels is so crucial.

Noise reduction is the attenuation of sound energy reaching the ear, achieved through either the creation of a physical barrier that stops the energy, known as Passive Noise Reduction (PNR), or by so-called anti-noise techniques called Active Noise Reduction (ANR) or even a combination of the two.

In the case of a communication headset, noise reduction, in its simplest form, is typically achieved by blocking the noise path using earcups sealed to the head by means of a soft earcushion.

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 mechanized 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 .

What is Active Noise Reduction?

ANR is an electronic means of attenuating lower frequency noise (applicable where passive techniques alone are inadequate) using the principle of anti-noise. Due to ‘the physics’, ANR is most effective below a frequency of 1kHz. It works best in confined acoustic environments and thus ideally suited for use in a headset earshell. ANR is based on closed loop feedback, whereby a small sense microphone, located in the earshell, detects noise appearing at the ear. The signal is filtered and amplified before being fed back to the earphone to generate an identical acoustic signal that is in anti-phase. This has the effect of cancelling the noise, thereby improving the audio signal. The circuit incorporates a pre-amplifier, feedback filter and drive amplifier. A stability control mechanism is essential for the correct operation of the ANR system in a dynamic environment, for example, soldiers in vehicles moving quickly over rough terrain.

There are two basic implementations:

• First: a baffle across the earshell that creates a front and a back cavity.
• Second: the self-contained ANR Module approach.

The module approach benefits by preserving a greater amount of the volume within the earshell, which, in turn, enables a higher level of passive noise reduction to be achieved. Additionally, a module is easier to retrofit to passive earshells on an upgrade or OEM basis.

Digital Active Noise Reduction

dANR is an exciting program that represents the future of Active Noise Reduction.

Watch this space for news of an upcoming product launch!

ANR Module

The electronic ANR Module uses a closed loop feedback circuit. A small 'sense' microphone is positioned in the earshell cavity to detect the noise appearing at the ear. The signal is filtered and amplified and is then used to drive the earphone, also inside the earshell, to generate an identical acoustic signal which is in anti-phase (180 degrees phase shifted to the original sound).

The introduction of the real time anti-phase signal within the earshell results in a high level of cancellation, which can reduce the low frequency energy by up to 97%. This, when combined with the passive attenuation of the headset, provides a broadband noise reduction performance of 30 dB(A).

The heart of the module is a Racal designed electronic circuit, which incorporates the speech pre-amplifier, feedback filter and drive amplifier, and additionally provides the overall system stability control essential for correct operation in a dynamic environment. The circuit is mounted on a small Surface Mount Technology PCB, which also accommodates the passive components needed to optimize and interface the ANR system for specific applications.

By using this approach, the overall size and weight of the ANR system is minimised, allowing its incorporation in the earshell of the headset without the typical degradation of passive attenuation associated with other methods of integration.

The electronic PCB is mounted on a small plastic moulding, which also houses the earphone transducers and sense microphone. The board is sealed to the moulding to maintain the integrity of the acoustic transfer function and the component side of the board is internal to the module to provide a higher level of handling and environmental protection.

Fail Safe

In addition to the ANR earphone, a second conventional service type earphone is provided for communication such that, should the ANR circuit or ANR earphone fail, communications will still be received. This reversionary mode enables the headset to be used as a conventional headset when required. If required, a switch can be provided on the headset to disable the ANR circuit when used in the reversionary mode.

Effects of Noise on Hearing

Good communication is essential in military activities. Apart from a reduction in effectiveness, miscommunication can have a major impact on personal safety and ultimately compromise the success of a mission. Exposure to noise, even for short periods of time, can lead to hearing impairment, known as temporary threshold shift (TTS).

The amount of TTS is a function of noise exposure time and the sound level of the noise source. Fortunately, hearing will recover if further exposure is prevented. The period of recovery is dependant upon the amount of TTS, ranging from just a few minutes through, in extreme cases, to a recovery time of many hours.

Noise Induced Hearing Loss

For personnel exposed to a high level noise with a high TTS or where the TTS has not completely recovered before further exposure to noise, a permanent threshold shift (PTS ) can occur. This is a result of the hearing organ beginning to degenerate and this damage is irreversible and can eventually result in a loss of hearing.

Further reading

• National Institute on Deafness and other Communication Disorders - http://www.nidcd.nih.gov/health/hearing/noise.asp
• Health and Safety Executive - http://www.hse.gov.uk/noise/keyfacts.htm
• American Academy of Family Physicians -http://www.aafp.org/afp/20000501/2749.html

Noise Legislation

The effects of work-related noise on hearing have been known about since the Industrial Revolution. Evidence of the effects of noise on hearing has been well documented in the U.K.’s shipbuilding industry, where it was known as ‘boilermaker’s ear’. However, it was not until studies were carried out in the 1950s that the relationship between high noise levels, length of exposure time and the increased risk of noise induced hearing loss (NIHL) was quantified. NIHL is insidious since it happens over time and often goes unnoticed.

Although workplace legislation in the U.K. and U.S. dates back to the 1970s, for many years ‘the military’ were exempt from such legislation and immune from prosecution. However, this position changed in the 1980s.

Exposure to Noise

The exact detail of the legislation differs from country to country but fundamentally it places maximum limits on exposure levels and the time operatives may be exposed. This combination of level and time forms the concept of noise dose and it is the accumulation of noise dose over time that increases the risk of NIHL.

For example, the same daily noise dose can be achieved by 90 dB(A) for 4 hours or 93 dB (A) for 2 hours or 87 dB(A) for 8 hours.

There is also a separate limit on the peak noise caused by impulsive sources, such as weapon discharge or explosives. Not only does repeated impulsive noise quickly lead to a high noise dose, temporary shift and the risk of permanent NIHL, it can also cause instantaneous damage to the ‘mechanical’ parts of the ear, as with a burst eardrum.

Physical Agents Noise Directive

Most recently, the Physical Agents (Noise) Directive has been imposed Europe-wide and has been implemented in the U.K. as the Control of Noise at Work Act (2005), replacing the earlier Noise at Work Legislation (1989).

The 2005 Regulations requires employers to take action to protect workers at levels of noise 5 dB(A) lower than in the 1989 Regulations and now require health surveillance (hearing checks) for those regularly exposed noise above 85 dB(A).

For more detail it is advisable to refer to the agency tasked with enforcing the legislation, which in the U.K. is the HSE, who have an informative website www.hse.gov.uk/noise.

Understanding Decibels (dBs)

When considering noise measurement, the range of levels of interest is in the ratio of about 1,000,000,000,000,000,000 to 1. With numbers of this range and magnitude it is much more convenient to use a logarithmic scale to provide numbers which are far easier to comprehend and use.

The decibel scale is thus a logarithmic ratio between any two sound levels. For example, the ratio given above may be written as 180dB. The addition or subtraction of decibels is different to normal linear calculations. Adding 3dB to an existing noise level doubles it. So 93dB of noise is twice 90dB of noise. Adding two noise levels of 80dB does not give a noise level of 160dB, but gives a level of 83dB; i.e. twice the 80dB noise level.

The decibel measure is also applied to hearing protected headsets to describe their 'average attenuation' or 'insertion loss' (noise reducing effect). For example, if an individual is working in a noise level of 105dB at a frequency of 1kHz then wearing a protective headset with an average attenuation of 30dB at 1kHz reduces the effective noise level to 75dB at that frequency.

Further reading

Wikipedia Link http://en.wikipedia.org/wiki/Decibel

Salford University http://www.acoustics.salford.ac.uk/acoustics_info/decibels/