Noise reduction for improved communications

Sennheiser Government Systems

Military communication systemSennheiser Government Systems is the military development arm of the global Sennheiser Group. For over 50 years, Sennheiser has been creating unrivaled microphones, headphones, wireless transmission technologies and components for unique applications for professionals the world over, winning many awards for these achievements including each of the prestigious Grammy, Oscar and Emmy for technical innovation and expertise.

Today, our products embody our dedication to applied technologies for higher performance, and our devotion to better quality military communication systems.

Clear First Time Communications

We understand that mission success depends on the awareness of an enormous array of constantly changing information. Successful mission completion requires continuous navigational / situational awareness, systems performance, monitoring of offensive and defensive weapon systems and clear first time communications in areas where equipment and aircraft are producing noise levels upwards of 130 dB.

Fixed wing aircraft on takeoff can generate levels 20 dB higher! ANR technologies and products are especially suited for these situations since they relieve the warfighter from high noise levels which inhibit effective command, control and communication. Using ANR equipped communication systems

  • Improves mission effectiveness
  • Gets the message through the first time
  • Reduces personnel fatigue
  • Prevents personnel hearing loss

Background Noise Reduction

Background noise contributes to stress levels as one strains to hear, process and understand the multitude of signals and data that are required in operational situations. It requires at least an 8 dB difference in the audio signal above the ambient noise levels to have 80% word recognition. The ability to process speech and sounds is highly affected by higher background noise.

Without sufficient hearing protection, the individual is subject to added stress and adverse physiological symptoms may develop. Kryter and Pazo, 1980 showed that blood pressure, heart rate and respiration all increase in the presence of noise. Even visual acuity suffers as documented in FEMA FA-118, 1992, a potentially disastrous event in a battlefield environment. Tension, fatigue, and irritability are also symptoms of noise exposure.

Background noise at frequencies below the normal speech band limits the comprehension of speech and audible signals through the “upward spread of masking” affect. High background noise

  • Reduces accuracy of comprehension and response speed
  • Makes time sharing of attention more difficult
  • Impairs recall
  • Reduces effectiveness of information processing

The focus and attention of aviators and sensor operators is improved by reduced noise and enhanced audio quality. The incorporation of active noise reduction and passive noise protection in a high quality headset relieves the stress induced by noise on a strategic or long tactical flight.

Sennheiser ANR communications performance can be packaged in a number of configurations for your requirement: aviator’s helmet, ground crew helmet, command/control/communications specialists, sensor operator headset, etc. Contact Sennheiser Government Systems to discuss your specific requirements, and provide communications solutions for your team.

Hearing Protection Requirements and Regulations

Hearing is a key ingredient in human perception. The speed and accuracy of decision making and situational awareness improves when audible cues are used. Many military activities require attention to communication in the presence of high intensity noise. Communication in the presence of that noise is required, as is protecting the hearing of the individual routinely subjected to that noise for significant periods of time. DOD directive 6055.12 states the official policy of the US defense department as

“It is DoD policy under DoD Directive 4715.1 (reference (b)) to protect all DoD personnel from hearing loss resulting from occupational noise exposure through a continuing, effective, and comprehensive HCP (hearing conservation program).”

In November 2002, the European Parliament / Council of the European Union adopted PE-CONS 3666/1/02 REV 1 (en), a directive “on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (Noise).” That directive, like the DOD policy requires hearing protection for most industrial applications that result in noise exposure greater than 85 dB(A) in any 8 hour period.

The objectives of these directives and policies, and the many specific specifications that flow from them is to prevent the hearing loss that results from prolonged exposure to moderate levels of noise, and from exposure to higher (impulse) noise levels. For a primer on noise induced hearing loss, see the article “How Noise Affects Human Hearing, Communication and Job Performance” (DelConte, 1999).

What Hearing Protection is Required?

The policies and directives mentioned above, and MIL-STD-1474D (1997) proscribes 85 dBA as the maximum level for exposure over an 8 hour period to preserve hearing in the majority of individuals. Using 3 dB as the doubling factor (DOD 6055.12 encourages the use of the more conservative 4 dB), approximately 88 dBA exposure must be limited to 4 hours per day; 91 dBA exposure limited to 2 hours per day, etc. Given that levels of military vehicles often exceed 120 dBA, significant hearing protection (~ 35 dB) is required for a reasonable working duration to be achieved.

Hearing loss has become a significant health issue for the international defense community. In the U.S., veteran’s disability benefits paid for hearing loss as a primary disability has topped $1.1 billion since 1968. Besides this cost however, is the very important quality of life cost of lost communication and appreciation of the audible world.

Hearing protection comes in the form of earplugs or ear defenders (circumaural earcups / headphones) to shield the ear. The effectiveness of earplugs depends on the design and the training of the user to install them correctly. Ear defenders are also prone to mis-use if significant adjustment is necessary to achieve the intended fit. The effectiveness of an ear defender is dependent upon the seal made between the wearer’s head and the earcup. For activities that require real-time communications, it should be noted that earplugs or ear defenders alone will inhibit the wearer’s ability to perceive audible instructions.

Hearing protection analysis

The effectiveness of hearing protection is often expressed as the Noise Reduction Rating or NRR. The NRR is defined as ‘C’ weighted level (100 dB pink noise) minus the ‘A’ weighted attenuation levels at octave frequencies minus 3 dB. While this is standard shorthand for discussing the effectiveness of hearing protection devices, it does not adequately describe the effectiveness of hearing protection in the complicated environment of high noise military applications.

The noise intensity vs frequency of the environment of interest should be measured (see figure) and compared with the ability of a given hearing protection device to perform against the frequencies of interest in that environment. For instance, a passive ear defender will not adequately protect an individual’s hearing from the low frequency machinery (e.g. engine) noise of a helicopter or armored vehicle, though it will receive a high NRR based on its higher frequency attenuation performance.

The ability to communicate in a high noise environment is referred to as “intelligibility.” A signal-to-noise ratio of 8 dB is required to achieve >80% intelligibility: in the presence of 80 dBA background noise, a signal level of 88 dBA is required to achieve working intelligibility. But if the signal required to achieve intelligibility is higher than the recommended noise exposure levels, even though the signal is intermittent, then the performance of the hearing protection / communication system is not adequate. Therefore, sufficient attenuation to achieve a reduced noise level of approximately 80 dBA where communication is required is necessary.

What Role Does Active Noise Reduction (ANR) Play?

Active Noise Reduction

ANR is an active electro-acoustic feedback system designed to reduce real-world noise levels before they impinge on the wearer. The article “Headset with Active Noise-Reduction System for Mobile Applications” (V. Bartels, JAES 1992) describes in more detail the nature and performance of ANR headset systems. ANR is particularly effective at the low frequencies where wavelengths are sufficiently long to allow detection and cancellation in real time. Therefore, a headset may be designed to attenuate higher frequencies, and therefore lighter and more comfortable, and the ANR will electro-acoustically reduce the noise at the lower frequencies (< 600 Hz).

How Does ANR Work?

Noise compensation

From Mr. Bartels’ paper “A small microphone inside the ear cups of the headphone, in front of the ear canal, picks up the sound reaching the ear’s entrance. This signal is led to the control circuitry, which is responsible for amplification and proper phase shift.” The reproduced signal is a phase inverted representation of the noise, which cancels the original noise in the ear cup prior to being received (heard). This simple sounding process is really a sophisticated control system, and the measure of how well an ANR system works depends on the understanding of the designer and producer of the critical elements in the system.

What Makes a Good ANR System?

Because ANR is a close loop control system, the characteristics of the critical elements in the ANR system make the difference between a system which dependably defeats noise and one which produces more noise. The pickup microphone, acoustic driver (speaker), ear cup, and transfer function of the ANR circuit must all be designed and produced to work together to optimize ANR performance. Poor ANR performance can result in increased noise at medium to high frequencies, unstable (feedback) gain and low frequency noise, and related acoustic disturbances.

The presence of these defects is known as ANR system “instability.” Normal activities such as chewing, talking, rapid head motion can change the acoustic environment seen by the ANR system by opening or modifying the ear cup’s seal to the head. To ensure stability, Sennheiser retains control of the design and manufacture of the critical closed loop elements as no other ANR manufacturer does. This provides the user with the state-of-the-art in active attenuation and consistency (stability) of performance in all normal conditions of use.

Sennheiser Product Development

Sennheiser Government Systems (SGS) is developing the next generation of ANR for military applications. Lighter, compact, more power efficient, more EMI and environmentally resistant, and interfaces to advanced audio technologies are features required in future military applications. With partners around the world, SGS is bringing improved active and passive attenuation in a compact, lightweight form, and demonstrating advanced microphone noise canceling capabilities.

Today, Sennheiser offers the widest range of ANR headsets for commercial and general aviation applications. For high noise helicopters and turbo-prop aircraft, to the most widely used headsets for commercial airlines, Sennheiser technology provides the best in comfort and performance available.

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