Programme for the NATO-RTO Lecture Series 244, “Personal hearing protection including Active Noise Reduction”
Introduction
Dr. Herman J.M. Steeneken, TNO-Human Factors, the Netherlands
Personal hearing protection and speech communication facilities are essential for optimal performance in military operations. High noise levels increase the risk of noise induced hearing loss and deterioration of communications. For many years passive hearing protection (earmuffs and earplugs) was used to reduce the noise dose exposure to personnel. Nowadays electronic systems, based on active noise reduction, have been used to improve the performance of personal hearing protection and speech communications.
In this lecture series, criteria for adequate hearing protection, the performance
of passive and active systems, the assessment and the applications will be discussed.
The lecture series consists of five lectures and a concluding panel discussion.
The program consists of:
1 Introduction (Dr. H.J.M. Steeneken)
2 Hearing and hearing protection (Dr. A. Dancer)
3 Passive hearing protectors and their performance (Mr. R. McKinley)
4 Active hearing protection systems and their performance (Dr. K. Buck)
5 Assessment and standardization (Dr. H.J.M. Steeneken)
6 Applications: overview of military noises, insertion loss, prediction of performance
(Miss. S. James, Mr. R. McKinley)
7 Final panel discussion (all lecturers),
The lecture series will be held in three countries: Poland (Warsaw at CIOP, 25-26 October 2004), Belgium (Brussels at the Royal Military Academy, 28-29 October 2004), and the Unites States (Portsmouth, Navy Environmental Health Centre, 9-10 November 2004).
Hearing and hearing protection
Dr. Armand Dancer, Institute de St. Louis, France
This presentation addresses the origins of Noise-Induced Hearing Losses (NIHL)
and their relation to the morphology and the physiology of the ear. Understanding
the mechanical and acoustical properties of the auditory periphery: i.e. the
transfer functions of the external and middle ears, and the input impedance
of the inner ear, allows the fundamentals of the A-weighted iso-energy exposure
criterion (ISO 1999) and its limitations (critical level, non-linearity’s)
to be explained. Exposure to intense noise induces two major types of damage
to the inner ear: (i) injuries to the hair cells (first to the stereo-cilia
of the outer and inner hair cells), and (ii) destruction of the connections
of the primary auditory neurons with the inner hair cells (glutamate excitotoxicity).
These types of damage have mechanical and metabolic origins (for impulse noise
of large peak pressure the mechanical damage is the first reason for NIHL).
They induce immediate and/or progressive destruction of the sensory cells of
the organ of Corti (necrosis, apoptosis). An analysis is presented of the influence
of the parameters of the noise (level, duration, frequency) on the Noise-Induced
Hearing Losses, and the main consequences of the cellular damage on hearing
(half-octave shift, loss of sensitivity: temporary and/or permanent threshold
shifts, loss of frequency selectivity, tinnitus, impairment of speech intelligibility).
A discussion is then presented on the importance (and the limitations) of the
mechanisms of protection: (1) by the middle ear acoustic reflex and the “intra-cochlear
acoustic reflex” (efferent feedback system), and (2) by the “training”
of the ear with the help of an acoustic stimulus of moderate intensity
.
Last but not least, some recent results are presented concerning the possibilities
to protect the ear against noise and to treat the acoustic trauma. New medical
treatments applied directly to the inner ear and taking advantage of the latest
advances in molecular biology (anti-oxidants, neurotransmitters: agonists or
antagonists, growth factors...) could prevent the destruction of the hair cells
by apoptosis, speed up and improve the recovery, and reduce the annoyance of
related side-effects like tinnitus.
Passive hearing protection systems and their performance
(Mr. Richard McKinley, HCEB Air force Research Labs, USA)
Whilst ideally noise should be reduced at source, in the military environment the most effective solution in terms of both cost and operational effectiveness, is to provide personnel with personal hearing protection. This protection may either be in the form of an earplug that occludes the ear canal or a circumaural protector that inserts a barrier between the ambient noise and the ear. For both devices the level of passive protection provided changes with frequency. A great deal of research was conducted in the 1940/50s to define the mechanisms and parameters that limit the performance of these types of protectors and this presentation will provide an overview of the findings of this early research.
By the 1970s the performance of such devices, particularly those used in military applications, had been best optimised for use with the types of cranial protection being worn by soldiers and aircrew. Since that time the major thrust in hearing protection enhancement has been the development and integration of Active Noise Reduction systems where an electronic circuit is incorporated into the device to provide additional active attenuation on top of the passive. ANR has provided significant benefits in low frequency attenuation and provides complementary performance to the passive device. However, for future military noise environments ANR headsets and ANR earplugs will not individually provide sufficient levels of protection and passive and active devices may have to be used in some combination.
This presentation will discuss some recent research showing the benefits deeply inserted earplugs can provide to reducing hearing damage risk and the possibilities new construction techniques, the use of double protection (earplug in combination with an ear muff), and ear seal design offer to enhancing the performance of circumaural styled protectors. Issues associated with the fitting of personal hearing protection and their performance in the field will also be discussed.
Active hearing protection systems and their performance
Dr. Karl Buck and Dr. Véronique Zimpfer-Jost, Institute de St. Louis,
France
A brief history of active noise cancellation shows, that the possibility of using ANR in hearing protection devices was proposed long before the first commercial devices became known. The basic theory of active noise cancellation is quite simple and was first described in the 1930’s. The main difficulty for an early implementation was the bulky electronics and the lack of reliable and adapted miniaturized transducers. With the arrival of new technologies the first functional ANR hearing protectors were proposed to the military community but for a long time were considered a “luxury” device and not really useful for the soldier. However, since, the usefulness of such devices has been proven, crews of armoured vehicles and aircraft regularly use this type of hearing protector.
Different approaches to obtain active noise cancellation are shown in the presentation. The techniques of feedback and feed-forward systems are discussed as well as different possibilities for their implementation. At most places were ANR is to be used an important need of communication is present. It is shown how the communication system may be implemented into ANR hearing protectors and in which way it may be optimised.
The present limitations of the systems are mainly due to the physical parameters of the passive protector and to the limitation of the transducers used. The loudspeaker used to create the “anti-noise” and the way it is built into the earmuff have a significant influence on the performance. Different parameters describing the performance (contribution of ANR to the insertion loss, stability and overload) are described in order to determine some basic design rules.
New military material, exposing its crews to higher noise levels than ever before, creates a need for the development of new types of active hearing devices. These devices need to provide higher levels of attenuation than can be obtained with earmuffs. Therefore these new devices need to incorporate earplugs. The problems that arise during such an implementation will be described.
Assessment and standardization
Dr. Herman J.M. Steeneken, TNO-Human Factors, the Netherlands
The performance of passive hearing protection is normally determined by subjective tests in which the threshold of hearing for a number of subjects is obtained with and without a hearing protector. The difference between the two threshold levels quantifies the insertion loss of the hearing protector. The insertion loss is determined at a number of frequencies.
Active noise reduction systems require a different assessment method. Due to
self-noise and noise-level dependency, methods operating at threshold level
cannot be used. The effect of high noise levels and impulsive noise may introduce
a non-linear behaviour of active systems. Therefore the use of artificial heads
(to avoid the risk of introducing hearing loss of subjects) is applied. Comparison
of results from subjective and objective test methods will be discussed.
A prediction of the noise dose representative for a certain condition can be
obtained by consideration of the environmental noise spectrum, the insertion
loss of the hearing protector and an estimation of the variance of the insertion
loss among individual users. Examples of such a prediction (by using a spreadsheet)
will be given.
Speech communication quality is an important issue for a user in operational conditions. The noise level at the ear is one of the major variables that define the speech communication quality. Subjective and objective assessment methods for speech communication systems will be presented and discussed. Prediction of the speech intelligibility of a communication system (in a similar condition as presented for the noise dose) will be demonstrated for an objective intelligibility measure.
Some performance measures for hearing protection, speech communication and criteria for speech quality are standardised by international bodies. Some relevant international standards, provided by ISO, CEN and IEC, will be discussed.
Applications overview of military noises, insertion loss, prediction
of performance
Miss. Soo James QinetiQ, United Kingdom
With a few exceptions, the cockpit noise levels in military aircraft, particularly in fast jets, are steadily increasing. This is true in both US and UK aircraft, and these increasing noise levels require greater and more ingenious methods for mitigating noise reaching the ear if hearing damage risk is to be contained and speech and non-speech communications are to remain intelligible during front line operations. In the current military environment it is essential that solutions are cost effective and hearing protection within a standard flight helmet provides an optimal solution for the majority of noise induced problems.
This presentation outlines the major problem areas in the military cockpit (including cockpit noise issues and the contribution the electrical communications signal makes to aircrew noise dose) and looks at the effectiveness of newer personal protection technologies in providing pragmatic solutions to the noise deluge. Results of both experimental trials and in-service operational trials of Active Noise Reduction are discussed and the significant levels of reduction in hearing damage risk shown. In-service operational results of such noise reduction techniques that have been introduced into service with the Royal Navy are also presented.
If the prediction of noise levels in the next generation fast jets are confirmed, then even more effective solutions will be needed and some results of UK longer term research into multiple layers of hearing protection are discussed as well as the potential limitations in conventional hearing protection devices.
Final panel discussion
All Lecturers
The lecture series will be concluded with a panel discussion. The audience
will be encouraged to raise issues and to contribute to these discussions.
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