Strategies of the auditory system

Sorry for the long delay, but I was really distracted by other activities.

In Nara, the ancient capital of Japan,  I had the honour to present a lecture at ICBEN2014. The facts presented indicate that the human auditory system has elaborate strategies to avoid noise-induced injury. Unfortunately these mechanisms are mostly not effective for impulses, particularly if they are extremely short and their pressure-time-history has the structure of a dagger. The pressure-time-history of acoustic impulses stimulate resonances of middle-ear structures, which in turn can permanently damage the organ of Corti in typical ways. – Masking effects are important, as well as training by the long-term acoustic environment. Middle-ear muscles have no protective functions, but they serve as an auditory equivalent of the visual task of accommodation.

It can be summarized that auditory training is very important for good sensitivity of hearing and that particularly very short impulses can be most harmful for hearing, and a cause for tinnitus.

Currently the ISO1999 is the basis for the protection of hearing, practically everywhere. It is based on the assumption that the acoustic energy received by the ear is the key parameter. However, the facts do not support this assumption. Orchestra musicians hear much better than nomadic people, and an extremely short impulse can permanently ruin the ear, even if it transmits only a small amount of energy. For effective auditory protection the basics of this standard should be replaced by principles derived from studies on human experience.

Here you can download the slides:

Fleischer-JP-Nara-ICBEN2014-short-(19Fig)

The Intelligent Ear

In 2002 an article was published on auditory performance of persons living in remote parts of the PR China. The study was performed in cooperation with the ENT Department of FMM university in Xian. Most of the persons examined were not exposed to technical noise, but a few of them had to endure extreme levels of noise, over a long time. Contrary to common assumptions, it could not be confirmed that persons not exposed to technical noise are hearing better than persons living in loud cities. Furthermore, it was apparent that acute acoustic trauma was wide spread, caused by fire crackers and other devices resulting in powerful impulses. The equal-energy-concept that is the basis for auditory protection does not explain these findings.

You can download the full article here: Fleischer-2002-TheIntelligentEar.pdf (2.2 MB)

Components for Protecting and Optimizing Good Hearing

In November 2010 I had the pleasure to give a lecture at the International Convention of Sound Designers on the marvelous capabilities of the human ear. The meeting took place at the Congress Center in Leipzig, Germany. One of the reasons for this topic is the fact that the EU-standards for protecting the ear at the workplace create much trouble for sound designers. The acoustic conditions at their work places are declared harmful, but sound designers hear much better than the normal population. This discrepancy is caused by wrong assumptions of the standard.

Here is the abstract.

Studies on the auditory performance of entire groups of persons reveal that people ex-
posed to low sound levels do not generally hear better than those living and working in
loud acoustic conditions. Hence, the basis for world-wide standards to protect the ear
are incorrect and insufficient. Reason for this are powerful neural mechanisms of the
auditory system that is working hard to prevent noise-induced auditory damage and to
optimize various aspects of hearing. Studying special conditions at the workplace, as
well as at places related to hobbies and other activities, indicates the existence of several
such functional components. Overall sensitivity of hearing can apparently be reduced in
loud environments. Anticipating exposure to short loud events the auditory system can
rapidly reduce the sensitivity. Masking effects of low frequencies can be scaled down in
order to focus on high frequencies. Strong low-frequency components protect against si-
multaneous high-frequency noise. Very short impulses are more harmful than longer ones.
Most dangerous are rarely occurring unexpected powerful impulses. Auditory threshold
and threshold of pain are both subject to training. Damage caused by impulses result in
characteristic forms of audiograms. — The auditory neural system can be damaged or
reprogrammed by a single powerful impulse.

The article is published under the CC BY-NC-SA 3.0 license.

You can download the original file, which is about 49 MB, or a compressed version of about 12 MB, lacking clickable links.