# Engineering Acoustics/International Space Station Acoustics Challenges

The International Space Station
(ISS) is a
research facility laboratory made up of several different modules in a
Low Earth Orbit. This facility represents a union of several different
space station projects from various different nations. The long-term
goals of ISS is to develop the technology necessary for human-based
space and planetary exploration and colonization (including life support
systems, safety precautions, environmental monitoring in space), new
ways to treat diseases, more efficient methods of producing materials,
more accurate measurements than would be possible to achieve on Earth,
and a more complete understanding of the Universe.

In order to achieve these objectives, human space flight centers train
highly qualified astronauts in order to best perform different tasks on
board the ISS. Astronauts spend months at a training facility and in a
variety of conditions which simulate the environment of space before
going to ISS. A part of astronauts are cross trained to perform a number
of tasks. A pilot, for example, might also be trained to carry out
scientific experiments, or to work on equipment repairs. Astronauts
schedule is usually very dense on ground, and this becomes even more
critical on board the ISS. Therefore, care needs to be taken in order to
provide astronauts a safe and habitable working environment once they
are on orbit.

One of the major problems on board the ISS is the presence of white
noise. Basically, each module of ISS have equipment such as fans, pumps,
compressors, avionics, and other hardware or systems that serve ISS
functionality and astronauts' life support needs. These equipment
present a significant acoustics challenge to the ISS because of
difficulties with controlling the noise produced by various elements
provided by international partners. The excessive noise levels from
machinery or equipment on ISS has shown to affect crews' hearing,
habitability, safety, productivity, annoyance, and sleep interference.
Crew performance concerns include inability to effectively communicate
and understand what is being said or what is going on around them (e.g.
intelligibility,
speech interference, inability to hear alarms or other important
auditory cues such as an equipment item malfunctioning, inability to
concentrate, strain in vocal cords).

## Normal Hearing Response and Hearing Threshold

The sensitivity of hearing mechanism is highly dependent upon the
frequency content of the received sound. Human ears can detect sound
between frequencies of 20 to 20000 Hz. Intensity of sound is measured in
decibels (dB). The decibel is a logarithmic unit used to describe a
ratio (10 log10 (p2/p1)2). The ratio may be power, sound pressure,
voltage or intensity or several other things. Sound power level (in dB)
of typical sources could be viewed
here.
There are several different units to quantize the perceived sound by
human ears. We will introduce some of them. The phon is a unit that is
related to dB by the psycho-physically measured frequency response of
the ear. At 1 kHz, readings in phons and dB are, by definition, the
same. For all other frequencies, the phon scale is determined by the
results of experiments in which healthy volunteers are asked to adjust
the loudness of a signal at a given frequency until they judge its
loudness is equal to that of a 1 kHz signal
exercise. To convert from
dB to phons, a graph of such
results is needed. Such a graph depends on sound level: it becomes
flatter at high sound levels. Hearing threshold defines the level at
which the ear barely perceives the sound. The threshold is
frequency-dependent and corresponds to the 0-phon curve. Damage to the
hearing mechanism has the effect of raising the hearing threshold, which
indicates that higher sound levels are required in order to be heard.
The degree of shift in the hearing threshold is used as an index of the
amount of hearing impairment incurred; criteria for hearing damage are
usually based on shifts in this threshold.

## dBA

The human ear does not respond equally to all frequencies: we are much
more sensitive to sounds in the frequency range about 1 kHz to 4 kHz
than to very low or high frequency sounds. For this reason, sound meters
are usually fitted with a filter whose response to frequency is a bit
like that of the human ear. If the sound pressure level is given in
units of dB(A) or dBA, the \"A weighting filter\" is used. dBA roughly
corresponds to the inverse of the 40 dB (at 1 kHz) equal-loudness curve.
Sound pressure level on the dBA scale is easy to measure
link and is therefore widely
used. It is still different from loudness, however, because the filter
does not respond in quite the same way as the ear. dBA different
sources

## Noise Exposures

![](Table_1_Permissible_noise_exposures.jpg "Table_1_Permissible_noise_exposures.jpg"){width="50"}

Exposure to excessive and prolonged noise is one major cause of hearing
disorders worldwide. Hearing damage result in an increase in hearing
threshold. Table 1 shows the duration of exposure to higher sound
intensities, set by Occupational Safety and Health Act (OSHA), which
will result in no more damage to hearing than that produced by 8 h at 90
dBA. In addition people must not be exposed to steady sound levels above
115 dBA, regardless of the duration.

When the daily noise exposure is composed of two or more periods of
noise exposure of difference levels, their combined effect should be
considered, rather than the individual effect of each. When people are
exposed to different sound levels during the day, the mixed dose (D)
must be calculated by using the following formula:

$D=C1/T1+C2/T2+...+Cn/Tn$

where Cn is the total exposure time at a given noise level and Tn is the
total exposure time permitted at that level. If the sum of the fractions
equals or exceeds 1, then the mixed exposure is considered to exceed the
limit value.
source

## Intelligibility

The listeners' speech comprehension is diminished by the ambient noise
and the distortion of the system. One could ensure that a message is
clear and intelligible in all situations by measuring the
"intelligibility" of the system. Speech intelligibility is the degree to
which speech can be understood by a listener in a noisy environment. For
satisfactory communication the average speech level should exceed that
of the noise by 6 dB, but lower S/N ratios can be acceptable (Moore,
1997). The most common method to rate the speech interference effect of
noise is called the Preferred Speech Interference Level (PSIL) shown in
the figure bellow.

![](PSIL.jpg "PSIL.jpg")

## Noise detection and protection on board the ISS

In order to determine the total exposure to noise during a given period
astronauts wear an audio dosimeter. The astronauts of the ISS are
exposed to an average noise level of 72dBA for the entire duration of
their stay on the ISS, which can last up to six months. One of the
medical flight rules sets noise exposure limits based on a 24-hour
exposure level. If the 24-hour noise exposure levels measured by the
audio dosimeters exceed 65 dBA, then the crewmembers are directed to
wear approved hearing protection devices. Design specifications of 60
dBA for work areas and 50 dBA for sleep areas have been agreed upon as
"safety limits" for ISS operations. The use of hearing protection
devices (HPDs) is suggested if noise exposure levels exceed 60 dBA, or
if the crewmembers are exposed to high intermittent noise periods (e.g.
use of exercise devices such as the treadmill, airlock repress, or other
short term high noises). The ISS specifications take into account the
impact of noise on crew hearing (both temporary and permanent threshold
shifts), as well as habitability and performance (disrupted
communication, irritability, impaired sleep, etc.). Use of HPDs during
sleep provides additional lowering of the noise input to the inner ear
and aids recovery from acoustic trauma sustained during the day.
Recovery is more robust in quieter environments such as in an adequately
quieted crew sleep station or with the use of hearing protection during
high noise exposure periods. Although hearing protection headsets are
available, astronauts do not use them all the time, as they are
uncomfortable to wear continuously and make communication with other
crewmembers difficult. Since hearing needs to be tested in a quiet
environment, researchers\' efforts to record in-flight changes in
hearing have not been successful because of the continuous noise on the
ISS. Some of the astronauts\' reports could be found in the following
website: SpaceRef

## Further Research

-   Acoustical Testing Laboratory
-   Leuven Measurement Systems
-   Test on
    Humans

## References

