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 noise. Basically, each module of ISS have equipments such as fans, pumps, compressors, avionics, and other hardware or systems that serve ISS functionality and astronauts’ life support needs. These equipments present a significant acoustics challenge to 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 dependant 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 psychophysically 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.
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
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:
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
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 6dB, 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.
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