Space Noise by Glen Grenier

$10,000 per pound is a pretty hefty price to pay anywhere for anything, however this is the estimated cost of a pound of material lifted to the International space station. This is but one of the challenges that David A. Nelson of Nelson Acoustics described to the audience at the Tuesday keynote luncheon at the SAE 2005 Noise and Vibration Conference and Exhibition in Traverse City.

The challenges Mr. Nelson faces working with NASA in noise control on the International Space Station (ISS) are varied. Starting with educating all parties involved in the importance and the realities of noise control, Mr. Nelson has produced a set of CDs to help illustrate the points he makes. The noise concern in the ISS relates to speech intelligibility, which is key to safety and efficiency of the astronauts as well as with respect to long term exposure and its effect on hearing loss.

So educating NASA, the astronauts, and to some extent the media, is a large challenge but not the only one. The technical challenges place some constraints on the material choices that eliminate many of the traditional acoustic materials. The materials must be obviously lightweight at the $10,000 per pound upload cost, but in addition, the materials must not shed any particulate nor may they off-gas any material that cannot be scrubbed by the atmosphere maintenance systems.

Noise in space is something that doesn't immediately ring clear as space is a vacuum and thus there is no medium. However, in the interest of astronaut preservation, spacecraft must provide an atmosphere and therefore the medium of acoustic transmission. So noise in spacecraft is as real as it is here on the surface.

One anecdotal point that Mr. Nelson notes is that humans often rely upon unconscious lip reading to augment speech intelligibility in higher noise environments. However in space, the communicators may be floating at odd angles relative to each other as we have all seen in movies. Nelson notes that at eye/mouth rotation angles greater than 90 degrees, the ability of this unconscious lip reading to take place is reduce to nil. This further increases the need for reduced background noise on spacecraft.

Mr. Nelson indicates that most of the noise on the Station is fan noise related to the cooling of the payload and support systems. The noise level is typically about NC-52. Mr. Nelson indicates NC-50 to be about the limit for reasonable speech intelligibility at 1 meter. The distances between communicators can be up to 5 meters. As well, this noise is continuous. No going home from the noisy job to get a break and recovery. When this noise level is budgeted out to the individual components of system and payload, it can mean that individual experimental modules must meet NC-32 at 600 mm and better.

Nelson has advised on the development of the NASA Glenn Research Center where through use of this facility, NASA and payload development companies can test and develop their designs to facilitate a quieter environment for the astronauts on the International Space Station.

In closing, Mr Nelson advises that the future of noise control in space is multifaceted because of the tremendous constraints on weight, space, power and noise budgets. Achievement of the goals will only be accomplished with improved thermal design, and an integrated approach to source control such that when the components (i.e. fans, ducts, etc.) are combined, they work as a tuned system to facilitate minimum acoustic emission.