| S & V OBSERVER
New Acoustical Testing Laboratory at NASA
Beth A Cooper, NASA
John H Glenn Research Centre, Cleveland, Ohio
Alan Eckel, Eckel Industries, Inc., Cambridge Massachusetts
Trent Butcher, National Instruments, Austin, Texas
David Nelson, Nelson Acoustical Engineering, Inc., Elgin, Texas
The NASA John H Glenn Research Centre at Lewis Field has designed
and constructed an Acoustical Testing Laboratory (ATL) to support
low-noise design of microgravity space flight hardware. This new
laboratory provides acoustic emissions testing and noise control
services for a variety of customers, particularly for microgravity
space flight hardware that must meet International Space Station
(ISS) limits on noise emissions. 1 The ATL provides a comprehensive
array of acoustical testing services, including sound power level
testing per ANSI S12.34 and ANSI S12.35. 2,3 A multichannel PC-based
acoustical data acquisition system allows simultaneous acquisition
and real-time analysis of signals to facilitate the identification
of equipment noise sources and transmission paths. Although the
ATL functions as a design verification facility by producing data
to document requirements compliance, it was constructed primarily
to provide an in-house laboratory environment where noise control
design strategies are actively pursued and integrated into the overall
design of flight hardware, early in the life of each project.
Acoustical Testing Laboratory Design Goals. Science experiment
payloads that will reside on the International Space Station are
subject to acoustic noise emission requirements, which have been
imposed by ISS to support hearing conservation, speech communication
and safety goals as well as to prevent noise-induced vibrations
that could adversely impact microgravity research data. These requirements
include meeting an NC-like maximum sound pressure level criterion
spectrum for acoustic emissions (the exact spectrum depends on the
nature of the payload and its intended operational characteristics)
as well as conducting sound power level testing in accordance with
any of several published standards. Although ISS policies require
the above testing for final verification and compliance purposes,
the primary motivation for the design and construction of the Acoustical
Testing Laboratory at Glenn Research Centre was the need for an
accessible, secure and flight-hardware-compatible acoustical testing
environment that would virtually ensure compliance by facilitating
the successful implementation of low-noise payload design strategies.
The ATL was designed as a reconfigurable hemi/anechoic chamber in
order to be able to meet either ANSI S12.34 or ANSI S12.35 (or both)
for typical experiment payloads up to a full rack in size (approximately
3ft by 3ft by 6ft high). Vibration isolation and very low background
and self-noise levels were dictated by the need to acquire accurate
and repeatable acoustic emission measurements on the least-noise-producing
component of a payload, typically a small fan.
Most larger payloads, especially full racks that serve as permanent
on-orbit test beds for a succession of payloads by providing common
utilities and service connections, usually require the operation
of test support equipment in order to generate noise under ground-based
laboratory conditions. It is necessary to prevent noise generated
by this support equipment, which can include high noise items such
as chillers, pumps and power supplies, from contaminating acoustical
measurements of the experiment itself. As a result, the support
equipment must be located in a remote, noise-attenuating enclosure
with the capability of running service hoses and cables between
the test support enclosure and test chamber to permit operation
and control of the test article without any attendant acoustic leaks
between the two rooms.
Facility Description.
The Acoustical Testing Laboratory consists of a test chamber with
27ft by 23ft by 20ft high outside dimensions (21ft by 17ft by 17ft
high interior working dimensions) and separate sound-attenuating
test support enclosure with outside dimensions of 23ft by 11ft by
12ft high. The ATL is located, along with two other engineering
verification facilities, inside a pre-engineered host building with
dimensions of 50ft by 150ft by 35ft high. The host building provides
the ATL with a conditioned exterior environment, overhead crane
support and utility services such as chilled water and shop (compressed)
air (see Figures 1 and 2).
Test Chamber Design
. NASA established an in-house design team to develop the technical
requirements for a laboratory design that would meet the specialized
demands of microgravity space flight hardware. In particular, the
laboratory was configured to be able to accommodate the Fluids and
Combustion Facility, a three-rack microgravity research facility
being developed for the International Space Station by the NASA
Glenn Research Centre's Microgravity Sciences Division under contract
to Federal Data Corporation. The NASA team worked closely with Jeff
Morse, Vice President of Engineering at Eckel Industries, Inc.,
who co-ordinated the efforts of Eckel's own design team.
Constraints on the available space in the host building dictated
a smaller chamber than would have allowed full anechoic testing
of the largest test articles per ANSI S12.35. Therefore, the capability
of configuring the chamber as either anechoic or hemi-anechoic was
incorporated into the design to ensure that even the largest test
articles would fall within the applicability of either ANSI S12.34
or ANSI S12.35. This is accomplished with removable floor wedges
mounted on rolling carts. The chamber design was tailored to support
reconfigurability by employing horizontally-split doors, shown in
Figure 3, and a set of personnel access steps built into one of
the floor wedge carts. Removable modular expanded-metal grating
sections attached to each floor wedge cart provide a walking surface
above the wedge tips.
The main test room consists of a 23ft wide by 27ft long by 20ft
high convertible hemi-anechoic chamber with a separate control room
test support enclosure. Absorptive 34 in. Eckel metallic wedges
(EMWs) in the test chamber provide an anechoic environment down
to 100Hz. The EMW anechoic wedge design incorporates high transparency
perforated 22 gauge perforated metal facing with a 53% open area.
The wedges have a fibreglass core and acoustically transparent fabric
layer between the perforated facing and fibreglass core for positive
fibre containment. The chamber's EMW lining is modular with two
34in. peaks per wedge unit with a 2ft by 2ft base.
An above-grade spring-isolated floor system affords vibration
isolation above 3Hz for test articles with a maximum weight of 5000
lbs (see Figures 4 and 5). Snubbers below the 6in. concrete slab
allow an 8000 lb forklift truck to safely manoeuvre test articles
into position.
Low background noise levels required for accurate acoustic measurements
are afforded by prefabricated 4in. thick wall panels with an internal
septum, which provide noise reduction, rated at STC 54, between
the host space and the test chamber. A silenced two-speed stand
alone ventilation system with 50% efficiency filtering provides
conditioned air from the host space.
To permit equipment access, the test chamber has one set of 9ft
by 10ft high double doors, shown in Figure 6, as well as a removable
8ft by 8ft panel in the ceiling that may be lifted by crane. A personnel
door functions as an emergency egress exit. When the chamber is
configured in the full anechoic mode, a set of portable steps is
positioned on the exterior of the chamber during laboratory operations
to permit entrance/egress between the walking surface above the
floor wedges (approximately 46in. high) and grade level in the host
building.
A "very early" smoke detection system continuously samples air
in the test chamber and the test support enclosure and notifies
a central lab-wide monitoring/dispatch station of any threshold
exceedances.
Test Support Enclosure Design.
The test chamber and adjacent test support enclosure are physically
and acoustically separate structures located on either side of a
4in. air space filled with fibreglass insulation. For most test
programmes, the test support enclosure functions as a control room
and houses acoustic data acquisition and customer test control equipment.
The test support enclosure is also a pre-fabricated room with 4in.
thick wall and ceiling panels rated at STC 49. Silenced (rated at
NC25) and filtered (30% efficiency) two-speed ventilation and absorptive
interior wall/ceiling panel surfaces provide the office-like environment
appropriate for most operations. The ventilation system is separate
from both the host building system and the stand-alone system that
serves the test chamber.
For test programmes where the test article requires remotely-located
test support equipment whose noise emissions could contaminate the
acoustic signals being measured in the test chamber, the test support
enclosure shown in Figure 7 functions as a noise control enclosure.
In this case, the acoustical data acquisition and customer test
control equipment and functions are relocated to the hallway outside
the test support enclosure by means of rolling equipment carts and
movable modular furnishings. Electrical receptacles on the exterior
walls of the chamber and test support enclosure facilitate easy
reconfiguration of the test control and data acquisition setup.
The floor of the test support enclosure is located at grade, enabling
equipment to be moved in and out easily through a set of 8ft by
10ft high double doors.
Test support equipment located in the enclosure is powered by 120VAC
or 208 V 3-phase electrical service supplied to the control room
and, in turn, provides services to the test article in the chamber
via temporary customer-supplied hoses, cables and tubing that pass
through silenced utility sleeves in the walls of the two rooms as
shown in Figure 8. Utility sleeves in the other walls allow phone,
Ethernet, shop air and water services to be temporarily supplied
to the test support enclosure from the host building and also allow
these services as well as data control functions to be routed, when
required, to the alternate test control location in the adjacent
hallway. Video and intercom systems facilitate remote test operation
as well as communication between the chamber and other locations
during setup and testing.
Personnel traffic between the test chamber and test support enclosure
must pass through a series of two aligned doors, one in each of
the rooms. A set of steps recessed into the floor wedge cart located
immediately inside the chamber provides a means of navigating the
change in elevation between the floor of the test support enclosure
and the test chamber walking surface above the floor wedges.
Instrumentation System. The National Instruments Sound
Power System (SPS) automates the process of noise emission testing
in free-field test environments. It incorporates PC-based instruments
with LabVIEW TM based software to provide sound power and sound
pressure measurements. Because it is PC-based, the system has a
higher performance and less opportunity for error than comparable
systems. The system implements acoustic measurement procedures defined
in ANSI S12.34 - 1988, ANSI S12.35 - 1990, ISO 7779 and EMCA-74
as well as a number of user-configurable options for other basic
sound power and sound pressure level measurement standards, such
as ISO3744 and ISO3745. |
Figure 1. Access door frame being installed. The test chamber complex
was assembled from pre-fabricated modular components.
Figure 2. Finished chamber complex within high-bay test facility
with crane and floor access services.
Figure 3. Test specimen access opening showing split door sections
and wedge/floor modules on movable platforms.
Figure 4. Test chamber seismic base with rebar and
vibration isolator forms in place ready for concrete pour.
Figure 5. Vibration isolator elements and levelling devices being
installed.
Figure 6. Inside view of test chamber configured for anechoic measurements
with test specimen access door wedge sections open.
Figure 7. Test support enclosure showing movable equipment and storage
modules.
Figure 8. Test specimen service utilities are provided
to the main test chamber interior by means of access sleeves with
silencer plugs.
|
