Air Force auditory research chamber upgraded with Eckel anechoic wedges
Ayer, MA. Wright-Patterson Air Force Base (Dayton, OH) is the home of the USAF’s Battlespace Acoustics Branch of the Air Force Research Laboratory (AFRL). Here, a broad spectrum of acoustic research takes place with the goal of providing operators in multiple airborne, ground-based, and cyber domains with technologies for hearing protection, for communication (particularly in high-noise levels), and for precisely pinpointing the location of sound sources in three dimensions.
In order to develop these advanced technologies, researchers at AFRL have been conducting basic and applied research aimed at understanding how humans determine the location of sounds and understand speech in complex acoustic environments. Much of this work takes place in AFRL’s Auditory Localization Facility (ALF). The ALF consists of a 22-channel sound system that is fully matrixed to an array of 277 loudspeakers mounted on the surface of a 14-ft. diameter geodesic sphere, all housed within a full anechoic chamber. This advanced sound system is designed to simulate complex, real-world acoustic environments, such as those that may be encountered by military personnel, where multiple sounds may be present concurrently, making it difficult for a listener to focus on a single, critical signal (for example, a targeting signal or threat warning). In this way, the system can essentially emulate what a fighter pilot or other operator might experience in combat. The development of virtual auditory displays is also accomplished in the ALF, where acoustic measurements are made on an individual listener to determine the direction-dependent acoustic cues used for sound localization for later integration in these technologies.
Anechoic chambers, like the one at AFRL’s ALF, are unique acoustic environments. They are designed to absorb nearly all the sound produced within a space above a specified cut-off frequency. There are two primary types of anechoic chambers: hemi-anechoic chambers—which feature wedges on the walls and ceiling and a solid floor; and full anechoic chambers—which also include wedges on the floor and a grid or cable-mesh suspended floor where the testing takes place. The wedge composition, geometry and dimensions all serve to determine the frequency characteristics of a chamber. Anechoic chambers are used in a wide variety of industries to precisely measure the sound level of products and systems and in a multitude of research settings. In the case of the ALF, housing the loudspeaker array in a full anechoic chamber allows researchers to very accurately control the sounds arriving at the subject’s ears.
The original ALF anechoic chamber was constructed in 1955, and is a cube-shaped room, 30 ft. in width, length, and height. It was constructed to be completely isolated from its surroundings, basically a room-within-a-room. The sound-absorbing wedges of the chamber had degraded over the years, both in appearance and effectiveness, and consequently, so did the acoustic absorption characteristics of the room. The existing 16-channel audio generation system and loudspeaker array dated from the 1980s and were also due for an upgrade. The old HVAC system was leaking, and mold had formed in the ductwork. The decision was made to refurbish the room.
Ball Aerospace (Fairborn, OH), the prime contractor for the project, contacted Eckel Noise Control Technologies to upgrade the chamber. Eckel selected Viking Enterprises (Waterford, CT) to prepare the chamber and perform the installation of the wedges and the new cable mesh floor. To start, Viking carefully removed all the existing wedges and the old HVAC ductwork. The room was then completely cleaned and sanitized down to the bare concrete walls. Wright-Patterson replaced the original 16-channel system with a 22-channel, state-of-the-art sound-generation system, and the loudspeakers in the array were replaced. The supports for the sphere were also upgraded, providing additional stability and increasing the space available to maneuver in the chamber. At this point a scaffold was carefully built around the sphere to facilitate the installation of the new wedges. New ductwork and a high-volume exhaust fan were also installed to increase airflow while maintaining the extremely low noise floor required for research. The cable floor was also replaced.
“The project was a challenge from the start, but the Wright-Patterson staff was incredibly helpful and patient in our efforts to get this project done on time,” says Dave Engdall, Viking’s senior superintendent, who oversaw the project.
Other innovations were necessary to get the job done. According to Jeff Morse, vice president at Eckel, “We had to design a wedge door that was a combination cage and interlocking door. We also had to design nine framed openings in the cable floor to accommodate the supports for the sphere. Both of those were firsts for us”
“The new chamber has an inside wedge-tip-to-wedge-tip dimension of 23 ft. and was designed to have a cut-off frequency of 80 Hz, but when we ran the qualification tests, we actually achieved a cut-off frequency of 63 Hz,” said Morse. The cut-off frequency is that frequency above which 99.9% of the sound energy hitting the wedges is absorbed.
According to Dr. Brian Simpson, technical advisor, Battlespace Acoustics, Air Force Research Laboratory, “The Auditory Localization Facility (ALF) is a one-of-a-kind, world-class laboratory and one of AFRL’s flagship facilities. Due to its unique design and requirements, upgrading the facility posed several challenges, necessitating innovative approaches to address design constraints, infrastructure repairs, and the scientific mission of the Battlespace Acoustics Branch. The team of Eckel Industries and Viking Enterprises was able to meet these requirements, re-establish the acoustic integrity of the environment, and enhance our overall research capabilities. They demonstrated a deep knowledge and understanding of our needs, a high level of professionalism, and a true desire to provide us with the best facility possible. We have been pleased thus far with our newly upgraded facility and look forward to testing and demonstrating its capabilities as we resume our research activities in this unique space.”