Page 11 - Jul2009

Research spaces—general characteristics
The four indoor research spaces have several features in common, which include low noise floors, acoustically dead- ened listening spaces, and flexible lighting systems. Calibration of the sound system in each room is accom- plished centrally via the Control Room. Three identically instrumented listener stations are available for use in any of the research spaces. Two of these stations can be moved to any location within the research spaces. All spaces use the Peavey Media Matrix signal processing equipment and com- mon loudspeaker models.
The EAR facility was constructed to block out as much external noise as possible. This is particularly important at Aberdeen Proving Ground where it is common to have ground vibrations from armored vehicles rumbling past the building, explosions from test range ordnance, and high intensity engine and propeller noise from low flying aircraft. The walls, doors, and ceilings were treated with Sound Transmission Class 64 (STC 64) rated construction material. The sub-floors are slab-on-grade concrete. Floors in the Sphere Room, Dome Room, and Distance Hall are floating floors constructed with fiberglass panels covered with ply- wood and finished with commercial carpet. To further mini- mize transferred acoustic energy, the floating floors in these three rooms do not contact the walls. As a result of this acoustic treatment to the spaces, most of the outside noises are inaudible inside the four research spaces.
Reflections from intentionally generated sounds inside the EAR facility are controlled with Black SonexTM Foam S.T.O.P.TM melamine foam wedges. The wedges cover all inte- rior surfaces and vary from four inches thick in the Distance Hall to ten inches thick in the Sphere and Dome Rooms. The walls in the Listening Lab are covered with fiberglass panels. The reverberation times for the Sphere Room, Dome Room and Listening Lab are less than 200 msec for frequencies of 250 Hz and higher.1 The reverberation times in the Distance Hall are approximately 400 msec for 250 Hz and below and 200 msec for 500 Hz and above. The
higher reverberation times in the
Distance Hall are due to the geome-
try of the room and the use of thin-
ner foam wedges. These carefully
chosen acoustic materials provide a
relatively quiet, flexible and nearly
anechoic space for conducting audi-
tory research.
Additionally, a hand-held laser pointer and response device adapted from the stock of a Glock handgun can be used by a participant to indicate the perceived location of a sound source. A head tracker can be used to record the participant’s head position and orientation. All reaction times and partic- ipant responses are transmitted wirelessly to the work station in the Control Room.
Calibration and validation of sound sources is automat- ed within each of the indoor research spaces. A centrally located microphone records a specific sound emitted from each of the loudspeakers used in a particular experiment and records its spectrum. The spectra from all loudspeakers are then compared and the average response across all loud- speakers is determined. Once a degree of acceptable toler- ance has been established by the user, any loudspeaker out- side of the established tolerance will have a correction applied to it to bring it in line with the average. Validations are accomplished through measurements made from two microphones mounted within the space. Spectra obtained from these microphones will be compared to the spectra obtained during the calibration. The software program will alert the experimenter if anything is outside of the tolerable range. The calibration procedure is intended to be carried out at the beginning of an experimental set up, and the validation procedure is intended to be carried out as a daily check to ensure no changes have occurred across the duration of a research study.
Control Room
The Control Room (Fig. 2) permits complete observa- tion and operation of instrumentation and research activities in each of the five research spaces. The Control Room con- tains the front-end of all of the instrumentation and stimulus generation systems. All switching devices and power supplies needed to operate the loudspeakers within the research spaces are mounted in racks installed in the Control Room. Experimental design and execution is controlled by software
The Sphere Room, Dome Room
and Distance Hall are each equipped
with an instrumented listener sta-
tion. The listener station is based on
a freely rotating and adjustable chair.
The chair is instrumented with
pushbuttons for recording a partici-
pant’s response, a laser pointer to
provide angular pointing feedback
to the participant, and an optical
shaft encoder to record the chair’s
orientation relative to the room. stimulus presentations in the research spaces.
10 Acoustics Today, July 2009
Fig. 2. A researcher configures an experiment in the Control Room. Rack mounted equipment is used to control the