In order to verify that the ISS and Shuttle flight
hardware meets acoustic emission requirements, acoustic
testing is required. In addition, acoustic emissions
testing is a major part of the hardware design and
fabrication process. Prototype and component hardware
acoustic data are used to help understand the acoustic
contributions of the hardware's components and configurations
so that the acoustic requirements of the end hardware
item can be achieved.
Acoustic testing can be performed in almost any
environment; however, the environment can easily influence
the measurement data, even to the point where the
data become meaningless. The two most important factors
pertaining to the measurement environment are background
noise and acoustic reflections, both of which are
functions of the acoustic frequency.
For optimum measurement accuracy, the background
noise should be at least 10 dB below the noise level
of the noise source of interest across the entire
frequency range of interest. In addition, the walls
of the room in which the measurements are being made
should absorb more than 99% of the acoustic energy
of the acoustic waves that impinge upon them; this
is so the waves will not reflect back to the measurement
microphones and affect the measured noise. These two
guidelines are optimal and are seldom ever met outside
of an anechoic chamber. Hence, in order to make meaningful
acoustic measurements, the effects of background noise
and reflections should be sufficiently considered.
There are also other factors to consider with acoustic
emissions testing. Some of these include: parameter(s)
to measure, test conditions, microphone positioning,
and microphone and instrumentation type. Many of these
items are specified in the acoustic requirements to
be met. For a more in-depth description of acoustic
emissions testing and recommendations for testing
processes and procedures, please follow the link to
Acoustic Emissions Testing. Also, see Acoustic Definitions.
With all there is to consider when performing acoustics
emissions testing, it may be cost-effective to go
to an acoustics testing facility in order to have
the testing performed. At NASA there are several acoustic
test facilities across the country. Some of them are
presented here so that the most convenient facility
can be chosen:
