In FY’09, significant progress was made with validation and development of the acoustic modeling technique. This included:
• Modified the acoustic reverberation time of the CM mockup to match the reverberation time inside the ISS US Lab in the speech bands, i.e. 0.5, 1, 2, and 4 kHz, for evaluating CM Snorkel Fan noise on post-landing crew communications. Thinsulate© sound absorption material was attached to the interior surface of the mockup to achieve this purpose. An SEA acoustic model of the CM mockup was used to predict the amount of acoustic treatment required, and reverberation time measurements were made to verify the match to the desired reverberation environment.
• Validated the SEA model in predicting mockup interior SPLs (Sound Pressure Levels) due to the emissions of realistic ventilation fan sources. The sound power levels of these fan sources were unknown a priori. Sound power levels of these fan sources were estimated using a sound intensity measurement technique. This is opposed to prior studies where a RSS (Reference Sound Source) with known calibrated sound power was used.
A measurement grid system of rectangular box shape was built for sound intensity measurements. The grid system enclosed the source to be measured with five surfaces, i.e., front, right, back, left, and top. Sound intensity at the center of each segment of the grid system was measured and time averaged for 15 sec. Sound intensity at the bottom surface reflected most of the incident sound energy so that it was accounted for at the other measured surfaces. Non-stationary background noise of the Chamber was of concern because it could introduce some net error on estimated fan sound power. The technique of estimating sound power via sound intensity measurements can cancel out only stationary background noise sources outside the grid system.
Acoustic testing was performed in the Orion CM mockup, and comparisons were made with acoustic modeling predictions. The comparisons showed very good agreement, plus or minus 3 dB above the Schroder frequency (where SEA is expected to give good results). The case studied was very realistic as the sound absorption material covered only part of the wall (i.e., 30% of the area of conical and vertical wall). The effect of such sound absorption material was predicted by:
• mockup cavity absorption predicted from measured mockup interior reverberation times and then using Sabine equation, or
• a two-layered Thinsulate© layup model, which attaches to the face (i.e., the interior wall surface) of the mockup cavity. The lay-up model was developed based on the results of impedance tube absorption testing. Furthermore, the effect of air absorption, which is notable in high frequencies (> 1 kHz), was also included.
Currently, the mockup only includes the bare pressure shell. The development of a fairly high-fidelity ECLS wall closeout panel was completed in FY’09. Installing the closeout panel into the mockup, updating the mockup acoustic model with the closeout panel, and validating the model with sound pressure measurement in the mockup will be performed.
In addition to the modeling development described above, other significant achievements were made with respect to Orion acoustics work. In particular, the Orion acoustic mockup, built as part of this acoustic modeling project, was used in demonstrations to protect the acoustic requirements (and the resulting Orion crew acoustic environment). Hamilton Sundstrand had indicated that the acoustic requirements were too strict and that they were not going to meet them. This included the continuous noise requirement, and the noise requirement for the Snorkel Fan (used after landing in a contingency situation). The Orion acoustics mockup was used to demonstrate to high level CxP management what the effects of higher noise levels would mean to Orion crews. As a result, a new commitment was made to hold to the acoustic requirements. An Acoustic Noise Control Plan will now be developed, and additional resources are being used to control noise during design of the vehicle. As part of this effort, the Snorkel Fan noise requirement was re-evaluated and updated to ensure adequate crew voice communications during a contingency landing situation.