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A parameter that is gaining significance in the evaluation of acoustic liner performance is the skin friction drag induced by air flow over the liner surface. Estimates vary widely regarding the amount of drag the liner induces relative to a smooth wall, from less than a 20% increase to nearly 100%, and parameters such as face sheet perforate hole diameter, percent open area, and sheet thickness are expected to figure prominently in the skin friction drag. Even a small increase in liner drag can impose an economic penalty, and current research is focused on developing 'low drag' liner concepts, with the goal being to approach the skin friction drag of a smooth wall. The issue of skin friction drag takes on greater significance as airframe designers investigate the feasibility of putting sound absorbing liners on the non-lifting surfaces of the wings and fuselage, for the purpose of reducing engine noise reflected and scattered toward observers on the ground. Researchers at the NASA Langley Research Center have embarked on investigations of liner skin friction drag with the aims of: developing a systematic drag measurement capability, establishing the drag of current liners, and developing liners that produce reduced drag without compromising acoustic performance. This paper discusses the experimental procedures that have been developed to calculate the drag coefficient based on the change in momentum thickness and the companion research program being carried out to measure the drag directly using a force balance. Liner samples that are evaluated include a solid wall with known roughness and conventional liners with perforated facesheets of varying hole diameter and percent open area.

The reverberance of a performance venue is often characterized by an important metric, reverberation time. In order for architects, acousticians, and performers to better understand the value of knowing and predicting the reverberation time in different spaces, the just-noticeable-difference (JND) is often cited. The established JND for reverberation time is approximately 5% of the reference reverberation, noted in the standard ISO 3382-1. This value was determined using impulsive sound, meaning it may not provide a complete picture for applications related to music. This study aimed to evaluate several elements of musical performance that may alter the perception of reverberation in a simulated listening environment. Primarily, the effect of instrumentation was investigated, including the use of solo, small ensemble, full orchestra, and vocal recordings for comparison. Results were generated using test participants listening to multiple soundfields and providing subjective evaluation. Reverberant soundfields were created in an anechoic listening environment and auralizations were produced using ODEON room acoustics software. Results indicated statistically significant mean differences in reverberation perception between musical instrumentation groups used during experiments. [Work supported by The Paul S. Veneklasen Research Foundation.]