Abstract:

Acoustic noise is something we would like to reduce or, if possible, get rid of all together. Noise is at best annoying and, in fact, very loud noise (large decibel noise) such as that generated by an airplane just taking off can be dangerous to human ear. Therefore, engineers are always seeking ways to reduce or eliminate noise. There are three basic approaches to reducing the effect of noise: (a) manipulate the source to reduce the generation of noise, (b) reduce the effect of noise at the ear level (ear protectors, etc) and (c) place a sound barrier in between the ear and the sound source to cut down the level of radiated noise coming to the ear. The first two approaches, in many instances, may not be easy or even possible. For example, reducing the emission of noise from an aircraft jet exhaust at full throttle is a difficult one, at best, without compromising thrust performance. Also, using ear protectors at all times may not be a practical approach either. Therefore, the most common method of noise management is to place a physical barrier between the source and the ear. One example that comes to mind is the sound barriers (or fences) you may see on the side of highways (usually made of wood or compressed wood chips) to protect adjacent communities. However, using physical barriers may not be practical or even possible in many applications. For example, it is not feasible to build sound barriers in communities adjacent to airports to protect them from noise generated by aircraft flying over them. Nor is it practical to add physical cowlings at the exhaust of jet engines big and long enough to deflect sound radiating sideways.

In a recent research project at Polytechnic, a futuristic concept was investigated to address this problem; the use of glow discharge plasma as a sound barrier. The idea is to turn on a plasma when sound protection is needed and turn it off when it is not. The main thrust of the investigation was the determination of the effectiveness of using glow discharge plasma as a sound reflector/absorber. The attenuation of sound by glow discharge plasma was demonstrated experimentally. The experiments were conducted in an anechoic chamber where the attenuation of a single frequency sound propagating through plasma formed between two electrodes was studied. The sound attenuation was measured at different locations inside the chamber with the plasma plane oriented normal to the direction of sound wave propagation. Some measurements were also carried out with the plasma boundary oriented 45 degrees to the sound wave propagation direction. The measurements clearly demonstrated that the plasma attenuates sound. The highest attenuation of about 23 dB was obtained when the plasma was oriented 45 degrees to the sound propagation direction. These interesting results will be summarized and mechanisms that are responsible for acoustic wave attenuation plasma will be discussed.

*This seminar series is geared towards students specifically but all are welcome.