Active Sound Transmission Control of a Double-Panel Partition
An actively controlled, bidirectional partition that can provide excellent low-frequency attenuation while remaining relatively light weight. Significantly reduces low-frequency noise in systems like aircraft fuselage trim panels, rocket payload fairings, large ceilings, and high-rise buildings without the undesirable increase in mass for the purpose of sound attenuation.
The Mechanical Engineering Department at Brigham Young University has developed a functioning prototype of an actively controlled partition (aka an active segmented partition (ASP) module). The ASP module is bidirectional; capable of attenuating noise from both sides of the partition. It is an active control which is able to block broadband random noise. Using robust analog feedback controllers, these capabilities were achieved at minimal cost.
As an acoustic disturbance form a source space impinges upon a double-panel partition, it imparts a distributed force upon the first panel. The force then induces vibration within the panel where the vibration is transmitted by structural or acoustic paths to the second panel causing an acoustic radiation into the receiving space. High frequency vibrations are easily blocked as they pass through the partition. However, low frequency sound is readily transmitted from the source space to the receiving space without significant attenuation.
Many passive methods have been proposed to increase the transmission loss at low frequencies. The most effective passive method is to add mass to the partition. Doubling the partition mass can eliminate low frequency sound up to approximately 6 decibels. It is easy to recognize that a moderate increase of the transmission loss, say 18 dB, would require a partition that is 8 times as massive as the original.
Clearly, adding mass to a partition is not a feasible option especially in a number of applications; including aircraft trim panels, rocket payload fairings, transportation vehicles, large distributed ceilings, and high-rise buildings. Each of these applications has severe penalties (both cost and function) associated with adding mass. A viable solution is ASPs, where electrical energy is utilized instead of mass in order to dissipate low frequency noise.
Prior ASP modules have been proposed, but unfortunately they lack two critical performance capabilities: bidirectional transmission loss and active control of broadband random noise. Also, the current modules require expensive digital controllers.
This technology is a very novel and cost-effective way of reducing noise in any environment. The lightweight advantage that it offers can significantly affect the design decisions in the aerospace industry and in the building industry. A partition capable of noise reduction without the need of adding additional mass will make the system versatile. With this technology aircrafts, homes and buildings can be lightweight, compact and not to mention, cost efficient.
The proposed design for an individual ASP module is shown below. It consists of two parallel panels separated by an air space. Each panel is connected to a rigid and lightweight interstitial structure which allows the panels to vibrate with some independence from the interstitial structure, reducing the mechanical coupling path between the two panels in the same module and between any two modules in the array. This design enhances the module’s ability to successfully implement decoupled controllers. An actuator and sensor pair is connected to each panel in the module. The actuators are contained inside the cavity and the sensors can be mounted on either side of the panel. The actuators are also connected to the interstitial structure by means of lightweight supports. The output of the sensor is fed into the controller and the output of the controller is fed into the actuator (shown schematically for the leftmost panel in the figure). Although not shown in the figure, the necessary electronics for the controllers could be compactly designed so that they can be contained within the module. The remainder of the cavity is filled with acoustically absorbent fiberglass insulation (not shown) to help improve the passive sound isolation performance of the module at high frequencies.
Jason D. Sagers, Jonathon Blotter, Timothy Leishman
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