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Published Articles

The Volume 4, No 2, June 1999

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A New Passive/Active Hybrid Headset for a Helicopter Application

M. Winberg, S. Johansson, T. Lago, I. Claesson


In helicopters, the low frequency noise generated by the rotors and engines often masks and jeopardizes safe communication. In addition, pilots are likely to suffer from damage to their hearing due to the high sound levels in the headset produced to overcome the noise caused by increased speaker levels. A feasible approach is to reduce the low frequency noise by using active techniques combined with a method for reducing the noise in the intercom microphone signal, with lower speaker levels as a result. Helicopter noise consists of tonal components embedded in broadband noise. In order to achieve an efficient attenuation of the primary noise inside the headset, a combination of a digital feedforward controller and an analog feedback controller is employed. Spectral Subtraction is used to suppress the background noise in speech signals. This paper evaluates a combination of the two techniques and their application to real data.

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Energy Absorption in the Hand and Arm System Exposed to Impact Vibration with High Frequency Contents

Anna Sorensson, Lage Burstrom


In the following study, ten subjects were exposed to four different authentic vibrations with impact and high frequency contents. The purpose was to investigate the energy absorbed in the hand and arm system from handheld vibrating tools. The tools studied were a chipping hammer, an impact drill, an impact wrench and a breaker. The frequency range chosen was 20 to 5000 Hz. The influence of two frequency-weighted acceleration levels, 3 and 6 m/s2, and two different grip and feed forces, 20 and 40 N, were also investigated. The study shows that there were differences in the amount of vibration energy between the four types of hand-held vibrating tools. The impact wrench and the impact drill contributed to higher absorbed energy in the hand than the chipping hammer and the breaker. The energy absorbed in the hand and arm increased with the vibration level. No clear relationship was found between the hand forces and the energy absorption. Furthermore, the results show a relationship between the rise time of the tools and the amount of energy absorbed in the hand and arm. A conclusion from this study is that tools which generate impacts with a short rise time increase the risk of vibration injuries.

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Pressure Rise Control in Axial Ducted Fan by Acoustic Noise Modeling

T.V. Rama Murthy, Y.G. Srinivasa, M. Govardhan, G.R. Srinivasa


A control strategy for maintaining the pressure rise within certain limits in an axial ducted fan has been described. Experiments reveal that the spectrum of the acoustic noise contains predominantly the peak frequency of the broad band rotor noise. The variation of the amplitude of the envelope of a bandpass filter output tuned to the peak frequency with changes in throttle position and impeller speed has been studied. An off-line Least Squares Estimation algorithm fits a discrete time linear model for predicting the amplitude of the envelope. An error signal between the predicted amplitude of the model and its expected value for maximum pressure rise activates the throttle valve servo. Gain Scheduling of the model parameters in terms of the throttle position and the impeller speed is implemented for wider operating range of the machine.

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Iterative Design for Vibration Attenuation

T. Meurers and S.M. Veres


Iterative design is introduced for disturbance compensation (IDDC) as a direct tuning method using closed-loop experimental data without modelling the plant. The method is based on numerical optimization of controller parameters through a sequence of experiments. The basic idea of this method is illustrated in simulation as well as in active control of a vibrating glass plate.

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