Headphones with an integrated active noise cancellation system have been increasingly introduced to the consumer market in recent years. When exposing the human ear to active noise sources in this striking distance, t...Headphones with an integrated active noise cancellation system have been increasingly introduced to the consumer market in recent years. When exposing the human ear to active noise sources in this striking distance, the ensuring of a safe sound pressure level is vital. In feedback systems, this is coupled with the stability of the closed control loop; stable controller design is thus essential. However, changes in the control path during run-time can cause the stable control system to become unstable, resulting in an overdrive of the speakers in the headphones. This paper proposes a method, which enables the real-time analysis of the current system state and if necessary stabilizes the closed loop while maintaining the active noise reduction. This is achieved by estimating and evaluating the open loop behavior with an adaptive filter and subsequently limiting the controller gain in respect to the stability margin.展开更多
Spread-F is a manifestation of ionospheric irregularities and generally takes place at nighttime. However, it can also be ob- served seldom at daytime, It is recognized that acoustic gravity waves （AGWs） play an imp...Spread-F is a manifestation of ionospheric irregularities and generally takes place at nighttime. However, it can also be ob- served seldom at daytime, It is recognized that acoustic gravity waves （AGWs） play an important role in triggering plasma instability which results in spread-F in the ionosphere. The typhoon is a main source of the AGWs. In this paper, two cases of ionospheric daytime spread-F in the period of typhoon were analyzed. One case was on July 29, 1988 and the other was on August 01, 1989. The results showed the following： 1） There were some wave-like disturbances appearing in the HF Doppler records firstly, consequently the Doppler echo traces became scattered, which indicated that the ionospheric spread-F was triggered; 2） the blurred echo traces in the both two cases appeared in the morning （08：30-11：30 Beijing time） and lasted for more than two hours; 3） with the blurred echoes gradually weakening, the traveling ionospheric disturbances （TIDs） still ex- isted and became clearer; 4） the frequency shifts in the two of the radio wave in the ionosphere moved downwards. spread-F during the typhoon period in Asian region. cases were both positive, implying the effective reflecting surface These results provide good observational evidence for daytime展开更多
文摘Headphones with an integrated active noise cancellation system have been increasingly introduced to the consumer market in recent years. When exposing the human ear to active noise sources in this striking distance, the ensuring of a safe sound pressure level is vital. In feedback systems, this is coupled with the stability of the closed control loop; stable controller design is thus essential. However, changes in the control path during run-time can cause the stable control system to become unstable, resulting in an overdrive of the speakers in the headphones. This paper proposes a method, which enables the real-time analysis of the current system state and if necessary stabilizes the closed loop while maintaining the active noise reduction. This is achieved by estimating and evaluating the open loop behavior with an adaptive filter and subsequently limiting the controller gain in respect to the stability margin.
基金supported by the National Natural Science Foundation of China (Grant Nos. 40974091, 41174135 and 40921063)the Specialized Research Fund for State Key Laboratories
文摘Spread-F is a manifestation of ionospheric irregularities and generally takes place at nighttime. However, it can also be ob- served seldom at daytime, It is recognized that acoustic gravity waves （AGWs） play an important role in triggering plasma instability which results in spread-F in the ionosphere. The typhoon is a main source of the AGWs. In this paper, two cases of ionospheric daytime spread-F in the period of typhoon were analyzed. One case was on July 29, 1988 and the other was on August 01, 1989. The results showed the following： 1） There were some wave-like disturbances appearing in the HF Doppler records firstly, consequently the Doppler echo traces became scattered, which indicated that the ionospheric spread-F was triggered; 2） the blurred echo traces in the both two cases appeared in the morning （08：30-11：30 Beijing time） and lasted for more than two hours; 3） with the blurred echoes gradually weakening, the traveling ionospheric disturbances （TIDs） still ex- isted and became clearer; 4） the frequency shifts in the two of the radio wave in the ionosphere moved downwards. spread-F during the typhoon period in Asian region. cases were both positive, implying the effective reflecting surface These results provide good observational evidence for daytime
