Abstract
This paper describes measurement of natural vibration of acoustic space by multi-point excitation using decentralized control. A set of acoustic excitation system consists of a speaker, a microphone, and a local feedback controller. The acoustic excitation system handles the frequency range from 100Hz to 500Hz higher than a mechanical vibration system does. So the phase lag of an actuator, the dead time by propagation of sound, and the dead time by sampling of a controller influence the local feedback control. First, the various phase lag in the acoustic excitation system are identified. The speaker has the second vibration property derived from mechanical resonance, and the first delay property derived from induced voltage of coil. The propagation of sound from the speaker to the microphone causes the dead time. Sampling time of the controller also causes the dead time from the input signal to the output signal. Second, the way to compensate these phase lag in the acoustic excitation system is proposed. Resonance of the speaker makes the phase decrease 180 degree at the resonant frequency. To suppress a change in the phase in the frequency range from 100Hz to 500Hz, the resonant frequency moves to less than 100 Hz. The phase lead and the phase lag after the resonant frequency movement are compensated by the second order phase lag-lead element. The natural vibration of one-dimensional/two-dimensional acoustic space is self-excited by the acoustic excitation system with the phase compensation. By closing the feedback loop, the first mode natural vibration is excited. By adding sinusoidal signal from outside, all natural vibration in the frequency range from 100Hz to the 500Hz is self-excited (forced entrainment).
