Tuning of vibration absorbers and Helmholtz resonators based on modal density/overlap parameters of distributed mechanical and acoustic systems

Comparison of smart panels for tonal and broadband vibration and sound transmission active control

“…This sensor-actuator pair can be suitably used to implement a constant gain velocity feedback loop, which is physically equivalent to adding a sky-hook point damper to the hosting structure [6,40]. At low frequencies where the flexural response of the structure is dominated by the resonant responses of low order modes [6,23], adding sky-hook dampers can be very effective, both in terms of controlling the response and the sound transmission of the structure [6,40]. In principle, the control performance of the feedback loop can be further improved by tailoring the control function to the response of the hosting system.…”

“…higher frequencies the spectrum is characterized by smoother crests and troughs, which are due to the overlap of the resonant responses of clusters of higher order flexural modes of the panel [6,23]. In parallel, the black solid line in plot (b) of Figure 8 shows that, up to about 800 Hz, the spectrum of the sound transmission ratio is also characterized by well separated sharp peaks, which, however, are due to the resonant responses of the volumetric flexural modes only.…”

“…However, to guarantee good noise control levels, a rather large number of channels had to be used, which made these systems rather expensive and complex to install and to maintain. Indeed, at each frequency, the acoustic response of an enclosure is given by the overlap of multiple resonant modes, whose number increases proportionally to the cube of frequency [23]. In general, to fully attenuate the sound field of an enclosure, the control system should encompass as many channels as the number of modes forming the acoustic field at the frequency of the disturbance [24].…”

“…The flexural response of thin panels at each frequency is also characterized by the overlap of multiple modes. However, for thin panels, the modal overlap grows linearly with frequency [6,23]. Therefore, for a desired control frequency range, the vibration control system should encompass a smaller number of channels compared to an acoustic control system.…”

On the use of dynamic vibration absorbers to counteract the loss of sound insulation due to mass-spring-mass resonance effects in external thermal insulation composite systems

“…A dynamic vibration absorber, DVA in short, basically consists of a seismic mass suspended on a spring -damper system. The same kind of device is sometimes also referred to as a tuned mass damper (TMD) [7]. The difference between a DVA and a TMD lies in the incentive for their use.…”