Abstract
Through experiments concerned with the transmission of sound under conditions of normal incidence of plane wave, it has been confirmed that attaching weights on a membrane improves the sound insulation ability in a frequency range from natural frequency of the whole membrane to that of the partial area divided by the additional weights. This mechanism is as follows : The additional weights act on the membrane to form divided modal shapes with the same phase, and that modal shapes interfere with the normal mode of the whole membrane. As a secondary effect, the additional weights prevent high-order vibrations of the membrane. By these total effects, a great deal of attention of sound occurs and the transmission loss is over thirty or forty decibels in the most efficient case. This method is simple, light-weight, possible to tune, and effective in a lower frequency range and hence the authors think that this method has the possibility of developing into a useful one in the field of noise control.
