Abstract
In the holographic recording of the vibrating mode of the cone of a loudspeaker which has a large diameter and a high compliance edge, reconstruction of the vibrating mode is generally impossible by reason of the random vibration of the cone driven by the external disturbance which consists of air motion and acoustic noise. For the purpose of improving this defect, a motion compensation method is already well-known. The optical arrangement of this method is indicated in Fig. 4. However, the amplitude of the cone recorded by this method cannot be computed from the mode. In the case of partial vibration, a different mode is sometimes obtained for the same vibration in accordance with a mirror position on the cone (shown in Fig. 6). In order to determine the exact vibrating mode recorded with the external disturbance, we developed three holographic recording methods. First, we attempted a time-averaged method using a soundproof box. In this method, the loudspeaker cone is freed from acoustic noise by using the soundproof box and, considering the volume of this box, air motion can be neglected. The appearance of this soundproof box is shown in Fig. 7. According to many experimental results, in high-frequency and in small-amplitude operation, such as the measurement of interference fringes, no modification of the vibrating mode could be found using this soundproof box (shown in Fig. 8). Generally, the cone vibration due to external disturbance is piston motion and its amplitude is maximum near the fundamental resonant frequency f_0. Accordingly, it can be considered that two loudspeakers with identical mechanisms are influenced identically in the same air disturbance. Fig. 10 shows the optical arrangement of the holographic recording method using a completely equal loudspeaker (the non-driven reference loudspeaker) as the object loudspeaker. In this method, since the reference beam is reflected by a mirror fixed on the cone of reference loudspeaker, its phase is modulated by the external disturbance. Therefore, when it interferes with the object beam, the effect of the external disturbance is cancelled. In Fig. 10, if the condition of Eq. (12) is satisfied, the same vibrating mode as recorded by the time-averaged method can be obtained (Fig. 11). In this case, it is difficult to prepare a reference loudspeaker having exactly the same characteristics as the object loudspeaker, but this can be approximately satisfied using a reference loudspeaker with an identical resonant frequency f_0. This method, a reference beam modulated method using a pair of loudspeakers, has the defect that reconstruction of the stationary part (the speaker holder) of the object cannot be achieved. In order to improve this defect, we developed a combined method using both a non-modulated reference beam and a phase-modulated reference beam with the reference loudspeaker. As indicated in Fig. 12, the reference beam is divided into two paths by a beam splitter. One beam is reflected by the stationary mirror and the other beam is reflected by the mirror fixed on the cone of the reference loudspeaker, and each beam interferes with the object beam on the hologram plate. Fig. 13 shows the effect of this combined recording method. This method is effective in recording the vibrating mode of a multiple loudspeaker system. Small diameter loudspeakers used as tweeters and large diameter loudspeakers used as woofers are generally influenced differently in the same external disturbance. Therefore, the complete vibrating mode of the multiple system could not be recorded on a single hologram. However, by using the combined method, the complete vibrating mode of the system could be recorded on one hologram plate and reproduced at the same time (Fig. 14).
