A convenient calibration apparatus for a miniaturized hydrophone is described which is primarily intended for intracardiac, intravascular, intrauterine and various other uses in vivo. The present apparatus is composed of two parts to calibrate in a vibrating column of water shown in Fig. 2 and in a closed water-filled chamber shown in Fig. 8. As the first step of the calibration process, a monitor hydrophone (Fig. 5) to be used for a secondary standard is calibrated at low-audio frequencies by immersing the hydrophone in the vibrating column of water. When the water column is vibrated vertically and sinusoidally, the pressure in the water is proportional to the water height h and the acceleration as given by Eq. (4). The values of the correction factor for this apparatus due to wave effects are tabulated in Table 1. The sensitivity of a monitor hydrophone is -125. 4 dB re. 1 V/μbar at 100 Hz, and the frequency response is shown in Fig. 7. The secondary calibration of a miniaturized hydrophone is performed by comparing the unknown output with that of the monitor hydrophone in a closed water filled chamber shown in Fig. 8. Sinusoidal pressure is obtained by driving the diaphragm of the chamber by an electrodynamic vibrator excited by a beat frequency oscillator. Two factors for designing the closed chamber are represented by Eqs. (12) and (13) with a nondimensional parameter χ. The response frequency characteristics of a miniaturized hydrophone was measured by the apparatus mentioned above over the frequency range from 20 Hz to 5 kHz at a sound pressure level over 10^4 μbar. The results of the calibrations are shown in Fig. 13 and 14. Furthermore, the response to a static pressure fairly agreed with that to a sinusoidal pressure. The sensitivity has been measured with an accuracy of about ±1 dB.
In a dynamic speaker, a particular supporting piece is used for positioning of the moving coil. Actual structure of this supporting piece is variously devised in practice to decrease the stiffness and to prevent generation of non-linear strain. In order to satisfy these two essential requirements, the length of the supporting piece in the radial direction is inevitably increased to a certain degree. Then, the frequency characteristic of vibration of the cone diaphragm gets more complicated because of an obstruction caused by higher mode of vibration above the second resonance of the supporting piece itself. Taking these possible problems into consideration, the vibration characteristic of the vibration system composed of the moving coil and the supporting piece has been analyzed to estimate the degree of this obstruction. Contrary to our previous expectation, it has been found that the obstruction from the higher mode of vibration is actually eliminated, if the weight of the moving coil (including the vibrating plate) is more than ten times that of the supporting piece. It has been made clear that the whole system is approximated as a simple resonant system of one-freedom. Therefore, it is apparent that a considerably long supporting piece can be used practically without any trouble.