8. DISCUSSION AND CONCLUSIONS

It has been shown, through both theoretical and experimental demonstrations, that single- and dual-diaphragm cardioid condenser capsules respond differently to low frequency sound sources. This can be attributed to the difference in the acoustic phase shift network, which for the dual-diaphragm capsule has the rear diaphragm as the first element. This diaphragm acts as a compliance at low frequencies (Fig. 6), and as such its impedance gets larger as frequency is lowered, partially blocking low frequency sound waves from reaching the back of the diaphragm. It follows that at lower and lower frequencies the dual-diaphragm capsule acts less as a pressure-gradient responding device and more like a pressure responding device. One real effect is that at a given source distance, the dual-diaphragm capsule has a polar response that is less directional than its single-diaphragm counterpart. Also, the dual-diaphragm capsule has reduced proximity effect and pop sensitivity.

This difference in low frequency directionality between dual- and single-diaphragm capsules translates into inherent advantages and disadvantages for near- and far-field applications. In the far-field, the single-diaphragm capsule has superior polar and on-axis responses, rejecting low-frequency rear-incident sound waves better and reproducing front-incident sound more accurately. The dual-diaphragm capsule has a low frequency directional behavior that is more suited for close distance applications where both the desired front-incident sound and unwanted rear-incident sound are in the near field. For intermediate distances either design might be suitable. Traditionally dual-diaphragm designed capsules have been preferred for vocal applications with references specifically made to the low frequency character. The findings of this paper offer a theoretically bases for this evident differentiation.

Every time a microphone is used, a three-dimensional sound field is integrated down to a one-dimensional signal. New knowledge on how transducer performance is effected by source distance can serve as a basis for new microphone characterization formats currently under consideration in the AES Microphone Characterization Committee SC-04-04. Ultimately, the goal is to provide the end user with application specific information.