Is there such as thing as a zoom microphone?
Your video camcorder has a zoom lens that can change from wide angle to telephoto according to how wide or narrow you want the field of view to be. But what about sound? Is there an equivalent of the zoom lens; a kind of zoom microphone?
Yes there is, and the most likely place you will find it is indeed in or mounted on top of a video camcorder.
A zoom microphone has two cardioid capsules that are placed one behind the other, both facing in the same direction but connected in opposite phase. Sound arriving from the side will strike both capsules at the same time, and therefore because of the opposite phase will be canceled out.
Sound arriving from the front however will not cancel if the wavelength is less than half the distance between the two capsules. However, as the wavelength increases towards the low-frequency end of the sound spectrum, the combined output of the capsules falls by 6 dB for every decreasing octave.
To compensate for this, a phase shifting circuit that has progressively increasing action towards low frequencies places the signals back into phase at the lowest frequency of interest. In-between frequencies are shifted by an amount proportional to the loss of level due to the spacing of the capsules. This has the effect of bringing the low frequency response back to normal.
The zoom effect is controlled by the amount of signal from the rear capsule that is mixed in. If none of the signal from the rear capsule is used, then the microphone will have a normal cardioid characteristic. If the signal from the rear capsule is used at full level, then the maximum zoom effect will be achieved.
A third cardioid capsule can also be used to broaden the response to omnidirectional, so the microphone can cover the full range from omni to highly directional.
Plainly, with the multiplicity of capsules and corrections involved, sound quality will not be optimum. However, zoom microphones do seem to work amazingly well for video cameras and one has to wonder why they are not used at all in pure audio work.