"Zooming in" on distant sounds

I have a digital recorder with this feature - and it works very well. I presume it works by increasing signals which appear equally in both channels (- and are therefore in the direction the stereo mic is pointing) and decreasing amplitude of signals which appear unequally and are therefore off to the side. In other words, weak signals are amplified if they appear equally in both channels. If this is how it works, then it should be possible to do it on the computer. Does Audacity already have something that will do this?

Sort of, but not quite.
Zoom microphones use a pair of cardioid capsules that are both forward facing with one directly behind the other. The rear microphone is wired up so that it is out of phase with the front microphone. As long as the distance between the two capsules is substantially bigger than half the wavelength of the sound, both microphones will contribute to the signal recorded from sounds that are in front of the microphones.

When used “zoomed in”, sounds that are arriving from the side will reach both microphones at exactly the same time and will produce a signal in the rear microphone that “cancels out” the signal from the front microphone.

When used “zoomed out”, the rear microphone is switched off and the front microphone works as an ordinary cardioid microphone.

To cope with low frequencies (long wavelength sounds), some fancy electronics are used that shift the phase of the rear microphone so that it is less than 180 degrees out of phase with the front microphone.

There is an additional problem in that a certain amount of “comb filtering” will occur causing a boost or cut of frequencies that have half wavelengths that are exact multiples of the distance between the microphones. I presume that this could also be compensated for electronically, but I guess that it is usually ignored as the effect will not be particularly noticeable on most “real world” sounds. (and also why zoom microphones are not used in professional audio applications)

Can this be reproduced in Audacity?
Theoretically yes, (I’ve never tried it), but you would need to have an original recording from 2 microphones with one placed directly behind the other. The physics involved depend on what is happening acoustically with the microphone capsules as much as what is happening in the signal processing. You should be able to reproduce the signal processing, but for the acoustic properties you need the physical microphones in the correct configuration.

My digital recorder has two mics at 90°:

from http://www.proaudioreview.com/article/17384

A stereo pair of microphones resides atop the LS-10; its cardioid diaphragms are fixed at a 90-degree angle . . . .
Zoom Mic utilizes DiMAGIC Virtual Microphone (DVM) technology for a directional microphone function

so as far as I can tell it seems to work on the signals from two normal mics, so this function could surely be performed just as well on the computer - or do I not understand it?

I presume the method is similar to the “classic” zoom microphone - using phase shifting to produce partial cancellation of sounds from the sides - though the mathematics are probably somewhat more complex. You also see something similar on some dual diaphragm condenser microphones which can have their pick-up response changed electronically (Omni / Figure of 8 / Cardioid).

In each case I’d guess, “yes” you can do it in Audacity, but;
a) The exact placement of the 2 microphones is critical
b) You need to work out the necessary phase shifts.

I’m not sure what phase shifting is - perhaps it is that the peak of a wave hits the second mic slightly later than the first mic? Thanks for the explanation thus far anyway and I’ll give up the idea of doing this on the computer for the time being.