Chris:
Hot microphones
Can't find what RR_NDB said right now. Can tell you, though, that the clearest signal on the BAC 1-11 CVR that I heard in 1980/81 was from the hot mikes. Also occurs to me that if you play back the P1 & P2 mics simultaneously, it may provide a stereo effect. Don't know how well the mics pick up distant noises, but am confident that it's good enough to hear a pilot talking even when his headset is lying on his side console. I have a feeling that hot mikes for CVRs were advocated in the UK from the start, whereas other states were not so keen.
The microphones on all headsets in use on commercial aircraft these days are of a type known as "noise cancelling" - this is not some electronic magic, the mic transducer is open at the front AND the back of the mic (with some mechanical delay introduced to the signal path) - when correctly positioned in front of the human mouth (within 1" or closer), the sound waves from the wearer arrive at the front of the mic first and then travel around to the rear of the unit some finite time later and correspondingly attenuated by the admitted small additional path distance (1/r sqrd law) and hence do not cancel, however sound sources at a greater distance arrive at the front and back of the mic transducer more or less simultaneously and at more or less the same volume and DO cancel. Hence "noise cancelling".
Some more explanation:
How do you make a mike directional?
Start by making an omni-directional mike. Take a mike transducer, made of a diaphragm and some hardware that changes diaphragm motion into a signal. Then put this transducer in the end of a sealed can, so that incoming sound contacts the diaphragm only on its front surface.
Sound from the front presses on the front of the diaphragm and makes a signal. Sound from the side or rear bends around to the front of the mike. This sound also presses on the front of the diaphragm and makes a signal. So the mike responds the same to sounds from all directions. In other words, it has an omni-directional polar pattern. Note that the omni mike becomes directional at high frequencies. That’s because the mike housing blocks high frequencies that arrive off-axis.
Now suppose we put some holes in the can behind the diaphragm. We carefully size these holes and add acoustic damping such as felt or foam to create an acoustic phaseshift network. It’s like an RLC circuit, which delays the signal passing through it. The holes or “rear ports” let sound into the back of the diaphragm.
How does this arrangement cancel sound from the rear?
Suppose a sound wave approaches the mike from the rear. It travels to the diaphragm by two paths: outside the mike and inside the mike through the ports. Some of the sound wave travels to the front of the diaphragm, outside the mike. The sound travel time, from the rear port location to the front, is what we call T. Some sound also enters the rear ports and is delayed. If the delay inside the mike is set the same as the delay outside the mike, sounds arrive at the front and rear of the diaphragm at the same time, in phase. Sounds push on opposite sides of the diaphragm, also in phase. The diaphragm cannot move, so sounds from the rear make a very weak signal. Rear sounds cancel out. You have created a cardioid polar pattern -also known as noise cancelling.
Sounds coming from the front do not cancel out. Why? Frontal sound waves travel to the rear ports during time T. Inside the mike, the phase-shift network further delays the sound by time T. The total delay is 2T. Since there is a big delay or phase shift between the signals at the diaphragm’s front and rear, a frontal sound makes a strong signal. High frequencies do not reach the rear of the diaphragm because they are filtered out by the rear port’s RLC filter. The cardioid mike is directional at high frequencies because its housing blocks high frequencies off-axis
Sorry for the long-winded explanation.....
However the point is that while a hot-mic recording might reveal significantly comments made 'off-PTT', the usefulness for distant cockpit sounds is doubtful. That's the job of the area mic. I do question why the area mic is not of greater bandwidth (currently limited to 6kHz I think). It is simple to find good omnidirectional mics with response out beyond 20kHz for a couple of hundred $, so cost is not the reason, at least for the mic itself.