Experiment 9 is going to give you the visual you need to better understand this concept. In the previous experiment, I was taking simple readings of the voltage between the electret and GND, and the reading was definitely in the millivolt range. Not much voltage.
But insert this LM741 op-amp hip and all of a sudden I'm seeing the millivolts converted to volts. The book calls it gain, and my results are amazing. In a quiet room (probably some background noise in there such as an AC running or just the movements of my chair or my handling of the camera) I was able to get the reading down to 0.007 volts. 7 millivolts in a quiet room. Just my voice alone talking in the video was causing an increase in voltage between 1 and 2 volts. Tapping on the electret (not recommended) would give a large jump... sometimes up to around 8v!
The discussion on how DC voltage is blocked with the coupling capacitors... very interesting! I don't recall that discussion from Make: Electronics, but I'm beginning to understand what's happening. I thought it was a pretty slick solution to pair two 100k resistors between 9V and GND and then use the midpoint as the Reference Voltage on pin 2 of the LM741 and then use that final coupling capacitor so that a valid voltage reading (with respect to GND) could be made. Pin 6 is the output for the LM741, btw.
You'll also need to understand the importance of finding matching pairs of resistors. I was fortunate to have a string of 100 resistors in the 100k value range and it only took about a dozen or so reads to find two pair of matching resistors. If I understand the experiment correctly, unless you can get those pairs to be very close in value, you might not get good results from this experiment. Fortunately, resistors are dirt cheap and 100k resistors always seem to be in demand, so grab yourself a package of them if you can find them.
Experiment 10 looks fun... I've already read over it, so I know what's coming. When I'm done, I think I'm going to take Experiment 10 and transfer it (with a 9V battery) to a box and make a little toy for my boys. (Think the old school favorite "Quiet Game" and you may have a hint.)
Here's the video: