sound varying in quality

What software are you using? If you have anything else open on your desktop while you're recording, you are sacrificing quality of audio. I run my mic through a USP Pre Amp and I have a high quality cardiod mic, everything gets edited in Audacity. You can apply compression and noise reduction to get the sound more even.

Are you sure there was no background noise when you made the second
and fourth slides?

Is it possible the mic wire was positioned close to something that could
have caused it to pick up interference?

Any electronic or electrical devices/equip running during the second/fourth
recordings that were not in the others?

Is your mic securely plugged in to the USB jack?

If it's an old mic, or maybe even slightly defective, try recording and wiggling
the wire/cable at termination points such as at the mic or even at the USB
connection. Could be slighly intermittent, but not open, connection or even
slightly cold solder joint or oxidized terminal connection between different
metal sources.


Ken

It may not be really apparent, but one of the things that makes a huge difference in sound quality of a recording is the distance from the microphone and the aim of the mike. Most mikes have a cardioid pattern, which is a sort of rounded heart shaped area of sensitivity in front of the mike. Another way to think about it is to imagine a loosely filled balloon, with the microphone poked into one side. The point on axis forward of the front of the microphone is the most sensitive, with some sensitivity to the sides, and relatively little to the back (where the wire comes out). If you are talking to slides that you are recording on your PC, you may be moving towards and away from the microphone. This can cause a pumping effect, or can make a huge difference between sections, depending on how close you get to the mike.

As someone already mentioned, if you can run the voice through a processor to compress and level the volume, you can get very consistent audio. I haven't looked for awhile, there's probably an application available for the PC that will do that. Years ago, I used separate outboard audio processing units for this function for live recordings.

Depending on your voice, there also may be a proximity effect involved. Bass and lower frequency tones are picked up better by a microphone when they are very close by. As you move away from the mike, there is a sharp dropoff as you move away, which is not nearly so noticeable for higher pitched tones or high voices.

If you aren't in the picture (for video) or doing audio only, I'd suggest trying a headset microphone so you can get a consistent distance from your mouth to the mike. Some microphones are much more sensitive than others, so sometimes changing microphones can make a huge difference in your recordings.

Also, if the microphone is a noise cancelling type, you can sometimes get some odd results depending on the ambient sounds. If I have to record in a relatively noisy area, I use two mikes, one on me and one for ambient, and mix in the ambient one out of phase (thereby canceling the ambient noise picked up by the main mike). It's an old trick I used to use for on-site live music recordings and can help, but takes at least a minimum sound board or mixer and some effort to ride the gain controls during the recording.