Two microphones, one hollow sound
When a thin, hollow sound appears the moment you add a second microphone, it has one clean cause, and the fix costs nothing.
Something is wrong with the sound and you cannot describe it. It is not noisy. It is not quiet. It is not echoey, exactly. It just sounds thin, a little hollow, slightly cheap, as though the voice has been played through something rather than recorded.
Hollow sound has other causes too, the room, a flipped polarity, a microphone in the wrong spot, but if yours arrived the day you added a second microphone, this is the one to check first, and it has nothing to do with your voice or the money you spent. The mute test below settles it in under a minute.
Two microphones sitting at different distances from the same source hear the same sound at slightly different moments. Where the two versions disagree, they cancel each other. What survives is thinner than either recording would have been on its own.
What is actually happening?
Picture two people clapping. Clapping together, you hear one loud clap. If one of them is a fraction behind, you hear something smeared and smaller, and if the delay lands exactly wrong, the two claps flatten each other into almost nothing.
Sound travels at a fixed speed. A microphone half a metre further from your mouth than another one receives everything a fraction later, every time, for the whole recording. The moment those two recordings are combined into a single track, which is what happens as soon as you play them together, some parts of the sound line up and add, and some arrive out of step and subtract.
Nothing is broken. This is arithmetic, and it happens whether or not anybody in the room notices.
Why does it sound thin rather than doubled?
Because the cancellation is uneven. Different parts of a voice are affected differently by the same delay, so some survive and some flatten, and what you hear is whatever was left over.
The effect has a name worth knowing, comb filtering, because an even delay strips out a regularly spaced row of frequencies and leaves the ones between them standing. It is the term almost everything else useful about this problem is filed under.
The parts that go first tend to be the ones carrying weight and body. What remains is the part of a voice that sounds like a phone call: intelligible, present, and somehow not a person in a room. That is why this reads as cheap rather than as faulty. Faulty would be easier, because you would have known to fix it.
How can you hear it for yourself?
The test takes under a minute and needs no software you do not already have.
Play both microphones together. Mute one and listen to the other alone. Then swap. If either microphone on its own sounds fuller than the pair, you have found it, and you have found it in the one way that leaves no room for doubt. If muting neither one helps, the fault is somewhere else, the room or the placement, and that is worth knowing too.
Do this before you touch a single volume control. A recording with this fault in it cannot be balanced sensibly, because every adjustment is being applied to a sound that is already fighting itself, and you will spend an hour chasing a problem that moves each time you get near it.
Where does the second microphone go?
Further away than instinct suggests. The rule of thumb comes from the wider craft rather than from anybody's house style, and it has a name, the three-to-one rule: the gap between the two microphones wants to be at least about three times the distance from the near one to the source.
The reasoning is proportion. If the close microphone is a hand's width from a voice and the second one is two hand's widths away, both are hearing that voice at similar strength, and similar strength is what makes cancellation severe. Push the second one out until the voice reaches it much more faintly than it reaches the first, and the two recordings stop competing over the same sound. The distant microphone now contributes the room, which is what you wanted from it in the first place.
More distance is safer than less. If you are unsure, move it further, then check.
What if it is already recorded?
Then it is repairable, which puts it in a considerably better category than most audio faults.
The two recordings can be slid against each other in time until they agree, and the body comes back. It is not clever work. It is careful work, and it has to be done by ear as well as by eye, because two recordings that look aligned on a screen can still be quietly arguing.
Alignment is the first thing this studio does to a redundant capture like this one, two microphones on one source meant to be summed, ahead of noise removal and ahead of any balancing at all. The reason is not tidiness. Every later decision is taken by listening, and until the two recordings agree with each other, the thing being listened to is not the recording. A deliberately spaced or stereo pair is the exception, because there the small timing difference is the stereo image itself, and aligning it away would flatten the recording rather than mend it.
What should you change before the next session?
Measure the two distances roughly, with your arm, and be honest about what you find. If the second microphone is not obviously much further from you than the first, move it until it is, and turn it so it is pointing somewhere other than at your mouth.
Then record a short test and run the mute check before the real take rather than after it. Thirty seconds of talking, listened to properly, will tell you more than any specification you can read about either microphone.
The reason this fault survives in so many recordings is that it never sounds like a fault. It sounds like the ceiling of what a home setup can do, which is a comfortable thing to believe and, in this one case, entirely wrong. If your audio has sounded thin since the day you added the second microphone, that date is the evidence. The mute test is the rest of it.
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