Is it some sort of effect or do they just record the same track at how ever many pitches they need?
I thought you meant Mongolian mouth music, where they can create multiple harmonics on a single note - if you’ve never heard this, you should hunt it down - it’s mad.
As to the question, unless you’re using a modal harmony, you’d have to multi-track it, since the harmony isn’t normally an exact interval away from the tune.
Having said that, I’m sure nowadays they could get computer effect boxes to make fabricated harmonies fit the key, but it’d probably be more trouble than it’s worth.
I thought this was about Mongolian mouth music too…
But as per the OP, yeah, they usually sing harmony on a different track. It’s the easiest way.
As jjimm mentioned you can electronically “pitch shift” a note – usually that is done in the editing room with fancy smancy software. But you can’t shift it too far without losing the “human-being-singing sound” so you’re much, much better off just singing the harmony on a separate track.
Pitch shifting is great for funky effects and to tidy up notes that were a little off (like if you sang a note a touch flat).
You can for example get pitch shifting devices that work in a live setting – those use algorithms to map what you sang to the nearest note, so if you were a bit flat, it would adjust the signal to be on target before it went out through the speaker. However, these are only good witin a demi-tone or so. If the singer is too far off, the note will be matched to the wrong one and it’ll make bad pitch sound even worse.
You can also “delay” a signal for a fraction of a second to make it sounds as if there are two singers. So it’ll sound like the singer is singing with him/herself, but that would be the same notes, not a harmony. Delay efects are used to create evffects like echo, reverberration, and chorus.
The pitch shifting effects can work within all keys/scales, made up keys or scales (if you have a good one), on static notes, or fixed intervals now. About 5 years ago you wouldn’t find many of them out there but now the lowest end vocal harmonizers start at around 100 bucks brand new for the harmony tracks. Cher and Metallica have both used them. However, the songs that I had heard were fairly old and the “fixed” notes sounded very electronic and fake.
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The DVD “Bela Fleck, Live at the Quick” has a Tuvan throat singer performing with them on a few songs. He does indeed sing multiple harmonics at the same time, and it is amazing.
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This seems relevant:
http://www.straightdope.com/classics/a1_078b.html
But of course, this is quite rare. The ordinary method, per the OP, is to record multiple tracks separately.
The first person to back up herself was Peggy Lee, with Les Paul producing the record.
That was, of course, through double-tracking (as digital recording didn’t exist at the time).