Sure A,
The best examples come from instruments.
Think of a trombonist and pay attention to the slide.
When the player hits the note and starts the vibrato it starts slowly and builds in speed and then ramps back down in speed.
I dont know if you younger guys have ever seen a Leslie speaker in action.
Usually used in conjunction with a Hammond organ (B3/C3/A100 etc) the top and lower part of the Leslie rotates. (Well the bottom does not actually rotate but there is a baffle that ‘swishes’ the sound around).
When the player activates the ‘Leslie’ function, this changes the speed of the rotation; but it is not instantaneous.
Like in the trombone example, the vibrato starts out slow, then builds in speed and then ramps back down.
To hears some great organ examples of this, check out some (old very old) recordings of Booker T and the MG’s (Time is Tight etc)
This technique can be achieved by a vocalist but the singer has to be very good or work really hard to do/learn it. Usually both. Tho’ one time I heard a maid in my hotel doing it as she was cleaning rooms. Some folks have the talent and don’t even know it. (lucky stiffs)
Now scale this up to a group of singers.
The Mills Brothers and Andrew Sisters or Glady Knight and the Pips, Alabama, Carpenters
could nail this.
This is called: ‘Blood Blend’ meaning that families grow up talking/listening and singing in the same way. ‘It’s in the blood’ was the way it was put to me.
Give it a try yourself.
Start singing a note and then apply vibrato.
Now try and change the speed of the vibrato. With practice (and some talent) it can be done.
Now if you can get a number of singers doing it at the same time, the result is magical.
These days I am practicing Mongolian throat singing. I’m terrible at it but I’ll keep trying.
(It’s fun to do and it annoys my wife:)
BTW
Notice that nothing I have written here (re: vocalist) has anything to do with recording gear.
Once you get the vocal source happening, then
it’s time to slap on your compression,eq, verb etc and ‘bob’s yer uncle’.
And the better the vocals, the less of that other stuff is needed.
HTH
{‘-’}