A classical-singer acquaintance of mine, many years ago, explained "pear-shaped tones" by an example of singers' training:
As a training exercise, the singer speaks, "How now, brown cow?" But it's said something like this "Hah-oh nah-oh, brah-own cah-ow?"
Each of those words is smoothed out in its diphthong, of course, rather than broken up, as I have had to do it, with hyphens. The result is an "ow" that smoothly changes shape in the saying, just as a pear changes shape from one end to the other.