determining the ripeness of (Annona) fruit by purcussing

[FlyingFoxFruits]

sometimes it can be difficult to determine when to harvest certain fruits (in my case guanabana)

in short, I have found that you can tell when certain fruits are ripe by percussing them with your finger.

gently tap the fruit, and you will hear a hollow sound...the same as you would with a watermelon.  I suppose the moisture in a ripe fruit allows the vibrations to reverberate a little bit, as opposed to when the fruit is still green, the dry unripe pulp will not allow for such a sound.

[Grapebush]

Here some people (me included) use a similar technique; gently hit the fruit the fruit a few times with your hand, and then shake it, if you can hear the sound of the seeds moving inside, you can pick it, and it will be ready in 3 to 5 days.
Fruits harvested that way, are usualy sweeter than the ones picked already ripe from the tree.

[Guanabanus]

Interesting   I hadn't compared sweetness of tree ripened versus mature-picked fruits.

I have usually gone by appearance-- spreading carpels, slightest yellowing.