Suppose you hear a sound during meditation, when you hear the sound you recognise the sound. You will narrow it down to sound only, do not differentiate it. Then you keep paying attention to the sound, and you will find that it is not static. It is changing. As the sound changes, your perception of the sound changes with it. Perception of the sound is Impermanence. If the sound is unpleasant, then your perception of the sound is Unsatisfactory. Since there is no way you can change the sound the way you like, we call it Selflessness. When you see the three characteristics of the sound, everything is like that. This is how we train our mind to perceive the sound as the object of our meditation.
– Bhante Henepola Gunaratana on Meditation on Perception