@jameslo It's an accurate and useful estimation method, thanks a lot!

@Balwyn Thank you so much. That's a really clear answer. I get it now!

The question is as shown in the title. Thank you in advance for your answer! ：）

@whale-av Thank you so much! It works! : )

In order to transition from a sine wave to a triangular wave, I combinined the two sound waves by adjusting the gain of the two waves to each other, i.e. the triangular wave gain (a) drops uniformly from 1.0 on the left to 0.0 on the right, while the sine wave gain increases accordingly (always 1-a).

It worked well at first. However, when I ran it again, the volume changed. The exact same procedure, which I can only explain in metaphysical terms

Maybe someone can answer it for me? Thanks so much
Here is the picture view of my problem：