# Which and how different factors affect the sound made by the collision of tiny solid particles?

In daily life, the sound of mixing flour is almost undetectable; the sound of hitting the sandbag is dull; The maracas used as a musical instrument has a bright and loud sound; the sound is clear and melodious. The sound produced by the earth's particles under the disturbance of external forces is affected by many factors and produces different sound characteristics.

So I wander which and how different factors actually affect the sound made by the collision of tiny solid particles such as sand. And I'm eager to learn the mathematical expressions between the sound and different physical factors.

P.S. The sound here is not restricted to a single factor; and the physical factors here should only be related to the particle itself, the container or the outer force that causes the collision.

Here is a stop-gap answer. Let's hope you get a better one!

Here are two key factors determining the noise made by colliding bodies.

1. Are the bodies set vibrating at audible frequencies by standing waves within them?

This is clearly the case for a drum, cymbal or triangle hit by a suitable 'hammer'. It would not be the case for grains of flour: because of their small size any standing waves set up inside them would be ultrasonic, that is of too high a frequency for us to hear.

1. Are the bodies large enough to set enough air moving when they collide for an audible amount of sound power to reach our ears?

Again, this will be fine for most objects large enough for us to see with the unaided eye, but will not be the case for grains of flour.

• Thanks a lot! Just one question, do you mean that the 'sound' created by the collisions are mainly caused by the standing waves other than other sources? Oct 1, 2021 at 14:03
• Standing waves are clearly important for bodies that give a musical sound when hit, and I suspect that heavily damped standing waves nay be involved when bodies emit various banging sounds, but I wouldn't want to claim that standing waves are always involved when you can hear a collision between bodies. That's why we want an answer from an acoustics expert. Oct 1, 2021 at 14:44