# B Collision sound

#### Borek

Mentor
Summary
looking for intuition on pitch and volume of a collision sound
I need to develop some kind of simplified formula that will allow me to estimate pitch and volume of a sound of two colliding objects in a simulation. It doesn't have to be exact, mostly it is enough that it follows the intuition - large object produce lower sounds, large and fast object produce louder sounds. To simplify things let's say objects are spherical cows ('moo' doesn't count). Physical engine returns impulse of the collision, I also know mass and volume of the object.

So far I had some success with the pitch - I assumed that the oscillations of a solid object will be in a way similar to the oscillations of air in the Helmholtz resonator (one of the modes of the oscillations of a solid can be contraction/swelling, doesn't it?), that suggested frequency being proportional to $V^{\frac {-1} 2}$ and after some tinkering with constants I am quite happy with the result - when I look at the collision and listen to the sounds it feels natural.

It is loudness that is a problem. I did some random checks making loudness linearly dependent on the impulse or impulse/mass ratio, but I don't like the effect. If nothing else works I will try to test exponential and logarithmic dependencies by brute force, but it will be time consuming. Do you have any idea about what the real thing is? Or some analogies that could be adapted?

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#### A.T.

large object produce lower sounds
The stiffness of the materials involved also affects the pitch.

Staff Emeritus
large object produce lower sounds
Is that true? It stands to reason, but I have heard some very low, almost thundery sounds from objects less than a half meter across, and much higher sounds from cars.

#### Borek

Mentor
The stiffness of the materials involved also affects the pitch.
Definitely. I am assuming all objects are made from more or less the same material. Also for the same volume/mass something long and thin will produce different pitch than a sphere, but I am trying to keep things simple (hence the reference to the spherical cow).

Is that true? It stands to reason, but I have heard some very low, almost thundery sounds from objects less than a half meter across, and much higher sounds from cars.
Simulation combines initially stationary objects that start to fall or roll when pushed, and they are close to each other to allow interactions like in a domino effect, so the speed of the colliding objects is rather low. Hopefully that allows the simplification. But I am open to suggestions.

#### 256bits

Gold Member
Your physics engine should be using something similar to this for ideal elastic collisions.
So knowing Young's modulus and Poison's ratio for the materials of the object, the time of interaction can be determined, and hence the impulse.

Yet no elastic collision is ideal, as evident by the energy given off by sound from the collision.
What mathematical relation impulse and the energy emitted by the collision have to one another as the sound will have an energy content, as well as the two bodies absorbing some energy as heat from the interaction.

Nonetheless, if that relationship is linear, or exponential, or whatever, the energy emitted with the sound may not be perceived again in a linear fashion with our ears. I am sure you have heard of sound decibel levels regarding intensity and pressure. Twice as much energy in the sound wave is most likely not seen as being twice the loudness, and is frequency dependent also if I recall.

I forget my sound knowledge so I can't give a better statement than that.
I t might lead you though with some keywords for a search on our ears perceiving sound.

#### Klystron

Gold Member
The author of this NASA publication develops three dimensional (3-D) audio simulations for human factor and crew studies. This sim includes spatialized simulated sound including speech synthesis and ambient audio but too long ago for me to remember coding details.

Probably more relevant if your cows talked to each other but may spark some ideas.

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"Collision sound"

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