Physical sound – how to

update: also available in Polish (slides) Bullet i dźwięki pdf

I decided to create short tutorial about physics based sound. In previous post video you was able to hear my audio. Now I would like to share my short conception with you. Example written in bullet physics engine.

Of course the only way to determine collisions is to search contacts points used by physics engine. But contact points aren’t obvious. We must to check if they meet interesting for us conditions. Currently  I am using two physics sounds:

Impact sound

It should be played when some object hits other object. Typically this is short, loud sound dependent on the materials which objects are made of. Browsing contacts should be done after stepSimulation method defined in collisionWorld class if you have implemented fixed time step physics (see fix-your-timestep article) if you don’t, you can do it using bullet’s internal tick callback.

So you probably want to play “hit” sound when contact point meets this contitions (they work well for me)

  • object really collide with other so contact point distance is less then zero
  • hitting force reaches certain threshold (you must try which is best for you)
  • optionally lifetime of contact point  = 1, so it is just created (I have this condition just in case)

Scratching sound

When some object is sliding on other. It also could depend on objects materials and as extension it can also depend on object sliding velocity (of course if you have such option in your sound library). If greater velocity then sound pitch (frequency) should be greater. Such action will give realism to the sound, like in real world. Here are my conditions for sliding sound effect:

  • length of difference of two object velocity vectors greater than some  small amount (depends on your physics world scaling). Vectors difference is needed to ensure that object is really sliding, this also prevents situation when one object is at rest on another moving object. In this case sliding sound shouldn’t be played, all two object have some velocity, but relatively to each other they aren’t moving and difference of this objects velocities is of course 0
  • contact point distance is less than small value but greater than zero, so one object is located on second

My code example ( magic values works for me, but i have the world scaled by 64 ):

[sourcecode language=”cpp”]
for (int i=0;i< dynamicsWorld->getDispatcher()->getNumManifolds();i++)
btPersistentManifold* contactManifold = dynamicsWorld->getDispatcher()->getManifoldByIndexInternal(i);
for(int t=0;tgetNumContacts();++t)
auto point=contactManifold->getContactPoint(t);
if(point.getDistance()<-0.5f && point.getLifeTime()==1 && point.getAppliedImpulse()>100.f)
//play impact sound
if(((contactManifold->getBody0()->getLinearVelocity()-contactManifold->getBody1()->getLinearVelocity()).length()>4.f) && point.getDistance()<1.5f )
//play sliding sound

This easy, short code can really improve audio of your applications. There is still issue – how to play this sounds. I’m using such rules: For impact sound, I’m just playing it on first available channel. For sliding sound I’m using resume / pause method, but You can handle it yourself.

sound, springs, trees and bushes

Ok, at last my game prototype has audio. To achieve that I used SDL_mixer which has quite easy api, and with this wasn’t problem. More difficult part was to determine when certain sound must be played. Of course I am not writing here about trivial cases like player jump, or when player shoots than play “boom” sound. The difficulty is in physics based sounds because of collision ambiguity.  I created two types of physical sound:

  • first is  impact sound e.g.  when box falls to the ground
  • second – sound of scratching e.g. when some object is sliding on ground

All this you can hear on video (sound was recorded little too loud, so there are some defects )



On the video you can also see upgraded graphics. Adding some plants can really refresh game appearance,  they don’t have physical body they can be displayed behind or before player (hiding places). They are only purely cosmetic aspect.

The last new thing is the spring implemented from bullet physics engine, which may have various frequency of oscillation.

Now I must think about player improvement. I am thinking about creating him legs or feet 🙂