$surfaceprop
$surfaceprop is a QC command available in all 
Source games.
…
It is also available as a shader parameter. It links the surface of either a material or model to a set of physical properties, including:
- What sounds are emitted during collisions
- What effects are emitted when the surface is shot
- Mass, buoyancy, bounciness...
- The effect of the surface on nearby sounds (reverberation, echo, absorption...)
Bug:(in 
) Not including a $surfaceprop in a vmt for brushes will cause the material to take on the sound (and perhaps the other properties as well) of whatever the last material that was interacted with which did have a $surfaceprop defined.
Workaround: Use $surfaceprop "default".
Contents
Syntax
Syntax is the same for both models and materials:
$surfaceprop <name>
Names are defined in <game>\scripts\surfaceproperties_manifest.txt (and the files it references). For a list of Valve's stock props, see Material surface properties.
Bug:
Left 4 Dead does not display bullet decals properly on brick surfaces. If you're okay with the decals and visual effects being inappropriate, you can change the $surfaceprop to something else to fix it, such as concrete.
Confirm: Is this fixed in 
Left 4 Dead 2?
In Materials
$surfacepropis not linked to any particular shader, so it can be used at any time.- Only brush based shaders require this parameter.
$surfacepropmust always be defined within the collision property of the QC file if anything other than the default is to be used, however.
In Models
- It is unclear whether a model's mass is calculated from
$surfaceproporprop_data. - Surface props can be defined for individual bones with
$jointsurfaceprop. - A static prop can only have one $surfaceprop.
