{
   "id":"_ID_",
   "method":"hub.scenes.expressions.set",
   "params":{
      "name": "HalfOfSoundLevel",
      "code": "return params.sound_level / 2",
      "params": {
        "items": [
            { "name":  "sound_level", "_id": "sirenSoundLevelItemIdentifier"}
        ]
      }
    }
}