Physijs double collision issue

<!DOCTYPE html>

<html>

<head>
    <title>Rigid body - Physijs</title>

    <link rel="stylesheet" type="text/css" href="css/styles.css" />

    <script type="text/javascript" src="js/three.min.js"></script>
    <script type="text/javascript" src="js/stats.js"></script>
    <script type="text/javascript" src="../physi.js"></script>

    <script type="text/javascript">

    'use strict';

    Physijs.scripts.worker = '../physijs_worker.js';
    Physijs.scripts.ammo = 'examples/js/ammo.js';

    var initScene, render, applyForce, setMousePosition, mouse_position,
        ground_material, box_material,
        projector, renderer, render_stats, physics_stats, scene, ground, light, camera, box, sphere, boxes = [];

    initScene = function() {
        projector = new THREE.Projector;

        renderer = new THREE.WebGLRenderer({ antialias: true });
        renderer.setSize( window.innerWidth, window.innerHeight );
        renderer.shadowMapEnabled = true;
        renderer.shadowMapSoft = true;
        document.getElementById( 'viewport' ).appendChild( renderer.domElement );

        render_stats = new Stats();
        render_stats.domElement.style.position = 'absolute';
        render_stats.domElement.style.top = '1px';
        render_stats.domElement.style.zIndex = 100;
        document.getElementById( 'viewport' ).appendChild( render_stats.domElement );

        physics_stats = new Stats();
        physics_stats.domElement.style.position = 'absolute';
        physics_stats.domElement.style.top = '50px';
        physics_stats.domElement.style.zIndex = 100;
        document.getElementById( 'viewport' ).appendChild( physics_stats.domElement );

        scene = new Physijs.Scene;
        scene.setGravity(new THREE.Vector3( 0, -30, 0 ));
        scene.addEventListener(
            'update',
            function() {
                //applyForce();
                scene.simulate( undefined, 1 );
                physics_stats.update();
            }
        );

        camera = new THREE.PerspectiveCamera(
            35,
            window.innerWidth / window.innerHeight,
            1,
            1000
        );
        camera.position.set( 60, 50, 60 );
        camera.lookAt( scene.position );
        scene.add( camera );

        // Light
        light = new THREE.DirectionalLight( 0xFFFFFF );
        light.position.set( 20, 40, -15 );
        light.target.position.copy( scene.position );
        light.castShadow = true;
        light.shadowCameraLeft = -60;
        light.shadowCameraTop = -60;
        light.shadowCameraRight = 60;
        light.shadowCameraBottom = 60;
        light.shadowCameraNear = 20;
        light.shadowCameraFar = 200;
        light.shadowBias = -.0001
        light.shadowMapWidth = light.shadowMapHeight = 2048;
        light.shadowDarkness = .7;
        scene.add( light );

        // Materials
        ground_material = Physijs.createMaterial(
            new THREE.MeshLambertMaterial({ map: THREE.ImageUtils.loadTexture( 'images/rocks.jpg' ) }),
            .8, // high friction
            .4 // low restitution
        );
        ground_material.map.wrapS = ground_material.map.wrapT = THREE.RepeatWrapping;
        ground_material.map.repeat.set( 3, 3 );

        box_material = Physijs.createMaterial(
            new THREE.MeshLambertMaterial({ map: THREE.ImageUtils.loadTexture( 'images/plywood.jpg' ) }),
            .4, // low friction
            .6 // high restitution
        );
        box_material.map.wrapS = ground_material.map.wrapT = THREE.RepeatWrapping;
        box_material.map.repeat.set( .25, .25 );

        // Ground
        ground = new Physijs.BoxMesh(
            new THREE.CubeGeometry(100, 1, 100),
            ground_material,
            0 // mass
        );
        ground.receiveShadow = true;
        scene.add( ground );


        box = new Physijs.BoxMesh(
            new THREE.CubeGeometry( 4, 4, 4 ),
            box_material
        );
        box.position.set(
            0,
            2,
            0
        );
        box.castShadow = true;
        scene.add( box );
        boxes.push( box );


        sphere = new Physijs.SphereMesh(
            new THREE.SphereGeometry( 2, 8, 16 ),
            box_material
        );
        sphere.position.set(
            0,
            8,
            0
        );
        sphere.addEventListener( 'collision', function ( other_object, relative_velocity, relative_rotation, contact_normal ) {

            // `this` has collided with `other_object` with an impact speed of `relative_velocity` and a rotational force of `relative_rotation` and at normal `contact_normal`
            //console.log('collision', new Date().getTime(), this.name, other_object.name);

                        console.log('speed', relative_velocity.length(), new Date().getTime(), relative_velocity);

        } );
        sphere.castShadow = true;
        scene.add( sphere );


        //renderer.domElement.addEventListener( 'mousemove', setMousePosition );

        requestAnimationFrame( render );
        scene.simulate();
    };

    render = function() {
        requestAnimationFrame( render );
        renderer.render( scene, camera );
        render_stats.update();
    };

    window.onload = initScene;

    </script>
</head>

<body>
    <div id="heading">
        <h1>Rigid Body Double Collision</h1>
    </div>
    <div id="viewport"></div>
</body>

</html>