added sound for drift velocity, see #479

js/balloons-and-static-electricity/view/BalloonNode.js

@@ -36,6 +36,7 @@ import BASEQueryParameters from '../BASEQueryParameters.js';
 import BalloonDirectionEnum from '../model/BalloonDirectionEnum.js';
 import PlayAreaMap from '../model/PlayAreaMap.js';
 import BalloonInteractionCueNode from './BalloonInteractionCueNode.js';
+import BalloonVelocitySoundGenerator from './BalloonVelocitySoundGenerator.js';
 import BalloonDescriber from './describers/BalloonDescriber.js';
 import MinusChargeNode from './MinusChargeNode.js';
 import PlusChargeNode from './PlusChargeNode.js';
@@ -207,6 +208,9 @@ class BalloonNode extends Node {
       }
     } );
 
+    // sound generation for drift velocity
+    soundManager.addSoundGenerator( new BalloonVelocitySoundGenerator( model.velocityProperty ) );
+
     // pdom
     balloonImageNode.focusHighlight = new FocusHighlightFromNode( balloonImageNode );
 
@@ -319,10 +323,6 @@ class BalloonNode extends Node {
       grabDragInteraction.reset();
     } );
 
-    model.velocityProperty.link( velocity => {
-      console.log( 'velocity.magnitude = ' + velocity.magnitude );
-    } );
-
     // Handle a query parameter that adds a line and a marker at the "charge center".  This can be useful for debugging.
     if ( BASEQueryParameters.showBalloonChargeCenter ) {
       const parentToLocalChargeCenter = this.parentToLocalPoint( model.getChargeCenter() );

js/balloons-and-static-electricity/view/BalloonVelocitySoundGenerator.js

@@ -0,0 +1,62 @@
+// Copyright 2018-2020, University of Colorado Boulder
+
+/**
+ * BalloonVelocitySoundGenerator is used to produce a sound that corresponds to the drifting velocity of the balloon.
+ * It does NOT produce sound when the balloon is being dragged by the user.
+ *
+ * @author John Blanco
+ */
+
+import LinearFunction from '../../../../dot/js/LinearFunction.js';
+import merge from '../../../../phet-core/js/merge.js';
+import NoiseGenerator from '../../../../tambo/js/sound-generators/NoiseGenerator.js';
+import balloonsAndStaticElectricity from '../../balloonsAndStaticElectricity.js';
+
+// constants
+const INITIAL_CENTER_FREQUENCY = 500;
+
+// function for mapping the speed of the balloon to the center frequency of the filter
+const mapSpeedToFrequency = new LinearFunction( 0, 2, INITIAL_CENTER_FREQUENCY, INITIAL_CENTER_FREQUENCY * 1.5 );
+
+class BalloonVelocitySoundGenerator extends NoiseGenerator {
+
+  /**
+   * {Property.<number>} balloonVelocityProperty - velocity of the balloon when drifting (i.e. when it is not being
+   * dragged by a user).
+   * {Object} [options]
+   */
+  constructor( balloonVelocityProperty, options ) {
+
+    options = merge( {
+        noiseType: 'pink',
+        centerFrequency: INITIAL_CENTER_FREQUENCY,
+        qFactor: 200,
+
+        // The output level has to be far higher than normal, since so much of the noise energy ends up getting filtered
+        // out.
+        initialOutputLevel: 15.0
+      },
+      options
+    );
+
+    super( options );
+
+    // monitor the molecule oscillation amplitude and update local state
+    balloonVelocityProperty.lazyLink( velocity => {
+      const speed = velocity.magnitude;
+      if ( speed > 0 ) {
+        if ( !this.isPlaying ){
+          this.start();
+        }
+        this.setBandpassFilterCenterFrequency( mapSpeedToFrequency( speed ) );
+      }
+      else if ( speed === 0 && this.isPlaying ) {
+        this.stop();
+      }
+    } );
+  }
+}
+
+balloonsAndStaticElectricity.register( 'BalloonVelocitySoundGenerator', BalloonVelocitySoundGenerator );
+
+export default BalloonVelocitySoundGenerator;