public void setRefDistance(float refDistance) {
        if (refDistance < 0) {
            throw new IllegalArgumentException("Reference distance cannot be negative");
        }

        this.refDistance = refDistance;
        if (channel >= 0)
            getRenderer().updateSourceParam(this, AudioParam.RefDistance);
    }
Set the reference playing distance for the audio node.
Does nothing if the audio node is not positional.
<br>
The reference playing distance is the distance at which the
audio node will be exactly half of its volume.

@param refDistance The reference playing distance.
@throws IllegalArgumentException If refDistance is negative
jme3-core/src/main/java/com/jme3/audio/AudioNode.java